Renal Hemodynamic Function and RAAS Activation Over the Natural History of Type 1 Diabetes

Abstract

Rationale & Objective

The renin angiotensin aldosterone system (RAAS) is associated with renal and cardiovascular disease in diabetes. Unfortunately, early RAAS blockade in patients with type 1 diabetes (T1D) does not prevent development of complications. We sought to examine the role of hyperfiltration and RAAS activation across a wide range of T1D duration to better understand the renal hemodynamic changes overtime.

Study Design

Post-hoc analysis of blood samples.

Setting & Participants

148 Canadian patients with T1D: 28 adolescents (16.2±2.0yrs), 54 young adults (25.4±5.6yrs), and 66 older adults (65.7±7.5yrs) studied in a clinical investigation unit.

Exposure

Angiotensin II infusion (ANGII, 1 ng∙kg⁻¹∙min⁻¹ – a measure of RAAS activation) during a euglycemic clamp.

Outcomes

Glomerular filtration rate (GFR_INULIN), effective renal plasma flow (ERPF_PAH), afferent (R_A) and efferent (R_E) arteriolar resistances, and glomerular hydrostatic pressure (P_GLO) estimated using Gomez’s equations.

Results

In a step-wise fashion, GFR_INULIN, ERPF_PAH and P_GLO were higher while renal vascular resistance (RVR) and R_A were lower in adolescents vs. young adults vs. older adults. R_E was similar in adolescents vs. young adults but was higher in older adults. ANGII resulted in blunted renal hemodynamic responses in older adults (RVR increase of 3.3±1.6% vs. 4.9±1.9% in adolescents, p < 0.001), suggesting a state of enhanced RAAS activation.

Limitations

Homogeneous study participants limit generalizability of findings to other populations. Studying older adult T1D participants may be associated with a survivorship bias.

Conclusions

A state of relatively low RAAS activity and predominant afferent dilation rather than efferent constriction characterize early adolescent and young adults with T1D. Given this state of endogenous RAAS inactivity in early T1D, may explain why pharmacological blockade of this neurohormonal system is often ineffective in reducing kidney disease progression in this setting. Older adults with longstanding T1D who have predominant afferent constriction and RAAS activation may experience renoprotection from therapies that target the afferent arteriole. Further work is required to understand the potential role of non-RAAS pharmacologic agents that target R_A in patients with early and longstanding T1D.

Nontechnical Summary

Renin angiotensin aldosterone system (RAAS) is associated with kidney and heart complications in type 1 diabetes (T1D), which are not completely prevented by RAAS blockers. We examined kidney function across a wide range of T1D durations by studying adolescents, young adults and older adults. The results of our analyses demonstrated that adolescents with T1D have low RAAS activity and have dilated arterioles flowing into the kidney. This may cause an increased glomerular filtration rate that is thought to mediate kidney injury. Older adults with T1D had increased RAAS activity and constricted arterioles flowing into the kidney. Our results suggest that medications that alter the tone of kidney inflow arterioles may protect or delay kidney complications in both early and longstanding T1D.

INTRODUCTION

In 1993, the Diabetes Control and Complications Trial (DCCT) demonstrated that intensive glycemic control reduces the risk of complications in patients with type 1 diabetes (T1D). Since that time, significant technological advances have been made leading to improved glycemic control and decreased rates of micro- and macrovascular complications in T1D ^1,2. Despite the declining rates of advanced chronic kidney disease and end-stage renal disease (ESRD) in T1D, diabetic nephropathy (DN) remains relatively common. In the absence of DN the 20 year mortality risk for T1D is comparable to the general population ³. The presence of DN, however carries a substantial cardiovascular risk, which is the leading cause of mortality in T1D ⁴. Changes in renal hemodynamic function, such as intraglomerular hypertension or hyperfiltration, are linked with the initiation and progression of DN via increased glomerular wall tension. Although most studies suggest that hyperfiltration is an early marker of renal disease, some studies fail to detect such an association ⁵. One of the key factors in DN pathogenesis is the chronic renal and systemic activation of the renin angiotensin aldosterone system (RAAS), which initiates tissue injury via fibrosis, production of reactive oxygen species, proinflammatory pathways, and increases in intraglomerular pressure ⁴.

Although RAAS inhibitors are the current standard of care for nephroprotection in DN, they do not completely protect patients with T1D. In the Renin–Angiotensin System Study (RASS) in adults with uncomplicated T1D, angiotensin converting enzyme (ACE) inhibitor or an angiotensin receptor blocker (ARB) did not prevent clinical or histological evidence of nephropathy over five years ⁶. RAAS blockade was also recently studied at even earlier stages of the disease in adolescents with T1D, since pathogenic processes leading to future complications may accelerate during puberty ^7,8. Unfortunately, in the Adolescent Cardiorenal Intervention Trial (AdDIT), ACE inhibition was shown not to be effective as a primary renal disease prevention strategy ⁹.

Improving our understanding of changes in renal physiology and hemodynamic function from the earliest stages of T1D over time, including the role of hyperfiltration, may allow us to better characterise the mechanisms underlying DN pathogenesis. We have previously compared renal hemodynamic function in young T1D adults with and without hyperfiltration ¹⁰ cross-sectionally in a homogenous patient cohort. Yet, little is known about changes in renal hemodynamic function over time and in response to RAAS activation over a much broader range of age and T1D duration. Accordingly, our aim was to perform an exploratory analysis of kidney function, hemodynamic changes and RAAS activation over the natural history of T1D in patients across a wide range diabetes duration, including adolescents, young adults and older adults with T1D.

METHODS

Subject Inclusion Criteria and Experimental Design

We conducted a post-hoc analysis to compare renal hemodynamic function in patients with T1D: adolescents (n=28), young adults (n=54) and older adults (n=66) using archived plasma samples from our earlier studies where ANGII infusions were performed and primary study results were previously reported ^4,11–17. Detailed baseline demographic characteristics were previously reported. All patients were studied under clamped euglycemic conditions (4–6 mmol/L). All participants from the older adult T1D cohort underwent RAAS inhibitor (ACE inhibitors, ARBs, direct renin inhibitors, aldosterone antagonists) washout 30 days prior to the study measurements. All studies were performed after a 7 day diet consisting of ≥150 mmol/day sodium and ≤1.5 g/kg/day protein. The sodium-replete diet was used to avoid circulating RAAS activation, volume contraction, between-subject heterogeneity and in an attempt to keep study conditions similar to typical North American dietary patterns. Pre-study protein intake was monitored to avoid the hyperfiltration effect of high protein diets. All study participants were instructed to avoid caffeine- containing products and to have the same light breakfast on the morning of each study visit. Studies were carried out in accordance with the Declaration of Helsinki, all study participants gave their informed consent and the study was approved by the University Health Network research ethics board.

Assessment of Renal Hemodynamic Function

Renal hemodynamic function (glomerular filtration rate [GFR] and ERPF) was measured using inulin and PAH clearance according to the plasma disappearance technique ^15,18. The mean of the final 2 clearance periods represented baseline GFR and ERPF, expressed per 1.73 m².

The following parameters were calculated:

$$\mathrm{Filtration}\mathrm{fraction}(\mathrm{FF})=\frac{GFR}{ERPF}$$

$$\mathrm{Renal}\mathrm{blood}\mathrm{flow}(\mathrm{RBF})=\frac{ERPF}{1-Hematocrit}$$

$$\mathrm{Renal}\mathrm{vascular}\mathrm{resistance}(\mathrm{RVR})=\frac{MAP}{RBF}$$

Due to the inability to differentiate between afferent (R_A) and efferent arteriolar resistances (R_E) in humans, studies of renal haemodynamic function have relied primarily on assessments of GFR. In 1951, Gomez et al derived equations for indirect estimates of R_A and R_E, filtration pressure (P_F) and glomerular hydrostatic pressure (P_GLO) ¹⁹.

Indirect intraglomerular hemodynamic parameters were estimated using Gomez formulae based on data from animal studies (Figure 1). These equations were used in a similar manner to evaluate patients with conditions such as hypertension, endocrine disorders and T1D ^20–22. Assumptions imposed by Gomez’s equations were the following: i) intrarenal vascular resistances are divided into afferent, post-glomerular and efferent; ii) hydrostatic pressures within the renal tubules, venules, Bowman’s space and interstitium (P_Bow) are in equilibrium of 10 mmHg; iii) glomerulus is in filtration disequilibrium; iv) the gross filtration coefficient (K_FG) = 0.1012 ml/s per mmHg (P_GLO = 56.4 mmHg) to reflect the P_GLO values in diabetic conditions observed in previous micropuncture studies in Munich-Wistar rats ²³. MAP (mmHg), ERPF (ml/s), GFR (mL/s) and total protein (g/dL) were used to calculate R_E (dyne•sec•cm⁻⁵) and R_A (dyne•sec•cm⁻⁵), P_GLO (mmHg), ΔP_F (mmHg) and π_G (mmHg).

Figure 1.

Summary of Gomez equations and assumptions used to calculate intrarenal hemodynamic parameters.¹⁰

The filtration pressure across glomerular capillaries (ΔP_F):

$$\mathrm{\Delta }{\mathrm{P}}_{\mathrm{F}}=\mathrm{GFR}/{\mathrm{K}}_{\mathrm{FG}}$$

The glomerular oncotic pressure (π_G) from the plasma protein mean concentration (C_M) within the capillaries:

$${\mathrm{C}}_{\mathrm{M}}=\mathrm{TP}/\mathrm{FF}\times \ln (1/1-\mathrm{FF})$$

$${\mathrm{\pi }}_{\mathrm{G}}=5\times ({\mathrm{C}}_{\mathrm{M}}-2)$$

Glomerular hydrostatic pressure (P_GLO):

$${\mathrm{P}}_{\mathrm{GLO}}=\mathrm{\Delta }{\mathrm{P}}_{\mathrm{F}}+{\mathrm{P}}_{\mathrm{Bow}}+{\mathrm{\pi }}_{\mathrm{G}}$$

R_A and R_E were estimated using principles of Ohm’s law, where 1328 is the conversion factor to dyne•sec•cm^{−5 19}:

$$\begin{gathered} {\mathrm{R}}_{\mathrm{A}}=[(\mathrm{MAP}-{\mathrm{P}}_{\mathrm{GLO}})/\mathrm{RBF}]\times 1328 \\ {\mathrm{R}}_{\mathrm{E}}=[\mathrm{GFR}/({\mathrm{K}}_{\mathrm{FG}}\times (\mathrm{RBF}-\mathrm{GFR})]\times 1328 \end{gathered}$$

Assessment of Angiotensin II Infusion Response

After a stable euglycemic clamp for at least 3 hours, plasma renin and aldosterone levels were measured. In the adolescents with T1D group, such measurements were performed in 11 patients only. After baseline renal clearance measurements were complete, ANGII (Clinalpha, Laüfelfingen, Switzerland) was administered at 1 ng/kg/min over 30 minutes, followed by a 30-minute recovery phase ²⁴. Oscillometric brachial artery blood pressure measurements were obtained in duplicates in a reclining position every 5 minutes during the ANGII infusion (Critikon, Tampa, Florida, USA). Blood was collected during the ANGII infusion period and after a 30-minute recovery for HCT, inulin and PAH to assess renal hemodynamic parameters. Renal and systemic hemodynamic responses to ANGII infusion were used to measure RAAS activation. Upon infusion, ANGII binds angiotensin 2 type 1 receptors and causes vasoconstriction, leading to decreased GFR, ERPF, RBF, and increased RVR and blood pressure. Blunted responses to the ANGII infusion, therefore, indicate saturation of the angiotensin 2 type 1 receptor due to high levels of endogenous RAAS activation.

Statistical Analyses

Data are presented as mean ± standard deviation (SD), unless otherwise specified. To assess for between-group differences, analysis of variance with post-hoc Tukey’s test was used. The difference between renal hemodynamic parameters at baseline euglycemic clamp and 30 minutes after the 3ng/kg/min ANGII infusion were used to compare the ANGII response between the patient groups. Sensitivity analysis was performed to compare renal, intraglomerular and systemic hemodynamic parameters between groups when adjusted for sex, HbA1c and BMI. All variables presented were normally distributed except for plasma renin and aldosterone levels. Non-parametric Kruskal-Wallis test was used to compare plasma renin and aldosterone levels. Statistical significance was defined as p<0.05. All statistical analyses were performed using SAS v9.1.3 and GraphPad Prism software (version 5.0).

RESULTS

Baseline Characteristics

At baseline, BMI was greater in older patients with T1D compared to adolescents and young adults. There was a stepwise decrease in HbA1c from adolescents to adults to older adults and an increase in plasma renin levels. Plasma aldosterone levels were increased in older patients with T1D compared to young adults.

Baseline Renal Hemodynamic Function

In a step-wise fashion, GFR_inulin, ERPF_PAH, RBF, and P_GLO decreased, while FF, RVR and R_A increased in adolescents vs. young adults vs. older adults with T1D (Table 1, Figure 2). Blood pressure, heart rate and R_E were similar in adolescents vs. young adults, but significantly higher in older patients with T1D. Similar results were obtained in the sensitivity analysis, where renal, intraglomerular and systemic hemodynamic parameters adjusted for sex, HbA1c and BMI were compared between groups (Table 2).

Table 1.

Baseline Demographic Characteristics of Adolescent, Young Adult and Older Adult Patients with T1D.

Parameter	Adolescents T1D (n=28)	Young Adults T1D (n=54)	Older Adults T1D (n=66)	p-value
Sex (% male)	13 (46%)	30 (56%)	30 (45%)
Age (years)	16.2±2.0#&	25.4±5.6*&	65.7±7.5*#	<0.001
Diabetes duration (years)	11.8±2.6&	14.1±6.7&	54.6±5.7*#	<0.001
Body mass index (kg/m2)	24.3±3.5&	25.1±3.3&	26.6±3.9*#	0.009
Hemoglobin A1c – mmol/mol (%)	69.1±13.5#& (8.5±1.2) #&	62.3±13.7*& (7.8±1.2) *&	57.0±9.3*# (7.4±0.8)*#	<0.001
Hematocrit	0.397±0.037&	0.383±0.033&	0.349±0.034*#	<0.001
Plasma Protein	71.4±23.8#&	61.7±6.3*	60.4±5.9*	<0.001
Renal Hemodynamic Function
ERPF (mL/min/1.73m2)	733±179#&	656±128*&	447±101*#	<0.001
GFR (mL/min/1.73m2)	150±40#&	118±20*&	103±17*#	<0.001
Filtration fraction	0.214±0.072#	0.187±0.039*&	0.235±0.041#	<0.001
RBF (mL/min/1.73m2)	1214±280#&	1065±212*&	689±166*#	<0.001
RVR (mmHg/L/min)	0.069±0.017#&	0.079±0.022*&	0.135±0.035*#	<0.001
Intraglomerular Hemodynamic Parameters
PGLO (mmHg)	60.4±8.7#&	53.3±4.7*&	51.4±4.26*#	<0.001
RA (dyne•sec•cm−5)	1424±990#&	2183±995*&	4546±1714*#	<0.001
(RE (dyne•sec•cm−5)	1928±867&	1690±407&	2403±544*#	<0.001
RA/RE	0.84±0.69#&	1.33±0.57*&	1.92±0.69*#	<0.001
Systemic Hemodynamic Function
HR (bpm)	68±11	67±11	66±10	0.7
SBP (mmHg)	115±12&	112±9&	131±16*#	<0.001
DBP (mmHg)	63±8&	66±7	68±5*	0.008
MAP (mmHg)	80±9&	81±8&	88±7*#	<0.001
Plasma RAAS Markers
Renin (ng/L)***	0.16 (0.10–0.33)#&^	5.20 (3.20–8.90)*&	11.50 (5.90–17.70)*#	<0.001
Aldosterone (ng/dL)***	130 (86–185)^	73 (37–122)&	165 (104–299)#	<0.001
Therapy
RAAS inhibitor use prior to study** (% of patients)	0 (0%)	0 (0%)	54 (82%)

n, number of participants. Data are presented as mean ± SD.

^*p<0.05 vs. Adolescents with T1D

^#p<0.05 vs. Young Adults with T1D

^&p<0.05 vs. Older Adult with T1D

^**All the older adult T1D participants underwent RAAS inhibitor (ACE inhibitors, angiotensin receptor blockers, direct renin inhibitors, aldosterone antagonists) washout 30 days prior to the study measurements

^***Data are presented as a median (interquartile range)

^{^}Data available for 11 patients only.

ERPF: effective renal plasma flow; GFR: glomerular filtration rate; RBF: renal blood flow; RVR: renal vascular resistance; P_GLO: glomerular hydrostatic pressure; R_A: afferent arteriolar resistance; R_E: efferent arteriolar resistance; HR: heart rate; SBP: systolic blood pressure; DBP: diastolic blood pressure; MAP: mean arterial pressure.

Figure 2. Baseline GFR_INULIN (A), ERPF_PAH (B), RVR (C), R_A (D), R_E (E), P_GLO (F), and R_A/R_E ratio (J) in adolescents, young adults and older adult patients with T1D.

Adolescents T1D n=28, Young Adults T1D n=54, Older Adults T1D n=66; GFR_INULIN: glomerular filtration rate measured by inulin clearance; ERPF: effective renal plasma flow measured by paraaminohippurate (PAH) clearance; RVR: renal vascular resistance; R_A: afferent arteriolar resistance; R_E: efferent arteriolar resistance; P_GLO: glomerular hydrostatic pressure. Values are mean ± standard deviation. The bars in each figure represent significance levels.

Table 2.

Standardized Baseline Characteristics of Adolescent, Young Adult and Older Adult Patients with T1D adjusted for sex, HbA1c and BMI.

Parameter	Adolescents T1D (n=27^)	Young Adults T1D (n=54)	Older Adults T1D (n=66)	p-value
Renal Hemodynamic Function
(ERPF (mL/min/1.73m2)	741±26#&	655±18*&	449±17*#	<0.001
(GFR (mL/min/1.73m2)	145±5#&	118±3*&	104±3*#	<0.001
Filtration fraction	0.203±0.009&	0.187±0.006&	0.236±0.006*#	<0.001
RBF (mL/min/1.73m2)	1234±41#&	1061±28*&	692±27*#	<0.001
RVR (mmHg/L/min)	0.066±0.006#&	0.079±0.004*&	0.136±0.004*#	<0.001
Intraglomerular Hemodynamic Parameters
PGLO (mmHg)	59.2±1.1#&	53.2±0.7*	51.6±0.7*	<0.001
RA (dyne•sec•cm−5)	1394±301#&	2181±192*&	4573±178*#	<0.001
RE (dyne•sec•cm−5)	1728±102&	1687±65&	2414±60*#	<0.001
RA/RE	0.88±0.14#&	1.32±0.09*&	1.93±0.08*#	<0.001
Systemic Hemodynamic Function
HR (bpm)	66±2	67±1	67±1	0.7
SBP (mmHg)	115±3&	112±2&	131±2*#	<0.001
DBP (mmHg)	63±1&	66±1	68±1*	0.009
MAP (mmHg)	79±2&	80±1&	89±1*#	<0.001

Data are presented as mean (adjusted for sex, HbA1c and BMI) ± SE. The numbers were calculated assuming 49% of male, HbA1c of 6.8% and BMI of 25.9 kg/m² in all three groups.

^*p<0.05 vs. Adolescents with T1D

^#p<0.05 vs. Young Adults with T1D

^&p<0.05 vs. Older Adult with T1D

^**All the older adult T1D participants underwent RAAS inhibitor (ACE inhibitors, angiotensin receptor blockers, direct renin inhibitors, aldosterone antagonists) washout 30 days prior to the study measurements.

^{^}Data from 27 out of 28 adolescent T1D patients were used for the sensitivity analysis as 1 patient was missing HbA1c data and was excluded from this analysis.

ERPF: effective renal plasma flow; GFR: glomerular filtration rate; RBF: renal blood flow; RVR: renal vascular resistance; P_GLO: glomerular hydrostatic pressure; R_A: afferent arteriolar resistance; R_E: efferent arteriolar resistance; HR: heart rate; SBP: systolic blood pressure; DBP: diastolic blood pressure; MAP: mean arterial pressure.

Renal Hemodynamic Function in Response to an ANGII Infusion

In response to infused ANGII, ERPF and RBF were most attenuated in the older adult T1D cohort compared to the other two cohorts (p<0.0001 for both comparisons), and intermediate in the young adult cohort compared to the adolescents (p=0.03 for ERPF and p=0.02 for RBF) (Figure 3, Table S1). RVR response to the ANGII infusion was also blunted the most in older patients with T1D (RVR increase of 0.033±0.016 vs. 0.049±0.019mmHg/L/min in adolescents, p=0.0006). R_E and diastolic blood pressure responses were suppressed in the older adult T1D cohort compared to both other groups, whereas the systolic blood pressure response was lowest in the older adult T1D cohort vs. the young adult cohort only; the difference in the response vs. the adolescent cohort did not reach statistical significance.

Figure 3. Changes in GFR_INULIN (A), ERPF_PAH (B), RBF (C), RVR (D), P_GLO (E) R_A (F), R_E (G), SBP (H), DBP (I), Heart Rate (J), and R_A/R_E ratio (K) in response to infused angiotensin II in adolescents, young adults and older adult patients with T1D.

Adolescents T1D n=28 (n=24 for PGLO, R_A, and R_E), Young Adults T1D n=53 (n=54 for GFR, SBP, DBP and HR), Older Adults T1D n=57; GFR_INULIN: glomerular filtration rate measured by inulin clearance; ERPF: effective renal plasma flow measured by paraaminohippurate (PAH) clearance; RBF: renal blood flow; RVR: renal vascular resistance; P_GLO: glomerular hydrostatic pressure; R_A: afferent arteriolar resistance; R_E: efferent arteriolar resistance; SBP: systolic blood pressure; DBP: diastolic blood pressure; HR: heart rate. Values are mean ± SD. The bars in each figure represent significance levels.

DISCUSSION

Animal models have provided crucial insights into the role of renal hemodynamic mechanisms that contribute to the development of DN. Data from animal models may not accurately reflect human physiology in health or disease, and observations from experimental models may not lead to the development of novel therapies ^25,26, as demonstrated in several clinical trial development programs. Renal hemodynamic function is controlled by a number of complex autoregulatory mechanisms including tubuloglomerular feedback, neurohormonal mediators and myogenic responses, and each of these mechanisms may ultimately translate into a better understanding of human disease and/or therapy. Whatever new physiological insights and/or therapies are elucidated in present or future work, it is critical to determine the role of these therapies in the context of RAAS activation characteristic of diabetes. It is therefore of significant importance to better understand the role of the RAAS throughout the spectrum of diabetes duration. In this analysis, we used the Gomez equations to better define renal hemodynamic function changes over a wide range of age and T1D duration, and to examine the relationship between RAAS activation and segmental resistances in the kidney.

RAAS activation is physiologically important for the regulation of blood pressure and effective circulating volume. In addition, RAAS activation on the basis of chronic hyperglycemia in the setting of diabetes increases intraglomerular pressure and the initiation and progression of DN ^4,27–30. From a hemodynamic perspective, RAAS inhibitors are therefore of use due to their ability to reduce efferent arteriolar resistance and hence intraglomerular pressure ³¹. Observations from our analyses suggest that the increase in efferent arteriolar resistance is present only in patients with long-standing T1D, but not in adolescents or young adults with shorter T1D duration. In addition to these baseline hemodynamic differences, patients with long-standing T1D exhibited evidence of RAAS overactivation compared to those with shorter duration of T1D, as was evident through the blunted RVR, R_E and blood pressure responses to an ANGII infusion and by significantly greater plasma aldosterone and renin levels.

The high levels of efferent resistance due to RAAS overactivation with longer duration of T1D may be the reason why RAAS inhibitors are effectively used for nephroprotection in patients with DN, but were shown to be ineffective as primary prevention agents in young adult and even adolescent patients with shorter duration of T1D without complications ^6,9. Moreover, our data suggest that the increased levels of afferent resistance are also predominant in the older longstanding T1D cohort, suggesting that pharmacologic targeting of the afferent arteriole could further improve nephroprotection already observed with RAAS inhibitors.

It is currently not known how to identify who is at risk for the development of DN prior to the onset of elevated albumin excretion or renal function decline. Elevated albumin excretion is one of the earliest manifestations to emerge clinically in patients with T1D and is evident in 12–16% of adolescents ^32,33. However, albuminuria is not an ideal biomarker due to significant variability and lack of predictive value. Furthermore, primary prevention strategies with RAAS inhibitors have proven to be ineffective in people with T1D. It is therefore important to identify early subclinical renal hemodynamic changes in otherwise healthy adolescents and young adults with T1D who may preferentially benefit from the early intervention strategies. It is of further interest to define renal hemodynamic profiles in younger patients with T1D to help determine where to target new therapies in the renal microcirculation. For example, previous animal work directly measuring hydraulic pressure in diabetic rat afferent and efferent arteries and human work in adults with T1D estimating segmental resistances showed that hyperfiltration (GFR ≥135 ml/min/1.73m², an early marker of DN) is a result of a decrease in afferent arteriolar resistance without changes to the resistance at the efferent arteriole during eu- and hyperglycemia ^20,34. In the current analysis, we demonstrated that the afferent resistance is lower than the efferent resistance only in adolescents, the early T1D cohort that had significantly greater GFR with a large proportion of patients with hyperfiltration. Moreover, the increase in afferent arteriolar resistance is observed earlier than changes at the efferent arteriole, since the R_A significantly increased and glomerular pressure decreased in a stepwise fashion from adolescents to young adults to older patients with T1D.

Resistance at the afferent renal arteriole is in part mediated by tubuloglomerular feedback mechanisms whereby hyperglycemia augments proximal renal tubular sodium reabsorption via sodium-glucose cotransporters (SGLT) including SGLT2. The resulting decrease in sodium delivery to the macula densa causes afferent renal arteriole vasodilatation and hyperfiltration. Accordingly, SGLT2 inhibition in young adult patients with T1D and hyperfiltration leads to an attenuation of GFR via an increase in R_A ^20,35. Given that hyperfiltration is a promising early marker of progressive diabetic kidney disease and the predominant afferent vasodilatation in the younger cohorts in the current analysis, our results suggest that pharmacologic agents that target R_A in patients with shorter duration of T1D may be an important step towards early therapy for the prevention of hyperfiltration and DN. Although long-term renal effects of SGLT2 inhibition in T1D are not known, clinical trials have explored the use of SGLT2 inhibition in T1D for glycemic control ³⁶. The use of SGLT2 inhibitors in patients with T1D is of particular importance given the cardiovascular and renal benefits that have been observed with SGLT2 inhibitors in cohorts of patients with T2D in the EMPA-REG OUTCOME and CANVAS Program trials ³⁷. Background RAAS inhibition therapies were taken by 80% of these patients, suggesting that simultaneous targeting of R_A and R_E may provide synergistic nephroprotective effects in patients with longstanding diabetes. Another class of agent that acts to increase R_A are the glucagon-like peptide-1 agonists (GLP-1RA), which are indicated for glucose control in patients with type 2 diabetes ³⁸. Similarly to SGLT2 inhibitors, 4 year treatment of patients with type 2 diabetes with GLP-1RA in addition to standard care, which largely include RAAS inhibitors, resulted in lower rate of development and progression of diabetic kidney disease and lower rates of new onset of severely increased albuminuria compared to placebo in the LEADER trial ³⁹. For patients with longstanding T1D with or without background CKD, although hyperfiltration at the whole-kidney level is exceedingly rare, hyperfiltration at the single-nephron level is likely to be present in a significant proportion of patients ⁴⁰ – which should also be ameliorated with SGLT2 inhibition or GLP-1RA treatment. Therefore, based on this physiological rationale, patients with longstanding duration of T1D may also benefit from agents targeting R_A – a possibility that merits exploration in future clinical trials.

Our study does have limitations. The relatively homogenous study groups limits the generalizability of our findings to other populations, such as those with proteinuria and overt nephropathy. We also acknowledge that in our older adult T1D group there may be a survivorship bias, where study participants had to survive 50 years with T1D to be eligible for the study. Moreover, improved glycemic control in the last 25 years has been linked to decreasing rate of developing complications in T1D. Therefore, due to the lack of longitudinal data for our study groups, we cannot determine what proportion of study participants, especially in the adolescent and young adult cohort, will eventually develop renal complications. Additionally, the lack of data for healthy control participants without diabetes in our cohort does not allow us to conclusively determine if our observations are a result of diabetes duration or age. Data on R_A and R_E in healthy controls compared to T1D are scarce. We previously observed that in a group similar to the young adult cohort there were no differences between R_A and between R_E when comparing T1D and age and sex-matched healthy controls ²¹. In another young adult cohort, we established that T1D with normofiltration have greater R_A and T1D with hyperfiltration had lower R_A, while no differences in R_E were observed when compared to healthy controls. The data for R_A and R_E comparisons of our older T1D cohort compared to healthy controls were previously published and showed that R_E is greater, but R_A does not appear to differ when comparing healthy controls with participants with T1D with resistance to diabetic kidney disease ⁴. While these previous observations suggest there may be R_A differences in the young adult T1D cohort and R_E differences in the older T1D cohort compared to healthy controls, a direct comparison of temporal changes in renal hemodynamic function in healthy controls to the changes in T1D would allow us to more accurately tease out the diabetic versus age specific changes. Moreover, our previous findings in healthy controls suggested that age-related exaggerated renal and systemic hemodynamic responses to acute clamped hyperglycemia and ANGII infusions are only observed in T1D, but not in healthy controls ²⁴. Although renal segmental resistances were not estimated in the aforementioned studies, observations suggest that the changes we observe in our cohorts may relate to duration of diabetes. To minimize the effects of small sample size, we used careful pre-study preparation and gold standard methods to quantify renal hemodynamic function. All study measurements were performed under clamped euglycemic conditions, which minimized the effects of our small sample size and concurrent glycemia on measured parameters. It should be noted that we previously showed an increase in GFR under clamped hyperglycemia in young adults with T1D, however our previous studies could not determine whether this observation is related to R_A or R_E ²². Therefore, the potential impact of hyperglycemia on our observations should be further examined. We also acknowledge that there are baseline differences between groups in glycemic control (HbA1c), blood pressure, heart rate and medication intake, however we believe these differences represent the natural progression of T1D that contribute to the outcomes presented in our analyses. To further limit intra-subject variability, future studies should be conducted that follow kidney function over time in each patient from diagnosis to long-standing T1D.

Additionally, we recognize that Gomez formulae are indirect estimates of intraglomerular hemodynamic parameters and are based on certain physiologic assumptions listed in the methods section and shown in Figure 1. These formulae have been developed in a canine model and are based on assumptions that could be generalized to human physiology. These formulae do not account for changes in intratubular pressure, such as tubular reabsorption of electrolytes or fluid, which may affect the pressure gradient across glomerular capillaries. While original formulae assume a K_FG based on normal kidney physiology (Winston’s indirect estimates in a canine model that P_GLO is roughly two-thirds of MAP and normal glomerular π_G is 25 mmHg), we modified the formulae in an attempt to better reflect the physiology of a diabetic kidney by using the K_FG obtained from diabetic Munich-Wistar rat models ²³. We do recognize, however, that π_G and K_FG vary across different stages of kidney disease and albuminuria as the glomerulus increases in permeability with kidney disease progression ⁴¹ suggesting that our analysis is limited by using static π_G and K_FG. Nevertheless, these formulate were applied in a number of healthy and disease human populations in the last decade, including in patients with T1D and appear to reflect hemodynamic changes in renal function ^21,22.

Our study uniquely characterises renal hemodynamic changes across a wide range of T1D duration without DN and in response to an ANGII infusion. T1D appears to first impact resistance at the afferent arteriole, which increases with longer duration of T1D. In contrast, the efferent arteriole resistance increases significantly only with longstanding T1D duration. Longer duration of T1D diabetes is associated with lower GFR, ERPF, elevated RVR, and evidence of RAAS activation as assessed by an infusion of ANGII and plasma renin and aldosterone levels. Further work is required to understand the potential role of non-RAAS, pharmacological agents that target R_A as a primary prevention strategy in patients with short-standing T1D and as an additive nephroprotective strategy in patients with longstanding T1D when used with classic medications.