Postprandial effects on electrolyte homeostasis in the kidney

Abstract

Insulin is known to be an important regulator of a number of different channels and transporters in the kidney, but its role in the kidney to prevent Na⁺ and volume loss during the osmotic load after a meal has only recently been validated. With increasing numbers of people suffering from diabetes and hypertension, furthering our understanding of insulin signaling and renal Na⁺ handling in both normal and diseased states is essential for improving patient treatments and outcomes. The present review is focused on postprandial effects on Na⁺ reabsorption in the kidney and the role of the epithelial Na⁺ channels as an important channel contributing to insulin-mediated Na⁺ reclamation.

INTRODUCTION

Although a bit antiquated, the common aphorism “You are what you eat” is worth considering, particularly in the context of a Western diet. Increased dietary sugar, salt, and fat over the past century has dramatically increased the incidence of obesity, diabetes, and hypertension. All three of these diseases are leading causes of death worldwide, are frequent comorbidities, and often have overlapping pathophysiology (43). Approximately 422 million adults were living with diabetes in 2014, ~8.5% of the adult population (35). Type I diabetes, which results from the unpreventable loss of the body’s insulin producing pancreatic β-cells and requires insulin supplementation, makes up ~5–10% of the diabetic population. Type 2 diabetes, which is caused by insulin resistance and has preventative intervention strategies, comprises the remaining 90–95% of the diabetic population. These differences in insulin, i.e., lack of insulin versus insensitivity to insulin, have important consequences for the development of disease progression and how we should approach treatment options. Importantly, both type 1 and type 2 diabetes are leading causes of diabetic kidney disease, which are associated with significant changes in renal hemodynamics and electrolyte transport (42).

INSULIN SIGNALING IN RENAL Na⁺ RETENTION

Even though our experimental understanding of the importance of insulin stimulation on Na⁺ transport in renal epithelial cells goes back over 40 yr (7, 8, 10, 20), we are still uncovering new information that is relevant and essential to our understanding of renal Na⁺ transport. The insulin receptor is expressed in many areas of the kidney, including the glomerulus, proximal tubule, thick ascending limb, distal convoluted tubule, and collecting duct, which suggests that it can have a broad range of effects on many different renal transporters (16, 40). The development of hypertension associated with obesity and metabolic disorders has been linked to increased Na⁺ retention (37); however, different research groups have presented evidence that the kidney both does and does not play a role in insulin-stimulated Na⁺ retention. Research supporting insulin-mediated Na⁺ retention demonstrated that under euglycemic and hyperinsulinemic conditions, both humans and dogs have reduced urinary Na⁺ excretion (9). Additionally, pharmacological inhibition of the epithelial Na⁺ channel (ENaC) and Na⁺/Cl⁻ cotransporter (NCC) in rats chronically infused with insulin increased natriuresis, suggesting insulin increases the activity of these distal nephron transporters (41). Finally, a mouse model with genetic ablation of the insulin receptor in renal epithelial cells resulted in increased systolic blood pressure as well as reduced natiuresis (46). On the other hand, studies refuting insulin-mediated renal Na⁺ retention are supported by the lack of hypertension in hyperinsulinemic patients with insulinoma (38, 47) and the absence of hypertension and Na⁺ retention in canines exposed to chronic insulin infusion (17, 18). To further address these inconsistencies, Manhiani et al. (27, 28) used a type I diabetes model in dogs to directly maintain a hyperglycemic state while altering insulin levels in dogs and analyzed urinary Na⁺ excretion. Dogs were treated with alloxan to eliminate their endogenous insulin production, placed on insulin replacement to maintain normal glucose levels, and then separated into two groups: one group that had hyperglycemia with reduced insulin levels (i.e., type 1 diabetes) and one group that had hyperglycemia with normal insulin levels. Both groups had elevated blood glucose; however, only the low insulin-treated group had prolonged increases in urinary Na⁺ excretion (25). Similarly, insulin infusion into the kidneys of dogs with type 1 diabetes prevented the sustained diabetic natriuresis, but when insulin was infused into the kidneys of normal dogs with normal blood glucose, there was no effect on Na⁺ excretion (26). These studies demonstrated the interdependency of circulating insulin and blood glucose levels, namely, that urinary Na⁺ wasting in the presence of hyperglycemia is ameliorated by insulin (27, 28). Further studies from this research group then went on to demonstrate that chronic renal artery infusion of insulin and glucose can trigger hypertension in Sprague-Dawley rats through a kidney-specific mechanism (24).

ROLE OF INSULIN IN NORMAL POSTPRANDIAL RENAL Na⁺ TRANSPORT

To date, most of the physiological experiments involving insulin signaling and renal Na⁺ reabsorption have focused on this interaction in disease models, specifically diabetes, or in cell culture models. On a day-to-day basis under normal physiological conditions, however, each of us undergoes transitional periods of fluctuating hyperglycemia and hyperinsulinemia after each meal as part of our basic metabolic existence. Evolutionarily, it makes sense for the body to have a way to couple meal consumption with renal electrolyte and volume control; otherwise, the osmotic load from a meal would result in Na⁺ and volume losses. The potential for insulin and hyperinsulinemia to regulate our normal Na⁺ excretion had previously not been examined until recently, when Irsik et al. (22) demonstrated that the normal increase in plasma insulin after postprandial hyperglycemia is important for preventing volume and Na⁺ loss after a meal. To do this, urinary electrolyte and volume excretion were measured over 24- and 4-h postprandial timeframes in control rats versus insulin-clamped rats. Insulin-clamped rats had higher basal insulin levels but also were unable to increase circulating insulin levels after a meal. It was found that insulin-clamped rats had significant Na⁺ and volume losses over the 24-h period and that preventing normal insulin increases after hyperglycemia resulted in Na⁺ and volume wasting (22). Furthermore, in a subsequent study (23) using similar insulin clamping but by infusing insulin directly into the renal artery, it was established that meal-induced plasma insulin signaling exerts its control on corporal Na⁺ homeostasis through direct effects on the kidney.

ENaC-MEDIATED Na⁺ RETENTION IN NONPATHOLOGICAL INSULIN SIGNALING

Now that it had been more firmly established that insulin does result in renal Na⁺ retention under the right glycemic conditions, the next question became “which channels and transporters along the nephron are involved?” While there are numerous channel and transporters whose activities are modulated by insulin (6, 12, 13, 36, 45, 49), one of the most well-established insulin-regulated channels is ENaC. Long before the cloning of ENaC (5), physiologists studied Na⁺ transport across a number of epithelia that was recognized as the amiloride-sensitive current. These classic studies found that in a number of different models, including reptile models, cultured mammalian tubule epithelial cells lines, and ex vivo, that the amiloride-sensitive current increases after insulin stimulation (2, 20, 21, 26, 33, 39). Additionally, a mouse with the insulin receptor genetically ablated in principal cells had no change in total ENaC expression; however, the open probability of ENaC was significantly reduced (33).

In the kidney, ENaC is located in principal cells of the collecting duct and is the rate-limiting factor for Na⁺ reabsorption in the distal nephron. ENaC is a trimeric channel composed of α-, β-, and γ-subunits (30, 44), and its importance in blood pressure regulation is demonstrated by hereditary gain- or loss-of-function mutations in the ENaC subunits that cause both hyper- and hypotensive diseases: Liddle’s disease and pseudohypoaldosteronism, respectively (19). At the molecular level, insulin upregulates ENaC activity through a phosphoinositide 3-kinase-dependent pathway, which results in increased serum and glucocorticoid-regulated kinase-1 (SGK1) activity (3, 14, 29, 32, 33). SGK1 is a key regulator of ENaC in a number of different signaling pathways, including aldosterone stimulation, and an increase in its expression and/or activity increases ENaC activity (4, 11, 25). To establish the potential involvement of ENaC or other distal nephron transporters in postprandial Na⁺ retention, we used fasted Sprague-Dawley rats that were given either a high carbohydrate meal supplement (Supplical) or nothing and examined distal nephron Na⁺ channel/transporter expression as well as ENaC activity in freshly isolated split-open collecting duct tubules 4 h after administration of the meal supplement (1). A key unique factor of this study is that we did not manipulate the insulin or glucose levels of the animals, that is, the rats used in the study were not treated with a chemical to eliminate β-cells or instrumented in any way. The observed changes were specific to fasted or postprandial conditions in normal, healthy animals. While there were no significant changes in total expression levels of the distal nephron Na⁺ transporters (Na⁺-Cl⁻ cotransporter, Na⁺-K⁺-2Cl⁻cotransporter, and Na⁺-K⁺-ATPase) or ENaC subunits (α, β, γ) 4 h postmeal supplement, there was a significant increase in the open probability of plasma membrane ENaC (1), demonstrating that ENaC activity likely plays a role in postprandial Na⁺ reclamation (Fig. 1). Additionally, since ENaC is classically regulated by the renin-angiotensin-aldosterone system (RAAS), and increased glucose and insulin signaling have been implicated in dysregulation of the RAAS, which may play a role in the development of hypertension in metabolic syndrome and diabetes, we used liquid chromatography-tandem mass spectrometry to determine circulating aldosterone and angiotensin metabolite levels. The circulating angiotensin peptides and aldosterone in postprandial animals were not significantly changed, establishing that the increase in ENaC activity within the postprandial period is independent of RAAS signaling (1); however, it should be noted that the meal supplement had no Na⁺ and minimal K⁺ levels, and future studies investigating postprandial responses should investigate whether dietary changes in the amounts of these ions has the potential to also activate the RAAS.

Fig. 1.

Postprandial effects in Sprague-Dawley rats. Increased renal epithelial Na⁺ channel (ENaC) activity as a result of increased channel open probability (P_o) helps prevent Na⁺ and volume wasting from the osmotic load after a meal. RAAS, renin-angiotensin-aldosterone system.

One limitation of this study is that expression of Na⁺-glucose cotransporters (SGLTs) was not assessed, which might be critical considering key role of SGLT2 inhibition in diabetic nephropathy, as recently reported (34). Additionally, these postprandial experiments were only conducted in male rats. Female rats have been shown to have higher expression levels of both Na⁺/Cl⁻ cotransporter and ENaC subunits as well as different urinary electrolyte excretion in response to a K⁺-rich meal (48), which provides sufficient evidence to investigate the potential differences between male and female animals in renal insulin-mediated Na⁺ handling. Another avenue of investigation worth considering is the potential effect of circadian rhythms to influence these experiments, particularly given the circadian modulation of αENaC expression (15) and the potential for disruption of normal circadian rhythm to contribute to the development of diabetic kidney disease (31). The experiments from this postprandial study were conducted after rats were fasted overnight, and the meal supplement was given early in the morning, roughly the human equivalent of fasting all day and then eating right before bedtime, which is the transition period between higher diurnal blood pressure and lower nocturnal blood pressure.

CONCLUSIONS

While large strides have been made toward understanding how insulin signaling affects our renal physiology, there is still a lot more work to be done to have a comprehensive understanding of both normal and pathophysiological changes to insulin-mediated renal Na⁺ transport. Establishing both normal and pathophysiological changes in renal Na⁺ control will greatly improve our ability to treat and manage diseases such as diabetes. For example, due to the absence or excess of insulin in type 1 and type 2 diabetes, respectively, improved understanding of renal Na⁺ handling may lead to different types of pharmacological intervention to better treat different secondary pathologies of these diseases and improve quality of life. As such, future work in this field may have significant clinical implications, so increasing the depth of our knowledge will benefit both our basic understanding of a complex physiological process and our clinical approaches to treatment in metabolic diseases.

GRANTS

Work in the laboratory was partially supported by National Institutes of Health Grants R35-HL-135749 and P01-HL-116264 (to A. Staruschenko) and T32-HL-134643 Cardiovascular Center A. O. Smith Fellowship (to C. A. Klemens), American Heart Association Grant 16EIA26720006 (to A. Staruschenko), and Department of Veteran Affairs Grant I01 BX004024 (to A. Staruschenko).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

C.A.K. and A.S. prepared figures; C.A.K. drafted manuscript; C.A.K., M.W.B., and A.S. edited and revised manuscript; C.A.K., M.W.B., and A.S. approved final version of manuscript.