Effects of Type II diabetes on exercising skeletal muscle blood flow in the rat

Abstract

The purpose of the present investigation was to examine the muscle hyperemic response to steady-state submaximal running exercise in the Goto-Kakizaki (GK) Type II diabetic rat. Specifically, the hypothesis was tested that Type II diabetes would redistribute exercising blood flow toward less oxidative muscles and muscle portions of the hindlimb. GK diabetic (n = 10) and Wistar control (n = 8, blood glucose concentration, 13.7 ± 1.6 and 5.7 ± 0.2 mM, respectively, P < 0.05) rats were run at 20 m/min on a 10% grade. Blood flows to 28 hindlimb muscles and muscle portions as well as the abdominal organs and kidneys were measured in the steady state of exercise using radiolabeled 15-μm microspheres. Blood flow to the total hindlimb musculature did not differ between GK diabetic and control rats (161 ± 16 and 129 ± 15 ml·min⁻¹·100g⁻¹, respectively, P = 0.18). Moreover, there was no difference in blood flow between GK diabetic and control rats in 20 of the individual muscles or muscle parts examined. However, in the other eight muscles examined that typically are comprised of a majority of fast-twitch glycolytic (IIb/IIdx) fibers, blood flow was significantly greater (i.e., ↑31–119%, P < 0.05) in the GK diabetic rats. Despite previously documented impairments of several vasodilatory pathways in Type II diabetes these data provide the first demonstration that a reduction of exercising muscle blood flow during submaximal exercise is not an obligatory consequence of this condition in the GK diabetic rat.

patients suffering from Type II diabetes are characterized by their compromised exercise tolerance (for review, see 33), a situation that is particularly tragic given the major therapeutic benefit of exercise to these individuals. Unfortunately, to date, the specific mechanistic bases for the exercise intolerance of this patient population remain to be defined.

Several manifestations of Type II diabetes, including the reduced maximal oxygen uptake (V̇o_2max) (5, 18, 34), slowed V̇o₂ kinetics (35) and impaired fractional oxygen extraction (5), could result potentially from compromised cardiovascular function and the inability to transport oxygen effectively to the mitochondria of the exercising muscles. This notion is supported by observations of low muscle blood flows at rest (35) and during muscular exercise (17, 18) in diabetic patients. Such reports are consistent with increased plasma endothelin-1 (39) and constrictor prostaglandin (4) concentrations as well as compromised endothelium-mediated vasodilation (17, 25, 42). Moreover, Type II diabetes can impact other aspects of skeletal muscle structure and function such as capillarity (decreased; 23, 24), Type IIb fiber composition (increased; 23, 43), and mitochondrial volume density (decreased; 36) and function (diminished; 16, 22).

In our laboratory, we have investigated the mixed-fiber type spinotrapezius muscle of the Goto-Kakizaki (GK) Type II diabetic rat and demonstrated that microcirculatory hemodynamics are severely perturbed (30) and the microvascular oxygen partial pressure (Pmv_O2) is reduced (31) at rest. However, during electrically stimulated contractions of that muscle, the steady-state Pmv_O2 is not different compared with healthy controls (31). This indicates that, despite microcirculatory dysfunction at rest and transiently following the initiation of contractions, the balance of O₂ delivery-to-O₂ utilization (which sets the Pmv_O2) during the contracting steady state is restored. In contrast to reports of decreased exercising muscle blood flow in Type II diabetic patients (17, 18) these latter findings suggest that microcirculatory dysfunction present in resting muscle might not necessarily portend compromised blood flow during exercise in the GK Type II diabetic rat.

Whereas exercise training improves vascular function and preferentially redistributes blood flow toward more oxidative muscle regions (2), conditions associated with vascular dysfunction [e.g., advancing age (Refs. 11, 28) and heart failure (Ref. 29)] often result in blood flow reduction and/or redistribution away from oxidative tissues and toward more glycolytic muscle regions. Given that oxidative muscle fibers are expected to be the most heavily recruited fibers during submaximal exercise, such effects may contribute to the reduced exercise capacity associated with these conditions. However, whether impairments in vascular control evidenced in the GK rat (7, 21, 37) result in altered hindlimb muscle blood flow during treadmill exercise has yet to be determined. Therefore, we sought to examine the integrated vascular response to exercise in the GK rat. Specifically, we tested the hypothesis that Type II diabetes would redistribute exercising blood flow toward less oxidative tissues of the hindlimb musculature. The GK diabetic rat was selected specifically as a model of Type II diabetes in which exercising muscle blood flow could be measured with high spatial resolution among the major muscles of locomotion and well-defined fiber type composition.

METHODS

Experimental animals.

The Type II diabetic model selected for these investigations was the male GK rat (Taconic Farm, Germantown, NY; 6–8 mo old; n = 10). Healthy male Wistar rats (n = 8) served as age-matched controls (CON). The GK model is a hyperglycemic, hyperinsulinemic nonobese rat strain developed by breeding glucose-intolerant Wistar rats selectively (i.e., ∼5 generations, 13). These GK rats require no specially formulated diet (14), and healthy Wistar rats are appropriate controls because they derive from the same original strain as the GK rat.

All rats were maintained in a controlled environment with a fixed 12:12-h light-dark cycle and room temperature maintained at ∼22°C. All rats were provided standard rodent chow and water ad libitum. All experimental conditions and surgical procedures were approved by the Kansas State University Institutional Animal Care and Use Committee and are consonant with federally mandated guidelines.

Exercise protocol and blood flow measurements.

All rats were familiarized with running on a motor-driven treadmill. During the period of familiarization (5–8 runs performed over 2–3 wk), rats exercised for 5–10 min/day at a speed of 20 m/min and 10% grade. Care was taken to ensure that total exercise volume for each rat was well below that which elicits physiological training effects (10).

After it was established that all rats were proficient runners, each animal was fasted overnight before acute instrumentation for the final experimental protocol. On the day of the experiment, each rat was weighed and anesthetized with 5% isoflurane. While being maintained on a 2% isoflurane-oxygen mixture, one catheter (PE-10 connected to PE-50) was placed in the ascending aorta via the right carotid artery and another in the caudal (tail) artery, as previously described (29). Both catheters were tunneled subcutaneously to the dorsal aspect of the cervical region and exteriorized through a puncture wound in the skin. After the closure of incisions, anesthesia was terminated and the animal was given 2 h to recover. This period of recovery was selected to avoid changes in tissue glycogen content as a result of the catheterization (26) that may impact running performance negatively. Additionally, the recovery time for all animals was similar to account for the possibility of the confounding effects of the anesthesia regimen (20), although, importantly, it has been demonstrated that a window of 1–6 h of recovery after anesthesia elicits stable cardiac and circulatory dynamics, regional blood flow, arterial blood gases, and acid-base status in the awake unrestrained rat (12).

Subsequent to the recovery period and before the final experimental protocol was initiated, an arterial blood sample (∼50 μl) was taken for analysis of blood glucose concentration (Accu-Check Advantage, Roche Diagnostics, Indianapolis, IN). Each rat was then placed on the treadmill, and after a period of stabilization (3 h after instrumentation), the tail artery catheter was connected to a 1-ml plastic syringe that was connected to a Harvard infusion/withdrawal pump (model 907, South Natick, MA). The carotid artery catheter was connected to a pressure transducer set at the same level as the rat for continuous measurement of mean arterial pressure (MAP) and heart rate (HR). Exercise was initiated, and the speed of the treadmill was increased progressively during the next 30 s to a speed of 20 m/min (10% grade). The rat then exercised steadily for another 3 min. After 3.5 min of total exercise time, blood withdrawal from the tail artery catheter was initiated at a rate of 0.25 ml/min. Simultaneously, arterial blood pressure and HR were measured via the carotid artery catheter. After 4 min of total exercise time, the carotid artery catheter was disconnected from the pressure transducer and ∼0.5–0.6 × 10⁶ microspheres with a 15-μm diameter (isotopes used were ⁴⁶Sc, ⁸⁵Sr, ¹¹³Sn, or ¹⁴¹Ce, in random order; Perkin Elmer Life and Analytical Sciences, Boston, MA) were injected into the aortic arch to determine regional blood flow. Approximately 30 s after the injection, blood withdrawal from the tail artery catheter was stopped and exercise was terminated. After a 60-min recovery period, hemodynamic parameters were measured as the rat sat quietly on the treadmill. A second microsphere injection was then performed with the same procedures as described above to determine resting hindlimb muscle blood flow. This sampling strategy minimizes the potential for blood loss to affect the exercise response and facilitates “resting” measurements that do not reflect the preexercise anticipatory response (1).

On completion of the study, each animal was given an overdose of pentobarbital (>50 mg/kg ia). The thorax was opened, and placement of the carotid artery catheter into the aortic arch was confirmed by anatomic dissection. The kidneys, organs of the gut, and muscles of both hindlimbs were identified and removed by a skilled research technician to ensure uniformity of the dissection process. The muscles examined in this investigation included ankle extensors [soleus (S), plantaris (PL), red portion of the gastrocnemius (G_R), white portion of the gastrocnemius (G_W), middle portion of the gastrocnemius (G_M), tibialis posterior (TP), flexor digitorum longus (FDL), flexor hallucis longus (FHL)], ankle flexors [red portion of the tibialis anterior (TA_R), white portion of the tibialis anterior (TA_W), extensor digitorum longus (EDL), peroneals (PER)], knee extensors [vastus intermedius (VI), vastus medialis (VM), red portion of the vastus lateralis (VL_R), white portion of the vastus lateralis (VL_W), middle portion of the vastus lateralis (VL_M), red portion of the rectus femoris (RF_R), white portion of the rectus femoris (RF_W)], knee flexors [anterior portion of the biceps femoris (BF_A), posterior portion of the biceps femoris (BF_P), semitendinosus (ST), red portion of the semimembranosus (SM_R), white portion of the semimembranosus (SM_W)]; and thigh adductors [adductor longus (AL), adductor magnus and brevis (AMB), gracilis (GR), pectineus (PEC)]. All tissues were blotted, weighed, and placed immediately into counting vials.

The radioactivity of each tissue was determined on a gamma scintillation counter (Packard Auto Gamma Spectrometer, model 5230, Downers Grove, IL). Accounting for the cross-talk fraction between isotopes, blood flows to each tissue were determined using the reference sample method (29). Given the differences in tissue and body mass between GK and CON rats (see results), all blood flow data were normalized to tissue mass and expressed as milliliters per minute per 100 grams of tissue (ml·min⁻¹·100g⁻¹). Adequate mixing of the microspheres was verified for each injection by demonstrating a <15% difference between blood flow to the right and left kidneys and/or to the right and left hindlimb musculature.

Statistical analysis.

All values are expressed as means ± SE. Primary endpoints (blood flow, heart rate, and MAP) were compared within (rest vs. exercise) and between (GK vs. CON) groups with two-way ANOVAs. When a significant F-ratio was demonstrated, a Student-Newman-Keuls post hoc test was performed to determine differences between mean values. In addition, secondary endpoints (blood glucose and tissue mass) with only a single between-group comparison were analyzed via unpaired Student's t-tests. Statistical significance was set at P ≤ 0.05.

RESULTS

Blood glucose, body and tissue mass, and cardiovascular variables.

Arterial blood glucose was significantly higher (GK: 13.7 ± 1.6; CON: 5.7 ± 0.2 mM; P < 0.05) and body mass lower (GK: 417 ± 14; CON: 516 ± 16 g; P < 0.05) in GK compared with CON rats, which is in agreement with previous studies (13, 43). Total hindquarter muscle mass was reduced significantly for GK rats compared with CON as were the tissue masses of all 20 individual whole muscles examined (P < 0.05 for all, Table 1). Moreover, muscle mass normalized to body mass was significantly reduced for the total hindlimb as well as in 17 of the 20 individual whole hindlimb muscles in GK vs. CON rats (Table 1). Total and normalized abdominal organ and kidney mass for both groups is displayed in Table 2. GK rats evidenced an increased (P < 0.05) small intestine mass/body mass ratio vs. CON rats, which is consistent with various rat models of diabetes (15, 38, 41). MAP did not differ between the CON and GK rats at rest or during exercise (P > 0.05, Table 3). On the other hand, HR was significantly higher in the GK rats at rest (P < 0.05). During exercise, HR increased from resting levels and was not different between groups (P > 0.05; Table 3).

Table 1.

Tissue mass expressed as absolute mass and normalized to body mass for the total hindlimb musculature and the 20 individual muscles of the rat hindlimb in Wistar control and GK (diabetic) rats

	Tissue Mass, mg		Tissue Mass/Body Mass, mg/g
	CON	GK	CON	GK
Ankle extensors
Soleus	221 ± 8	158 ± 6*	0.43 ± 0.02	0.38 ± 0.01*
Plantaris	483 ± 62	300 ± 11*	0.93 ± 0.12	0.72 ± 0.01*
Gastrocnemius	2,520 ± 54	1,813 ± 57*	4.92 ± 0.19	4.36 ± 0.08*
Tibialis posterior	271 ± 11	204 ± 5*	0.53 ± 0.03	0.49 ± 0.01
Flexor digitorum longus	46 ± 2	19 ± 2*	0.09 ± 0.01	0.05 ± 0.01*
Flexor halicus longus	721 ± 16	529 ± 12*	1.41 ± 0.06	1.28 ± 0.03*
Ankle flexors
Tibialis anterior	984 ± 25	621 ± 20*	1.92 ± 0.09	1.49 ± 0.02*
Extensor digitorum longus	260 ± 10	180 ± 6*	0.51 ± 0.02	0.44 ± 0.02*
Peroneals	573 ± 21	413 ± 9*	1.12 ± 0.06	1.00 ± 0.03*
Knee extensors
Vastus intermedius	451 ± 53	333 ± 16*	0.89 ± 0.12	0.80 ± 0.04
Vastus medialis	528 ± 29	293 ± 12*	1.02 ± 0.05	0.71 ± 0.02*
Vastus lateralis	2,060 ± 74	1,345 ± 42*	4.03 ± 0.19	3.23 ± 0.05*
Rectus femoris	1,678 ± 70	1,077 ± 42*	3.29 ± 0.19	2.59 ± 0.06*
Knee flexors
Biceps femoris	4,041 ± 139	2,544 ± 86*	7.90 ± 0.40	6.11 ± 0.09*
Semitendinosus	1,396 ± 61	929 ± 48*	2.74 ± 0.18	2.23 ± 0.06*
Semimembranosus	2,914 ± 103	1,895 ± 67*	5.70 ± 0.29	4.46 ± 0.06*
Thigh adductors
Adductor longus	132 ± 5	110 ± 4*	0.26 ± 0.02	0.26 ± 0.01
Adductor magnus and brevis	2,316 ± 81	1,550 ± 57*	4.53 ± 0.23	3.72 ± 0.04*
Gracilis	733 ± 30	525 ± 18*	1.44 ± 0.09	1.26 ± 0.03*
Pectineus	369 ± 13	247 ± 7*	0.72 ± 0.03	0.60 ± 0.01*
Total hindlimb musculature	22,920 ± 674	15,086 ± 475*	44.89 ± 2.26	36.24 ± 0.48*

Data are means ± SE. CON, Wistar control rats. GK, Goto-Kakizaki Type II diabetic rats.

^*P < 0.05 vs. CON.

Table 2.

Abdominal organ and kidney mass expressed as absolute and normalized to body mass in Wistar control and GK (diabetic) rats

	Tissue Mass, mg		Tissue Mass/Body Mass, mg/g
	CON	GK	CON	GK
Pancreas	1,172 ± 102	820 ± 46*	2.30 ± 0.22	1.96 ± 0.08
Adrenals	36 ± 4	28 ± 1*	0.07 ± 0.01	0.07 ± 0.01
Small intestine	4,209 ± 230	4,373 ± 158	8.23 ± 0.53	10.58 ± 0.46*
Large intestine	1,636 ± 50	1,163 ± 67*	3.19 ± 0.13	2.81 ± 0.18
Stomach	2,202 ± 50	1,880 ± 27*	4.32 ± 0.22	4.56 ± 0.17
Right kidney	1,895 ± 84	1,526 ± 79*	3.72 ± 0.25	3.65 ± 0.11
Left kidney	1,886 ± 71	1,613 ± 54*	3.70 ± 0.23	3.88 ± 0.09

Data are means ± SE.

^*P < 0.05 vs. CON.

Table 3.

Heart rate and mean arterial blood pressure measured at rest and during exercise of Wistar control and GK (diabetic) rats

	HR, beats/min	MAP, mmHg
CON
Rest	372 ± 12	131 ± 5
Exercise	500 ± 6†	139 ± 3
GK
Rest	439 ± 19*	133 ± 7
Exercise	506 ± 8†	134 ± 5

Values are means ± SE. HR, heart rate; MAP, mean arterial blood pressure.

^*P < 0.05 vs. same condition in CON.

^†P < 0.05 vs. at rest.

Muscle blood flow.

Blood flow measured to the total hindlimb musculature at rest (GK: 30 ± 7, CON: 20 ± 3 ml·min⁻¹·100g⁻¹; Fig. 1) and to the majority (24/28) of the individual muscles or muscle parts investigated (Table 4) was not different between CON and GK rats. Specifically, higher blood flows in GK rats were found in the gracilis, semitendinosus, soleus, and the red portion of the gastrocnemius. During exercise, blood flow to the total hindlimb musculature increased to a similar degree in both the CON and GK rats and was not different (P = 0.18) between groups (GK: 161 ± 16, CON: 129 ± 15 ml·min⁻¹·100g⁻¹; Fig. 1). Moreover, exercising blood flow was not different in 20 of the 28 individual muscles or muscle parts examined (Table 4). However, in 8 of the 28 tissues (i.e., semitendinosus, white portions of the vastus lateralis, rectus femoris, tibialis anterior, extensor digitorum longus, flexor digitorum longus, tibialis posterior, and flexor hallucis longus; Fig. 2) that normally contain a majority of fast-twitch glycolytic (IIb/IIdx) fibers (range from 68 to 100%, Ref. 9), blood flow was higher during exercise in the GK rats compared with the CON.

Fig. 1.

Blood flow to the total hindlimb musculature measured at rest (black bars) and during moderate intensity (20 m/min, 10% grade) exercise (white bars) for Wistar control and Goto-Kakizaki (GK) diabetic rats. †P < 0.05 vs. at rest. No differences were found between Wistar control and GK diabetic rats within the rest and exercise condition. Values are means ± SE.

Table 4.

Resting and exercising blood flows to the individual muscles or muscle parts of the Wistar control and GK (diabetic) rat hindlimb

	Rest		Exercise
	CON	GK	CON	GK
Ankle extensors
Soleus	76 ± 14	136 ± 17*	331 ± 28	311 ± 23
Plantaris	17 ± 3	25 ± 6	260 ± 38	318 ± 26
Gastrocnemius, red	27 ± 5	63 ± 11*	479 ± 64	523 ± 37
Gastrocnemius, white	17 ± 3	20 ± 4	54 ± 11	73 ± 11
Gastrocnemius, mixed	17 ± 2	24 ± 6	169 ± 22	200 ± 23
Tibialis posterior	19 ± 3	32 ± 9	120 ± 13	230 ± 22*
Flexor digitorum longus	32 ± 9	47 ± 9	102 ± 13	212 ± 27*
Flexor halicus longus	18 ± 3	28 ± 6	91 ± 14	132 ± 14*
Ankle flexors
Tibialis anterior, red	39 ± 13	67 ± 22	283 ± 36	301 ± 22
Tibialis anterior, white	22 ± 5	41 ± 13	101 ± 11	132 ± 12*
Extensor digitorum longus	20 ± 5	35 ± 10	64 ± 7	108 ± 11*
Peroneals	20 ± 5	27 ± 7	146 ± 15	160 ± 14
Knee extensors
Vastus intermedius	79 ± 23	90 ± 26	373 ± 34	406 ± 35
Vastus medialis	21 ± 4	19 ± 4	171 ± 32	153 ± 17
Vastus lateralis, red	57 ± 15	102 ± 29	381 ± 76	427 ± 41
Vastus lateralis, white	12 ± 2	17 ± 3	44 ± 4	71 ± 12*
Vastus lateralis, mixed	20 ± 4	37 ± 10	204 ± 39	250 ± 34
Rectus femoris, red	40 ± 12	55 ± 19	250 ± 39	306 ± 29
Rectus femoris, white	20 ± 5	31 ± 10	117 ± 15	166 ± 17*
Knee flexors
Biceps femoris, anterior	13 ± 2	17 ± 4	84 ± 12	101 ± 13
Biceps femoris, posterior	13 ± 2	22 ± 5	99 ± 14	126 ± 18
Semitendinosus	12 ± 3	29 ± 7*	42 ± 6	92 ± 15*
Semimembranosus, red	17 ± 3	22 ± 5	163 ± 20	198 ± 22
Semimembranosus, white	12 ± 3	15 ± 2	45 ± 6	59 ± 9
Thigh adductors
Adductor longus	126 ± 14	114 ± 9	320 ± 28	309 ± 32
Adductor magnus & brevis	19 ± 3	28 ± 5	98 ± 9	125 ± 15
Gracilis	12 ± 1	22 ± 4*	48 ± 6	71 ± 13
Pectineus	36 ± 6	39 ± 11	51 ± 9	58 ± 11

Values are means ± SE of resting and exercising blood flows (ml·min⁻¹·100 g⁻¹).

^*P < 0.05 vs. CON.

Fig. 2.

Individual muscles and muscle parts that evidenced significantly different blood flows during moderate intensity exercise (20 m/min, 10% grade) in Wistar control (black bars) vs. GK diabetic (white bars) rats. Blood flow was elevated in 8 muscles in the hindlimb musculature when GK diabetic rats were compared with Wistar controls. ST, semitendinosus; VL_W, white portion of the vastus lateralis; RF_W, white portion of the rectus femoris; TA_W, white portion of the tibialis anterior; EDL, extensor digitorum longus; FDL, flexor digitorum longus; TP, tibialis posterior; FHL, flexor hallucis longus. Values are means ± SE. *P < 0.05 vs. Wistar control.

Abdominal organ and kidney blood flows.

Blood flows to the individual abdominal organs and kidneys at rest and during exercise are presented in Fig. 3. Blood flow to the kidneys at rest was similar in the CON and GK rats. During exercise, blood flow to the kidneys was reduced significantly (P < 0.05) in both CON and GK rats compared with resting values, and these reductions were similar such that blood flow to the kidneys was not different (P > 0.05) in the CON vs. GK rats during exercise.

Fig. 3.

Blood flow to the kidneys and organs of the gut measured at rest (top) and during moderate-intensity (20 m/min, 10% grade) exercise (bottom) for Wistar control (black bars) and GK diabetic (white bars) rats. Small Int, small intestines; Large Int, large intestines. Values are means ± SE. *P < 0.05 vs. Wistar control. †P < 0.05 vs. at rest.

Blood flow at rest was reduced (P < 0.05) in both the small intestine and liver of GK rats compared with CON. During exercise, blood flow was reduced (P < 0.05) in the kidneys, stomach, and small intestines for the CON rats compared with resting values. In contrast, blood flow was reduced (P < 0.05) in the kidneys, small intestines, and pancreas for the GK rats. In addition, blood flow was significantly (P < 0.05) lower in the stomach, small and large intestines, and pancreas during exercise for the GK rats compared with their CON counterparts.

DISCUSSION

The most important original finding of the present investigation is that exercising muscle blood flows are not reduced during submaximal treadmill exercise in Type II diabetic GK rats. Surprisingly, despite the overt hyperglycemia, blood flow to many of the predominantly low oxidative muscles comprised of Type IIb and IId/x muscle fibers was increased significantly without compromising blood flow to highly oxidative locomotory muscles. These results indicate that, at least in this model of Type II diabetes, bulk blood flow and therefore oxygen delivery to muscles is not impaired during the steady state of submaximal exercise.

In human Type II diabetes patients the molecular and cellular consequences of the disease manifest in reductions in exercise tolerance, and the GK Type II diabetic rat serves as a powerful investigative tool of the mechanistic determinants of these impairments. However, whether the molecular and cellular alterations evident in the GK rat result in global physiological impairments during exercise as evidenced in human diabetes patients has not been resolved. In this regard the present work constitutes an important initial step in determining the physiological responses to conscious exercise in the GK rat.

Agreement with existing literature.

Type II diabetes induces elevated plasma endothelin concentrations (39), impaired endothelium-mediated vasodilation (17, 25, 42), and autonomic dysfunction (8) coupled with overt microcirculatory dysfunction (30). Collectively, these observations support the premise that exercising muscle blood flow might be expected to be reduced in Type II diabetes. Indeed, the sparse measurements of exercising muscle blood flow suggest that this might be the case in Type II diabetic patients (17, 18). In contrast to human diabetes patients, the present investigation demonstrated either the same or increased blood flow in all muscles examined during exercise in the GK rat model of Type II diabetes. This finding is particularly surprising given that marked impairments of known vasodilatory pathways have been reported in the GK diabetic rat (e.g., 7, 21).

Based on the Kingwell et al. (17) and Lalande et al. (18) findings of a lower exercising leg blood flow in Type II diabetic patients, we were surprised presently to find no differences in blood flow between GK Type II diabetic rats and healthy controls. Important considerations are the differences in blood flow measurement techniques utilized and that the GK rat represents a nonhyperinsulinemic diabetic model whereas human patients are typically hyperinsulinemic. Moreover, in the present investigation we found differences in tissue and body mass between GK and CON rats whereas body mass and composition were similar between the diabetic and healthy humans, which may reflect slightly different underlying pathology (17, 18). Conversely, the present finding that blood flow, and therefore bulk O₂ delivery, to the exercising muscles is not reduced in the GK diabetic rat is generally consistent with the unaltered Pmv_O2found in the steady state (i.e., beyond the transient behavior associated with contractions onset) of electrically stimulated spinotrapezius muscle contractions (31). Together, these data indicate that the GK rat can overcome the consequences of its symptoms (i.e., overt hyperglycemia) to maintain or even increase the steady-state hyperemia to all individual muscles compared with CON rats. As pointed out by Laughlin and Korzick (19), there is a high degree of redundancy in the control of the exercise hyperemia, and the present results suggest that, despite the pernicious effects of Type II diabetes on certain vasodilatory pathways (3, 4, 17, 25, 39, 40, 42), other pathways may be recruited during the exercising steady state as a compensatory strategy to defend exercising muscle blood flow.

The GK diabetic rat evidences a slightly greater proportion of glycolytic Type IIb and fewer more oxidative Type I and IIa skeletal muscle fibers compared with control WKY rats (43). This phenomenon per se is not expected to account for the blood flow responses observed between GK and CON rats in the present investigation, but it should be considered that the similar or increased muscle blood flows to predominantly glycolytic muscle regions during exercise reported presently for the GK diabetic rat may reflect an altered distribution of O₂ delivery to O₂ utilization among muscles. If this is the case it suggests a mechanism for the reduced arterial venous O₂ content difference (i.e., ↓ fractional O₂ extraction; a-vO₂ difference) evident during exercise in human diabetic patients (5).

The present observation that individual muscle mass normalized to body mass was significantly lower in 17 of the 20 individual hindlimb muscles in GK vs. CON rats extends similar observations from a report which investigated the soleus and plantaris only (43). Moreover, the ∼25% greater (although not statistically significant) hindlimb blood flow in the GK rats reported presently is explicable based on the ∼20% decrease in total hindlimb muscle mass/body mass ratio. Importantly, this further supports the conclusion that skeletal muscle blood flow is not impaired in GK rats.

Methodological considerations.

The radiolabeled microsphere technique is a powerful technology that allows the measurement of blood flow distribution to different organs of the body at distinct timepoints. However, one of the primary limitations of the technique is that neither frequent blood flow measurements nor those during transient changes in blood flow as seen in the transition from rest to exercise can be made. Therefore, despite hindlimb blood flows that are not different between GK and CON rats in the present investigation, it is possible that slow blood flow kinetics from the onset of exercise may exist in this population (31) as has been suggested in human diabetes patients (6).

In the present study we chose to examine the skeletal muscle blood flow response to a given level of submaximal treadmill exercise that corresponds to ∼55% of the healthy rat's maximal aerobic capacity (27). However, Type II diabetes produces a reduction in the V̇o_2max of human subjects afflicted with this disease (35). Therefore, although possible V̇o_2max reductions in the GK rat vs. healthy counterparts have not been evaluated specifically, GK rats may have been performing at a greater relative work rate during exercise even though the absolute running speed was the same for both groups. Based on this assumption, it is plausible to consider that the increases in blood flow found in several individual muscles of the GK rats may have related primarily to recruitment factors (and the greater blood flow reductions to several organs of the gut support this conclusion). If either of the above phenomena (i.e., slow blood flow kinetics at the onset of exercise and/or reduced V̇o_2max) exist in GK rats it might provide a mechanism for reductions in exercise tolerance despite hindlimb muscle blood flows that are not different from healthy rats at the same absolute level of exercise.

Conclusions.

The GK Type II diabetic rat has been utilized extensively to examine the physiological and molecular adaptations to insulin resistance (reviewed in Ref. 32). Considering the experimental utility of this specific model of Type II diabetes and the importance of exercise to humans afflicted with this disease, it is surprising that, to our knowledge, the physiological responses of GK rats to conscious exercise had yet to be characterized in more detail. Despite the plethora of impaired vasodilatory mechanisms that attend the GK Type II diabetic rat the present investigation demonstrates that, within the principal locomotory muscles, the exercise hyperemia measured during the steady state of submaximal running is not impaired. Not only was the blood flow not reduced in any of the muscles or muscle parts examined, but in several of the muscles comprised predominantly of low oxidative Type IIb and IId/x fibers, blood flow, and therefore O₂ delivery, was increased significantly. These findings suggest that, in the face of impairments of basic vasodilatory mechanisms in this model of Type II diabetes (7, 21, 37) there is sufficient redundancy in the system to facilitate a normal, or even enhanced, muscle hyperemia, at least during the steady state.

GRANTS

This work was supported in part by National Institutes of Health Grants HL-50306 and AG-19228, and a Grant-in-Aid from the American Heart Association, Heartland Affiliate (0455582Z).

DISCLOSURES

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