Contribution of the sympathetic nervous system to hypertensive response to insulin excess in spontaneously hypertensive rats

Sodium-retaining effect of insulin in diabetes

Insulin has long been hypothesized to cause sodium retention, potentially of enough magnitude to contribute to hypertension in obesity, metabolic syndrome, and Type II diabetes. There is an abundance of supportive evidence from correlational analyses in humans, acute insulin infusion studies in humans and animals, and chronic insulin infusion studies in rats. However, the absence of hypertension in human insulinoma patients, and negative results for sodium-retaining or blood pressure effects of chronic insulin infusion in a whole series of dog studies, strongly refute the insulin hypothesis. We recently questioned whether the euglycemic, hyperinsulinemia model used for most insulin infusion studies, including the previous chronic dog studies, was the most appropriate model to test the renal actions of insulin in obesity, metabolic syndrome, and Type II diabetes. In those circumstances, hyperinsulinemia coexists with hyperglycemia. Therefore, we tested the sodium-retaining effect of insulin in chronically instrumented, alloxan-treated diabetic dogs. We used 24 h/day intravenous insulin infusion to regulate plasma insulin concentration. Induction of diabetes (∼400 mg/dl) caused sustained natriuresis and diuresis. However, if we clamped insulin at baseline, control levels, i.e., prevented it from decreasing, then the sustained natriuresis and diuresis were completely reversed, despite the same level of hyperglycemia. We also found that 24 h/day intrarenal insulin infusion had the same effect in diabetic dogs but had no sodium-retaining action in normal dogs. This new evidence that insulin has a sodium-retaining effect during hyperglycemia may have implications for maintaining sodium balance in uncontrolled Type II diabetes.

Effects of chronic chromium picolinate treatment in uninephrectomized rat

Chromium picolinate [Cr(pic)3] is a nutritional supplement that is advocated as an adjuvant therapy for impaired glucose tolerance/type 2 diabetes because it improves glucose homeostasis. Because renal dysfunction is a major complication of type 2 diabetes, the potential impact of Cr(pic)3 on kidney function requires due consideration. This investigation takes added importance because the kidney is not only the principal route of elimination for chromium but also an organ that preferentially accumulates it. To avoid the confounding influence of chronic hyperglycemia, and its associated complications, we used the unilaterally nephrectomized (UNX) rat that shows impaired kidney function with age. We tested the hypothesis that chronic treatment of the UNX rat with Cr(pic)3 exacerbates the age-related decline in renal function. Accordingly, UNX rats were fed a diet lacking (eg, control; n=5) or containing 5 mg/kg of Cr(pic)3 (n=7) for 60 days. The treatment did not affect glucose tolerance as reflected by lack of any effect on changes in blood glucose concentration during glucose tolerance testing. Although nonfasting blood glucose concentrations were similar between the 2 groups, plasma insulin concentration was lower in the Cr(pic)3-treated group (P<.05), suggesting improved insulin sensitivity. Body weight, blood pressure, heart rate, daily food and fluid consumption, daily urinary fluid and electrolyte excretions, urine osmolality, and daily protein excretion were similar between the 2 groups before and during Cr(pic)3 treatment. Although renal excretory responses to acute administration of a 5% isotonic saline volume load were similar between the 2 groups, the Cr(pic)3-treated group displayed a more robust ability to excrete a 10% isotonic saline volume load, an effect primarily related to reduced tubular reabsorption of the filtered fluid and sodium loads. In conclusion, chronic Cr(pic)3 did not adversely affect renal function. Rather, the treatment improved the ability of the animal to dispose of an acute isotonic saline volume load, suggesting preservation of renal function in the UNX rat.

Insulin-exacerbated hypertension in captopril-treated spontaneously hypertensive rats: role of sympathoexcitation

Insulin excess exacerbates hypertension in spontaneously hypertensive rats (SHR). This study examined the relative contribution of the renin–angiotensin system and the sympathetic nervous system in this phenomenon. In SHR, daily subcutaneous injections of insulin were initiated either before short-term angiotensin-converting enzyme inhibition with captopril or after lifetime captopril treatment. Insulin treatment resulted in significant increases in mean arterial pressure and heart rate and captopril treatment lowered arterial pressure, but captopril did not lower arterial pressure more in the insulin-treated compared with control rats. To test the contribution of the sympathetic nervous system to this form of hypertension, each rat was intravenously infused with either a ganglionic blocker (i.e., hexamethonium) or a centrally acting α2-adrenergic receptor agonist (i.e., clonidine). Administration of either agent largely eliminated the differences in mean arterial pressure and heart rate between the insulin-treated and saline-treated SHR, irrespective of captopril treatment. These data indicate that in SHR, the ability of insulin to increase blood pressure is closely related to sympathoexcitation, which is unresponsive to blockade of angiotensin-converting enzyme.Key words: blood pressure, insulin, captorpil, hexamethonium, clonidine, rat.

Hypertension and prolonged vasoconstrictor signaling in RGS2-deficient mice

Signaling by hormones and neurotransmitters that activate G protein-coupled receptors (GPCRs) maintains blood pressure within the normal range despite large changes in cardiac output that can occur within seconds. This implies that blood pressure regulation requires precise kinetic control of GPCR signaling. To test this hypothesis, we analyzed mice deficient in RGS2, a GTPase-activating protein that greatly accelerates the deactivation rate of heterotrimeric G proteins in vitro. Both rgs2+/- and rgs2-/- mice exhibited a strong hypertensive phenotype, renovascular abnormalities, persistent constriction of the resistance vasculature, and prolonged response of the vasculature to vasoconstrictors in vivo. Analysis of P2Y receptor-mediated Ca2+ signaling in vascular smooth muscle cells in vitro indicated that loss of RGS2 increased agonist potency and efficacy and slowed the kinetics of signal termination. These results establish that abnormally prolonged signaling by G protein-coupled vasoconstrictor receptors can contribute to the onset of hypertension, and they suggest that genetic defects affecting the function or expression of RGS2 may be novel risk factors for development of hypertension in humans.

NaCl-induced hypertensive rat model of non-insulin-dependent diabetes: role of sympathetic modulation

Systemic hypertension is common in individuals with non-insulin-dependent diabetes (NIDD) and, in this population, markedly increases the risk for cardiovascular complications. The aims of this study were to develop a rat model of combined NaCl-induced hypertension and NIDD, and to determine the contribution of the sympathetic nervous system to the development of the manifest hypertension. Two-day old male Wistar-Kyoto rats were injected with either streptozotocin (90 mg/kg, ip; NIDD) or vehicle (citrate buffer; control). At 4 weeks of age, the animals underwent either a right nephrectomy or a sham operation. Animals in each group were further subdivided, with one group maintained on normal (0.72 %) NaCl diet whereas the other was placed on a high (8%)-NaCl diet. At 6 months of age, diabetes was confirmed by glucose tolerance testing. Hemodynamic parameters were measured in the freely moving animal (ia) before and after the administration of prazosin (peripheral alpha1-adrenergic antagonist, iv) or clonidine (central alpha2-adrenergic agonist). The NIDD rat displayed a higher (P < .05) blood glucose concentration than the nondiabetic control rat during the glucose tolerance test. Elevated dietary NaCl significantly increased mean arterial pressure (MAP) in the uninephrectomized, but not the sham-operated groups. Acute administration of prazosin resulted in a significantly greater reduction in MAP of both hypertensive groups than of their normotensive counterparts. Moreover, clonidine caused a significant reduction in MAP of the hypertensive control rat but not in the normotensive controls. By contrast, both the hypertensive NIDD and the normotensive NIDD rats showed a similar reduction in MAP in response to clonidine administration. The data suggest that the combination of uninephrectomy and dietary NaCl excess confers hypertension on the NIDD rat. Moreover, enhancement of the sympathetic pathway plays an important role in the regulation of arterial pressure in the hypertensive NIDD rat.

Renal function in spontaneously hypertensive rats with insulin-exacerbated hypertension

This study tests the hypothesis that in spontaneously hypertensive rats (SHR), insulin decreases natriuresis and diuresis and thereby contributes importantly to the hypertensive response to exogenous insulin administration. Seven week old SHR were given daily subcutaneous injections of either insulin (mixture of 5 U/Kg regular and 10 U/Kg NPH) or vehicle (isotonic saline). Within one week of treatment, systolic arterial pressure (SAP) was significantly higher in the insulin, compared to saline, treated SHR (184.2 +/- 2.5 vs. 158.3 +/- 4.0 mm Hg). However, twenty-four hour sodium and fluid excretion and the natriuretic and diuretic responses to an intravenous saline load were not affected either before or after the insulin-induced exacerbation of hypertension in SHR. Insulin treatment did not affect glomerular filtration rate, effective renal blood flow, or fractional excretion of Na+ or fluid. Therefore, our data do not support a major role for sodium and fluid retention in the insulin-induced exacerbation of hypertension in SHR.