Maintenance of baseline angiotensin II potentiates insulin hypertension in rats

Chronic insulin infusion in rats increases mean arterial pressure (MAP) by a mechanism dependent on angiotensin II (Ang II). However, the fact that plasma renin activity (PRA) decreases with insulin infusion suggests that Ang II sensitivity is increased and that the parallel reduction in Ang II may partly counteract any hypertensive action of insulin. This study tested that hypothesis by clamping Ang II at baseline levels during chronic insulin infusion. Sprague-Dawley rats were instrumented with artery and vein catheters, and MAP was measured 24 hours per day. In seven angiotensin clamped rats (AC rats), renin-angiotensin II system activity was clamped at normal levels throughout the study by continuous intravenous infusion of the angiotensin-converting enzyme inhibitor benazepril at 5 mg/kg per day (which decreased MAP by 18±2 mm Hg) together with intravenous Ang II at 5 ng/kg per minute. Control MAP in AC rats after clamping averaged 99±1 mm Hg, which was not different from the 101±2 mm Hg measured before clamping Ang II levels. Control MAP in the 8 vehicle-infused rats averaged 105±2 mm Hg. A 7-day infusion of insulin (1.5 mU/kg per minute IV) plus glucose (20 mg/kg per minute IV) increased MAP in both groups of rats; however, the increase in MAP was significantly greater in AC rats (12 ± 1 versus 5 ± 1 mm Hg). This enhanced hypertensive response to insulin in AC rats was associated with a greater increase in renal vascular resistance (153± 10% versus 119±6% of control) and a significant increase in renal formation of thromboxane (149±11% of control). Thus, decreased Ang II during insulin infusion limits the renal vasoconstrictor and hypertensive actions of insulin, and this may be caused, at least in part, by attenuation of renal thromboxane production.