The Cardiac Renin-Angiotensin System in STZ-Induced Diabetes

To determine the effect of diabetes on the cardiac renin-angiotensin system, we compared angiotensin II binding density and renin, angiotensinogen, and type 1 angiotensin II (AT1) receptor mRNA levels in hearts of Sprague-Dawley rats 14 days after the administration of streptozotocin (STZ), in vehicle-treated control rats, and in STZ-administered rats made euglycemic with insulin. Myocardial angiotensin II receptor density, determined using an in situ autoradiographic technique, was increased significantly in hyperglycemic diabetic rats in comparison with control rats and euglycemic diabetic rats (P < 0.01) as a result of an increase in both AT1 and AT2 (type 2 angiotensin II) subtypes. The myocardial AT1 receptor mRNA level, determined by slot blot hybridization, was also significantly greater in the hyperglycemic diabetic rats (P < 0.005). Neither plasma renin concentration nor cardiac renin or angiotensinogen mRNA levels differed among the three study groups. In an additional experiment, control and diabetic rats were infused with angiotensin II (200 ng · kg⁻¹ · min⁻¹ i.p. for 7 days) or vehicle. Plasma renin concentration decreased significantly, whereas no significant changes occurred in cardiac renin or angiotensinogen steady-state mRNA levels. As in the first experiment, levels of AT1 receptor mRNA were significantly greater in the diabetic rats. Thus, myocardial angiotensin II receptor density is increased in diabetic rats in association with an increase in steady-state AT1 receptor mRNA levels, an abnormality that appears to be independent of changes in the circulating renin-angiotensin system.