A contrasting effect of the diabetic state upon the contractile responses of aortic preparations from the rat and rabbit.

Diabetes was induced in rats by a single intraperitoneal injection of streptozotocin (65 mg kg-1). Rabbits were rendered diabetic by injecting alloxan (100 mg kg-1) into the lateral ear vein. Diabetes was confirmed by a significant elevation of serum glucose in both species 8 weeks after injection. The maximum contraction to noradrenaline (NA), 5-hydroxytryptamine (5-HT) and KCl was markedly diminished in thoracic aortic rings (AR) from diabetic rats with no change in the EC50 of the agonists. There were no differences in the contractile properties of AR from diabetic rabbits to NA, 5-HT or KCl. Diabetes did not alter the responsiveness of AR from the rat to angiotensin II (AII). However, AR from diabetic rabbits displayed a decreased maximal contraction and an increased EC50 to AII. The magnitude of the acetylcholine-induced relaxation to precontracted AR was not different between diabetic and control rats and rabbits. The contractile responses of AR to NA, 5-HT and KCl were depressed in diabetic rats, regardless of the control tissue to which they were compared. The decrease in maximal contraction to NA, 5-HT and KCl seen in diabetic animals was prevented by insulin replacement. The results demonstrated that while both rats and rabbits exhibited a similar degree of hyperglycemia after treatment with a diabetogenic agent, aortic preparations from the rabbit are not affected in the same way as the aorta from the diabetic rat when exposed to NA, 5-HT and KCl. This feature may be related to the marked differences between the extent of sympathetic innervation of the aorta in the rabbit and rat. Furthermore, the decrease in maximal contraction in rat aorta was non-specific with respect to agonists since it could also be demonstrated with KCl. Therefore, it follows that the diabetic state may affect processes responsible for contraction beyond the level of receptor activation.