Influence of diabetes on the reactivity of mesenteric microvessels to histamine, bradykinin and acetylcholine.

1 Noradrenaline (NA) evoked a vasoconstrictor response in rat mesenteric microvessels in situ, the latency and nature of which was analogous in normal and alloxan-diabetic animals.2 Histamine and bradykinin (Bk) were capable of antagonizing the response to NA in normal but not in diabetic animals. In contrast, acetylcholine (ACh) was equally effective as an antagonist to NA in both groups of animals.3 The altered responses to histamine and Bk were not associated with hyperglycaemia since fasting rendered the diabetic animals normoglycaemic and yet did not restore the reactivity of microvessels. Previous administration of insulin to diabetic animals corrected the impaired responses to histamine and Bk.4 A similar condition of impaired responses to histamine and Bk was produced in normal animals by the intravenous injection of 2-deoxyglucose although ACh remained fully active.5 Apparently, the functional changes observed in the response to histamine or Bk, as antagonists of the vasoconstrictor reaction to NA, were not associated with a defective response of all smooth muscle. First, because ACh remained active in diabetic animals, and, second, because extravascular smooth muscles obtained from either normal or diabetic rats were equally relaxed by histamine or Bk in vitro.6 It is suggested that histamine and Bk antagonized the vasoconstrictor response of microvessels to NA through an action on lining endothelial cells resulting in increased vascular permeability and hyperosmolarity of extracellular fluids.7 The process depended on the availability of insulin, and, therefore, might be affected by intracellular glucopaenia as occurring in diabetes.8 Intracellular glucopaenia markedly affected other structures. Reduced atria rates were observed in diabetes, despite the fact that the isolated preparation responded normally to NA, ACh or tyramine. Partial substitution of glucose in the bathing fluid by 2-deoxyglucose or addition of NaF to the organ bath evoked similar changes in atria from normal animals.9 ACh which has little effect on vascular permeability must exert its vasodilator effects through mechanisms which are different from those influenced by the biochemical changes occurring in diabetes.