Diabetes mellitus: progress and directions

Role of vascular adrenergic mechanisms in the haemodynamic and PGI2 stimulating effects of angiotensin in diabetic dogs

In aware of the well-known altered vascular responsiveness in the diabetic vasculature, this study aimed to compare the haemodynamic and PGI2 releasing effects of angiotensin in metabolically healthy (12) and alloxan-(560 umol/kg) diabetic (12) dogs as well as to analyze the role of vascular adrenoceptors in this. In vivo the effect of intracoronarially administered angiotensin (63-125-250-500-1000 pmol/kg/min) on coronary blood flow, mean arterial blood pressure, myocardial contractile force and heart rate was investigated without and with pretreatment of 2 umol/kg phentolamine. In vitro PGI2 release by isolated coronary rings was induced by 50 nmol/l angiotensin before and after pretreatment with 5 umol/l phentolamine and measured by radioimmunoassay. Angiotensin enhances dose-dependently both the mean arterial blood pressure and coronary blood flow, while it provokes a considerable (p < 0.05) increase of PGI2 formation by isolated coronary arterial rings. These alterations could be prevented by phentolamine administration both in vivo and in vitro, while this drug did not affect the angiotensin-induced enhancement of diabetic coronary blood flow. On the other hand the increase of blood pressure by angiotensin was found to be more (p < 0.05) expressed in diabetes and it could be further potentiated by phentolamine. PGI2 synthesis by isolated diabetic coronary rings could not be modified either by angiotensin alone or in combination with phentolamine. On the basis of above data, the lack of stimulated vascular PGI2 formation mediated by alpha-adrenergic mechanisms is supposed to causatively contribute to the diminished sensitivity of diabetic coronary arteries to vasodilation.

Disturbed lipid metabolism in diabetic coronary vessels

The aim of this study was to clarify whether or not arachidonic acid metabolic disorders are caused by a substrate inavailability and whether such disorders might contribute to circulatory disturbances in the diabetic myocardium. Norepinephrine induced a decrease in the conductivity of both coronary arterial bed and myocardial microcirculation in alloxan-diabetic dogs. It was markedly (p less than 0.05) attenuated both by indomethacin and acetylsalicylic acid pretreatments indicating an imbalance among the vasoactive prostanoids in diabetes. TXA2 release from the diabetic coronary rings was found to be elevated and could be normalized after the blockade of vascular adrenoceptors by phentolamine (p less than 0.05). PGI2 synthesis was also enhanced by adrenergic blockade in the diabetic arterial rings. After pretreatment with 14C arachidonic acid, in order to measure substrate availability, the arachidonic acid metabolic rate was less in the diabetic coronary arteries than in healty vessels (p less than 0.05). Ten mumol/l norepinephrine decreased arachidonic acid metabolism in the presence of prelabelled substrate in the diabetic animals, compared to an increase observed in metabolically healthy dogs. Therefore diabetes appears to diminish arachidonic acid metabolism and uptake independent of adrenoceptors and to induce an imbalance between vasoconstrictor and vasodilator cyclooxygenase products, resulting in elevated TXA2 release controlled by adrenergic mechanisms which may contribute to an impairment in myocardial microcirculation.