1 Diabetes mellitus is known to produce alterations in vascular reactivity. The present study examined the effects of endothelium-dependent and endothelium-independent relaxing substances on thoracic aorta from control and spontaneously diabetic rats. 2 Endothelium-dependent relaxation produced by acetylcholine or the calcium ionophore, A23817, in aortic rings precontracted with phenylephrine was significantly attenuated in diabetic vessels. 3 Relaxations produced by sodium nitroprusside or adenosine in diabetic preparations were comparable to those in control vessels. 4 The results show that diabetes leads to a specific impairment of endothelial-dependent relaxation.

Activation of protein kinase C has been implicated in the regulation of a variety of cellular reactions. Although we, and others, have found protein kinase C and its substrate proteins to be present in both membrane and cytosolic fractions in the heart, the physiologic role of this kinase in the regulation of cardiac functions remains unknown. In the present study, we found that in isolated perfused rat heart, administration of phorbol esters 4/3-phorbol 12,13-dibutyrate(PDBu) and 12-0-tetradecanoylphorbol 13-acetate(TPA), which are specific activators of protein kinase C, produced marked dose-dependent negative changes in inotropy and chronotropy. A dose-dependent decrease in coronary flow was also observed. The diacylglycerol analogues, 1,2-oleoylacetyl-glycerol and 1,2-dioctanoylglycerol, produced similar effects as the active phorbol esters on these isolated perfused hearts. An inactive analogue of phorbol ester, 4a-phorbol, failed to produce any effect. Protein kinase C activity in both membrane and cytosolic fractions prepared from rat heart could be activated by TPA and PDBu at the same concentration range as used in the experiments with perfused hearts. Following perfusion of the hearts with PDBu, a rapid translocation of protein kinase C from cytosolic to membrane fractions was also observed. Our findings provide the first direct evidence that protein kinase C may play a potentially important role in the regulation of cardiac functions. (Circulation Research 1987;61:372-378) C ardiac functions are regulated by various hormones, neurotransmitters, and drugs, which are thought to interact with specific receptors located on the sarcolemmal membrane.1 Activation of /3-adrenergic receptor elevates cyclic adenosine 3',5' monophosphate (cAMP) in the myocardium and induces positive inotropic and chronotropic responses. Cyclic AMP-dependent protein kinase in the heart phosphorylates Ca 2+ channels, thereby increasing Ca 2+ concentration inside the cell. 2 However, the cellsurface-transducing system of Ca 2+ -mobilizing receptors in the heart, and the second messenger system that mediates the inhibitory response of cardiac tissue, have yet to be elucidated.Nishizuka and his colleagues 3 first demonstrated that diacylglycerol (DAG) activates protein kinase C. Since then, a link between phosphatidylinositol (PI) turnover and protein kinase C activation in a variety of cellular responses to hormonal stimulation has been established. Intracellular activation of protein kinase C can be achieved by the addition of cell-membrane- This work supported by grant Tl-51 from the Ontario Heart Foundation. S. Y. was the recipient of a studentship from Connaught Award Foundation, University of Toronto.Address for correspondence: Amar K. Sen, MD, PhD, Department of Pharmacology, Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada M5S 1A8.Received July 7, 1986; accepted April 10, 1987. permeable DAG analogues. Tumor-promoting phorbol esters such as 4/3-phorbol 12,13-dibutyrate (PDBu) and 12-0-tetradecanoylp...

In the isolated perfused rat heart, the dose-related cardiostimulation produced by norepinephrine (NE) or calcium chloride (Ca2+) was followed by a corresponding increase in coronary flow (CF) and in the cardiac level of adenosine 3',5'-cyclic phosphate (cAMP). Prolonged prostaglandin E2 (pge2) infusion did not change the basic force of contraction, CF, or cAMP level but when NE or Ca2+ were administered, only the responses of the CF and the cAMP were diminished. A phosphodiesterase inhibitor, diazoxide (Dx), caused insignificant increase in the basal cAMP, without affecting the force of contraction or CF. With NE or Ca2+, during Dx both the changes in CF and cAMP were augmented compared to the nontreated hearts. The inhibitory effects of NE or Ca2+ remained unchanged. Propranolol abolished the NE but not the Ca2+ effects. It is suggested that PGE2 modulates the cardiac cAMP level and that the latter plays an important role in the adaptive regulation of the CF. It is also postulated that changes in cAMP levels may be brought about by the hyperactivity per se produced by a variety of cardiostimulating agents.

There is increasing evidence that patients with diabetes mellitus are prone to ischaemic heart disease. This study examined cardiac hyperactivity and its consequent metabolically induced coronary dilatation in isolated, perfused, electrically paced rat hearts from control and spontaneously diabetic Bio-Breeding (BB) rats. Cardiostimulation produced by noradrenaline, calcium, or by tachycardia elicited increases in coronary flow that were significantly lower in diabetic hearts. However, the inotropic responses to noradrenaline and calcium in diabetic preparations were comparable to control. When coronary vascular reactivity was tested with sodium nitroprusside and adenosine, a decreased dilator response was observed with adenosine in diabetic hearts while no difference was observed with sodium nitroprusside. It is suggested that failure in the adaptive coronary flow response to cardiac hyperactivity in diabetes may, in part, be responsible for the higher incidence of ischaemic heart disease in the diabetic population.

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