Effect of chronic experimental diabetes on vascular smooth muscle function in rabbit carotid artery.

To understand the effects of diabetes on vascular smooth muscle function and the underlying mechanism(s) involved, we examined the responses to alpha-adrenoceptor agents, serotonin (5-HT), K+, and prostaglandins in the carotid artery of male New Zealand white rabbits with chronic diabetes (16 weeks) induced chemically by alloxan (100 mg/kg, intravenously) treatment. Isolated ring segments of diabetic rabbit carotid artery exhibited an increased (20-60%) maximal response to norepinephrine (NE), methoxamine, phenylephrine, and K+ as compared with controls. Responses to 5-HT were not significantly increased. Nevertheless, there were no significant differences in ED50 values of the agonists in either of the groups. Putatively selective alpha 2-adrenoceptor agonists (clonidine and guanabenz), prostaglandin E1 (PGE1) and prostaglandin I2 (PGI2) did not elicit any response in control vessels. In the diabetic state, however, these drugs contracted the artery in a dose-dependent fashion. Isoproterenol (0.1-10 microM) relaxed arterial rings previously contracted with all the agonists except PGE1 and PGI2, which were potentiated by isoproterenol. Contractions to PGE1 or PGI2 alone or in the presence of isoproterenol were reduced or abolished by 10(-5) M phentolamine. Under these conditions, isoproterenol exhibited its typical relaxatory action. Nifedipine was more potent in inhibiting the K+ response in diabetic carotid artery than in the controls. These results suggest an increased reactivity of diabetic rabbit carotid artery to alpha 2-adrenoceptor agonists, K+, PGE1, and PGI2 which may, at least in part, be due to an increased sensitivity of calcium channels in diabetic vessels. Contractile responses to PGE1 and PGI2 could be attributed to their action on adrenergic neurotransmission, thereby facilitating the release of NE from presynaptic nerve terminals. Furthermore, isoproterenol at a high dose (1 microM or more) may directly stimulate alpha-adrenoceptors. Whether or not this effect of isoproterenol is only prostaglandin-dependent is not clear.