Effects of alpha 1- and alpha 2-adrenoceptors on basal and adrenaline mediated insulin secretion in rats

Groups of male rats were injected phentolamine, prazosin or yohimbine and infused with saline or adrenaline for 4 h under pentobarbital anesthesia. Blood samples were taken at 30 min intervals during the infusion, the isovolemia being maintained by replacing the blood volume with the blood obtained from normal donor animals. Plasma glucose and insulin levels were estimated. In saline infused animals a severalfold increase in plasma insulin levels was found after pretreatment with phentolamine, while such increase after prazosin or yohimbine was much less, though significant. In adrenaline infused animals the increased insulinemia after phentolamine pretreatment was still more remarkable than in those infused saline. After prazosin treatment, however, no changes in insulinemia were found during adrenaline infusion, while yohimbine pretreatment resulted in a significant insulin increase in similarly infused rats. It is concluded that alpha 2-receptors apparently play a major role in mediating inhibitory effects of adrenaline on insulin secretion. However, even some participation of alpha 1-receptors in the complex regulation of insulin secretion in vivo cannot be excluded, since the effect of phentolamine pretreatment on either basal or adrenaline induced insulinemia was higher than that of yohimbine, though a higher molar dose of the latter was used. In addition, no changes in insulinemia were found during the infusion of adrenaline only which may be explained by a prevailing effect of inhibitory alpha-adrenoceptor tone over a stimulatory beta-adrenoceptor tone under the conditions of unblocked adrenoceptors.