Influence of the sympatho-adrenal system and somatostatin on the secretion of insulin in the rat.

1. The effects of somatostatin on insulin secretion in anaesthetized rats subjected to different manipulations of the sympatho-adrenal system have been investigated.2. Somatostatin (0.1 mug/min) inhibited the secretion of insulin in intact rats both in the basal state and after inducing an enhanced insulin release by infusion of the alpha-adrenoceptor-blocker phentolamine.3. Combined surgical splanchnicotomy and adrenalectomy caused an increase in the basal plasma insulin concentration. Somatostatin (0.1 mug/min) inhibited basal insulin release also in these rats. After infusion of phentolamine, however, the dose of somatostatin had to be raised five fold (0.5 mug/min) to achieve a comparable inhibition of insulin release. On the other hand, a similar rate of insulin secretion induced by glucose in intact rats could be inhibited by the lower dose of somatostatin.4. Administration of the beta-adrenoceptor-blocking agent propranolol to splanchnicotomized-adrenalectomized rats lowered basal insulin secretion to the same level as seen in intact rats. In these beta-adrenoceptor-blocked rats somatostatin (0.1 mug/min) inhibited insulin release both in the presence and absence of alpha-adrenoceptor blockade.5. Rats subjected to chemical sympathectomy through pre-treatment with 6-hydroxydopamine together with adrenalectomy displayed plasma insulin concentrations slightly above the normal range, but the values were much lower than in splanchnicotomized-adrenalectomized rats. Infusion of phentolamine to the chemically sympathectomized rats did not further increase insulin secretion, and somatostatin (0.1 mug/min) depressed insulin release both in the absence and presence of alpha-adrenoceptor blockade.6. It is suggested that an inhibitory tone exerted by the splanchnic nerves modulates the basal insulin secretion in the rat. Somatostatin and the sympathoadrenal system show a complex interaction on the insulin cells in that the sensitivity to somatostatin in splanchnicotomized-adrenalectomized rats with intact beta-adrenoceptors is decreased in the presence of the alpha-adrenoceptor-blocker phentolamine. The exact mechanism behind this decreased sensitivity remains unclear.