Limiting Effect of Adrenaline on Output of Adrenal Medulla

Abstract

Transmission in sympathetic ganglia can be reduced by physiological quantities of adrenaline. It has, therefore, been suggested that adrenergic ganglionic inhibition could render a sympatho-adrenal discharge self-limiting and thus provide a homeostatic mechanism whereby adrenaline would serve to limit the output of the adrenal when it became excessive. A potential site of action that would be crucial for this effect would be at the adrenal medulla. To test for an adrenal action, the blood pressure response in cats/with the splanchnic vascular bed greatly reduced by partial evisceration, was used as a measure of epinephrine output upon stimulating the splanchnic nerve. With a constant stimulus intensity it was thus possible to obtain reproducible elevations of blood pressure reflecting output of epinephrine and serving as an index of synaptic transmission in the adrenal medulla. Small amts. of epinephrine could be introduced into the arterial supply of the adrenal gland without producing a significant change in systemic blood pressure. It was shown by close arterial injn. of 0.1-2.5 ug. that epinephrine transiently reduced the output from the adrenal medulla. In most positive expts., the synaptic inhibition indicated by the reduction of epinephrine output could be repeated several times. Measurement of venous outflow from the adrenals showed that the effect was essentially independent of alterations in adrenal blood flow. In keeping with the idea of primary adrenergic inhibition previously demonstrated in other sympathetic synapses, a period of postinhibitory facilitation was observed. The data were considered positive evidence for the existence of a mechanism for homeostatic regulation of the adrenal medulla under conditions of sympathetic stimulation and are in keeping with the idea of primary adrenergic inhibition at all sympathetic ganglionic synapses.