Catecholamines and adenine nucleotide material in effluent from stimulated adrenal medulla and spleen: A study of the exocytosis hypothesis for hormone secretion and neurotransmitter release

Abstract

The exocytosis hypothesis for neurotransmitter release from sympathetic nerves was experimentally tested by studying the effect of nerve stimulation on the out-flow of adenine nucleotide material from the cat spleen, and by comparing the adenine nucleotide content of stimulated and control portions of the splenic capsule. The adrenal medulla was used as a reference organ, assumed to secrete by exocytosis. The method used involved labelling the adenine nucleotides of the catecholamine storage particles in sympathetic nerves and adrenal medulla by daily injections of radioactive orthophosphate ( 32P) for one week. Results obtained by sucrose density gradient centrifugation and analysis by anion exchange column chromatography indicated that the technique used had resulted in significant labelling of the adenine nucleotides of the adrenal medullary amine storage particles, which have previously proved to be highly resistant to labelling with 32P. Induction of secretion in the perfused adrenal by carbachol consistently resulted in a largely parallel increase in catecholamines and 32P-labelled material in the effluent from the gland. Chromatographic analysis showed that the labelled material consisted mainly of inorganic phosphate, with only small amounts of AMP and no ADP or ATP. In the splenic perfusion experiments contraction of the organ, whether induced by injection of angiotensin or by nerve stimulation, resulted in increased outflow of 32P-labelled material, exclusively as inorganic phosphate. However, when contraction was blocked by phenoxybenzamine, nerve stimulation caused a large rise in noradrenaline outflow, but had no effect on the level of 32P-labelled material in the effluent. Moreover, prolonged nerve stimulation, known to cause release of about one half of the noradrenaline in the spleen, did not in any way affect the content of labelled adenine nucleotides of the splenic capsule. The present findings appear to be compatible with the exocytosis hypothesis for hormone secretion from the adrenal medulla, but they do not support the same hypothesis for neurotransmitter release from sympathetic nerves.