Control of catecholamine storage and release in teleost fish.

Abstract

This thesis examined a variety of factors controlling the storage and release of the catecholamine hormones, adrenaline and noradrenaline, from chromaffin cells in teleost fish. In these fish, chromaffin cells line the walls of the posterior cardinal vein (PCV) in the anterior kidney region (head kidney). To examine the process of catecholamine release, an in situ saline-perfused PCV preparation first was developed and validated in the rainbow trout (Oncorhynchus mykiss). The levels of stored catecholamines were approximately 4X greater in trout than in eel, and trout chromaffin cells released greater quantities of catecholamines in response to both 60 mmol 1$\sp{-1}$ K$\sp+$ and carbachol administration, in situ. Thus trout chromaffin cells appear to exhibit a greater capacity to release catecholamines than do eel chromaffin cells which may explain the different plasma catecholamine levels during acute hypoxia. The proximate stimulus for catecholamine release in rainbow trout, during exposure to acute hypoxia, was examined by acclimating trout to either 5 or 15$\sp\circ$C in order to modify haemoglobin oxygen-affinity. At 15$\sp\circ$C, catecholamines were released into the circulation at a PaO$\sb2$ threshold of 34.5 torr while at 5$\sp\circ$C the threshold was 24.0 torr. Release thresholds, calculated on the basis of arterial blood oxygen-saturation were similar at both temperatures (approximately 60% Hb O$\sb2$-saturation). Thus the lowering of blood oxygen content, rather than PO$\sb2,$ appears to be the proximate stimulus causing catecholamine release in rainbow trout during acute hypoxia. In the rainbow trout, intra-arterial injections of serotonin in vivo caused an increase in both plasma adrenaline and noradrenaline levels. In situ, bolus injections of catecholamines which was attenuated by pre-treatment with the serotonergic receptor antagonist, methysergide but unaffected by hexamethonium. Thus serotonin is capable of causing the release of catecholamines by interacting with serotonergic receptors on the chromaffin cells. The effect of adrenocorticotropic hormone (ACTH), the normal secretagogue of cortisol, on catecholamine release, was investigated in the rainbow trout. Intra-arterial injections of ACTH, in vivo, caused an elevation of plasma adrenaline levels. In situ, adrenocorticotropic hormone (ACTH) caused the release of both catecholamines. ACTH-induced release was unaffected by pre-treatment with hexamethonium or methysergide but abolished in Ca$\sp{2+}$-free media. Injections of cortisol in situ did not elicit catecholamine release. ACTH, therefore, is capable of eliciting catecholamine secretion from the chromaffin cells in rainbow trout. (Abstract shortened by UMI.)