THE EFFECTS OF CHRONIC HYPOXIA ON THE ACUTE ADRENERGIC STRESS-RESPONSE IN THE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

We have investigated the effects of chronic hypoxia on the acute adren ergic stress response of adult rainbow;rout (Oncorhynchus mykiss). The goal of this study was to determine whether a prior 5-d exposure of f ish to lowered environmental oxygen levels (60 or 80 Torr) would influ ence the nature of catecholamine secretion from chromaffin tissue in s itu. Using a saline-perfused posterior cardinal vein preparation, it w as demonstrated that the basal (unstimulated) secretion of noradrenali ne and adrenaline was increased at 60-Torr hypoxia. In response to cho linergic (carbachol-elicited) stimulation, noradrenaline and adrenalin e secretion were significantly affected by prior exposure to hypoxia. The construction of dose response curves revealed that noradrenaline s ecretion was enhanced at the lowest doses of carbachol (1 - 5 x 10(-7) mol kg(-1)) and that this was reflected by an approximate 10-fold red uction in the ED50 (the dose of carbachol eliciting half-maximal norad renaline secretion). The effect of chronic hyp oxia on in situ carbach ol-evoked adrenaline secretion was similar but less pronounced. The re sults of this study suggest that during chronic moderate hypoxia, incr eased basal catecholamine secretion and enhanced responsiveness of chr omaffin cells to cholinergic stimulation, as well aiding the ongoing s tress, may assist the physiological adaptations to subsequent bouts of more severe acute stress.