Storage and Differential Release of Catecholamines in Rainbow Trout (Oncorhynchus mykiss) and American Eel (Anguilla rostrata)

Abstract

Rainbow trout (Oncorhynchus mykiss) and American eels (Anguilla rostrata) were exposed (15 min) to two levels of external hypoxia (water Po₂ [Pwo₂] = 5.3 and 4.0 kPa for trout; 4.0 and 2.7 kPa for eels). Arterial Po₂ (Pao₂) fell to 1.7-2.0 kPa in trout and 1.1-1.3 kPa in eel. Plasma catecholamines were elevated in trout during both levels of hypoxia but were unaltered in hypoxic eels. The goals of this study were to elucidate the mechanisms for the differences in these plasma catecholamine levels. Levels of stored catecholamines were measured in the kidney, posterior cardinal vein (PCV), and the heart. In both species the anterior region of the PCV was the primary storage site, with levels being greater in trout than in eels. The ability of the chromafin cells to respond to two chemical stimuli, 60 mmol L⁻¹ K⁺ or carbachol (10⁻⁸ to 10⁻⁵ mol), was assessed with an in situ saline-perfused PCV preparation. Trout released greater quantities of catecholamines in response to both stimuli than did eels. Differences in the cholinergic stimulation of the chromafin cells may account for the differences in plasma levels during hypoxia. In trout, carbachol selectively caused the release of adrenaline over noradrenaline, which suggests the presence of two populations of chromaffin cells, those containingpredominantly adrenaline and others containing predominantly noradrenaline.