Effect of acute and chronic epinephrine administration on clearance and metabolism of [³H]-epinephrine in trout (Oncorhynchus mykiss)

Abstract

Several studies have measured the rate of catecholamine clearance, metabolism, and tissue accumulation in fish. However, no information is available on the effect of repeated stress or high circulating catecholamine levels on catecholamine clearance and metabolism. We measured the clearance and metabolism of [³H]-epinephrine (approximately 0.1 μg·kg^-1) in SW-acclimated rainbow trout subjected to acute (five injections in 1 day) and chronic (4 days; five injections per day) administration of 4.0 μg·kg^-1 epinephrine or saline. In addition, a saturation experiment, where 4.0 μg·kg^-1 of unlabelled epinephrine was injected concurrently with [³H]-epinephrine, investigated whether catecholamine clearance and metabolism are affected by high circulatin levels. Neither the rate constants for catecholamine clearance, nor the post-injection proportions of unmetabolised [³H]-epinephrine, deaminated [³H]-epinephrine and O-methylated [³H]-epinephrine were affected by the acute or chronic injection protocols. The concurrent injection of [³H]-epinephrine and 4.0 μg·kg^-1 of unlabelled epinephrine resulted in an elevated postinjection ³H:¹⁴C ratio, but increased proportions of O-methylated [³H]-epinephrine and reduced proportions of unmetabolised [³H]-epinephrine. We conclude that in fish (1) catecholamine clearance and metabolism are unlikely to be compromised by repeated exposure to acute stressors; (2) catecholamine extraction and/or metabolism is enhanced when circulating levels are high; and (3) there is a marked capacity to rapidly (minutes) clear and inactivate catecholamines that are released in response to stressful stimuli.