METHOD FOR MEASURING METABOLIC-CLEARANCE RATE AND CORTISOL SECRETION RATE IN EEL (ANGUILLA-ANGUILLA L)

Classical methods for the analysis of steroid dynamics were adapted to measure the metabolic clearance rate and the secretion rate of cortisol in the European eel using [1,2-3H]cortisol as a tracer. Measurements were made while the eels were in the steady state; stresses which usually result from anesthesia and handling were avoided by chronic catheterization of the blood vessels of the swim bladder. In this way the plasma cortisol levels were constant; it was possible to obtain valid measurements of the cortisol secretion rate knowing the metabolic clearance rate and the plasma cortisol level. In the eel cortisol and cortisone together form a chemical compartment, derived originally from cortisol secreted by the interrenal gland and subsequently transformed partially into cortisone in the plasma and maintained thenceforth in dynamic reversible equilibrium. Consequently, the metabolic clearance rate of cortisol was calculated from known changes in concentration of both [1,2-3H]cortisol and [1,2-3H]cortisone in the plasma deduced from dichloromethane-extractable plasma radioactivity, 72% of which is attributable to these 2 steroids from the moment that the radioactivity begins to decline regularly as a function of time (15-30 min after the injection). After a single injection of [1,2-3H]cortisol, the interpretation of the disappearance curve by compartmental analysis is complicated by the delay of tracer distribution and by the evident variability in the initial portion of the curve. Disappearance curves were accordingly analyzed in a more rigorous fashion either by integrating the area under each curve which allows one to calculate individually the metabolic clearance rates, or by integrating according to Normand, M. and Fortier, C. which allows 1 statistical treatment by data. Estimates of the metabolic clearance rate of cortisol were made in a satisfactory manner by either 1 of the 2 integration methods. They were very similar to estimates obtained by prolonged infusion of [1,2-3H]cortisol to constant specific activity into eels maintained under identical experimental conditions.