MAXIMUM CARDIAC-PERFORMANCE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) ATTEMPERATURES APPROACHING THEIR UPPER LETHAL LIMIT

Numerous studies have examined the effect of temperature on in vivo an d in situ cardiovascular function in trout, However, little informatio n exists on cardiac function at temperatures near the trout's upper le thal limit, This study measured routine and maximum in situ cardiac pe rformance in rainbow trout (Oncorhynchus mykiss) following acclimation to 15, 18 and 22 degrees C, under conditions of tonic (30 nmoll(-1)), intermediate (60 nmoll(-1)) and maximal (200 nmoll(-1)) adrenergic st imulation, Heart rate increased significantly with both temperature an d adrenaline concentration, The Q(10) values for heart rate ranged fro m 1.28 at 30 nmoll(-1) adrenaline to 1.36 at 200 nmoll(-1) adrenaline, In contrast to heart rate, maximum stroke volume declined by approxim ately 20 % (from 1.0 to 0.8 ml kg(-1)) as temperature increased from 1 5 to 22 degrees C. This decrease was not alleviated by maximally stimu lating the heart with 200 nmoll(-1) adrenaline. Because of the equal a nd opposite effects of increasing temperature on heart rate and stroke volume, maximum cardiac output did not increase between 15 and 22 deg rees C. Maximum power output decreased (by approximately 10-15 %)at al l adrenaline concentrations as temperature increased. This reduction r eflected a poorer pressure-generating ability at temperatures above 15 degrees C, These results, in combination with earlier work, suggest ( 1) that peak cardiac performance occurs around the trout's preferred t emperature and well below its upper lethal limit; (2) that the diminis hed cardiac function concomitant with acclimation to high temperatures was associated with inotropic failure; (3) that Q(10) values for card iac rate functions, other than heart rate per se, have a limited predi ctive value at temperatures above the trout's preferred temperature; a nd (4) that heart rate is a poor indicator of cardiac function at temp eratures above 15 degrees C.