TEMPERATURE AND PH EFFECTS ON CA2- A COMPARISON OF TROUT WITH MAMMALS( SENSITIVITY OF CARDIAC MYOFIBRILS )

Active salmonids maintain myocardial contractility at temperatures tha t are cardioplegic for mammals. We postulated that myofrbrillar Ca2+ s ensitivity in the trout heart might 1) exhibit lower temperature depen dence and/or 2) be greater over the range of physiological temperature s. Temperature-induced changes in intracellular pH may also play a rol e as alkalosis typically increases calcium affinity of myofibrillar ad enosinetriphosphatase (ATPase). Ca2+ sensitivities of ventricular myof ibrillar ATPase were determined in rats and in rainbow trout (Oncorhyn chus mykiss) over a physiological range of pH and temperatures. Maxima l myofibrillar ATPase activities of each species were similar and equa lly affected by temperature. Trout myofibrillar ATPase lost Ca2+ depen dence at 37 degrees C. At constant pH, reduced temperature decreased c alcium affinity more in trout (0.35 pCa/10 degrees C) than in rat (0.0 8-0.16 pCa/10 degrees C). Under a-stat conditions, the effects of temp erature were reduced in both trout (0.2 pCa/10 degrees C) and rat (no significant effect). Over trout physiological temperatures, Ca2+ sensi tivity was greater than rat at 37 degrees C. Qualitatively similar res ults were observed in studies measuring tension in skinned trout ventr icular fibers. One mechanism by which the trout heart is able to maint ain contractility at low temperatures is through the inherent higher C a2+ sensitivity of the contractile element compared with mammalian spe cies.