Effects of pH and temperature on myocardial calcium-activation in Pterygoplichthys (catfish)

Fish are chronically exposed to a wide range of temperatures and acidic environments. Fish hearts have to therefore adapt to these changes in order to maintain contractility. Myofibrillar responsiveness to Ca2+ is exquisitely sensitive to both temperature and pH in mammalian myocardium. To evaluate myofilament calcium-activation, we chemically skinned ventricular myocardium from catfish (Pterygoplichthys). A decrease in pH from 7.5 to 6.8, irrespective of temperature change, shifted the calcium-force curve towards higher calcium concentrations without affecting maximal Ca(2+)-activated force. The contractile elements are therefore sensitive to changes in pH. In intact muscle preparations the active twitch force was decreased with increasing temperature (10-22 degrees C). However, the sensitivity of the myofilaments to Ca2+ was independent of temperature. These data suggest a possible role of the sarcoplasmic reticulum (SR) in mediating the effects of temperature. The response of intact muscle preparations to changes in temperature is therefore not likely due to temperature-dependent changes in myofilament calcium responsiveness.