THE EFFECT OF TEMPERATURE ON THE CRITICAL OXYGEN PRESSURE FOR HEART-BEAT FREQUENCY IN EMBRYOS OF ATLANTIC SALMON AND SPECKLED TROUT

Abstract

Intact embryos of speckled trout and Atlantic salmon were exposed to solutions of oxygen and nitrogen in distilled water. It was observed that as the partial pressure of oxygen was reduced a pressure was found below which the frequency of the heart-beat was not maintained at the normal level characteristic of higher partial pressures of oxygen. A "critical" partial pressure of oxygen for heart-beat frequency can therefore be said to exist. Data have been obtained from which its value at five different temperatures can be determined. The critical pressures for the two organisms are similar, rising from approximately 3 to 5 mm. of mercury at 1.5 °C. to 40 to 50 mm. of mercury at 20 °C. Possible mechanisms leading to the establishment of a critical oxygen partial pressure are discussed. It seems likely that diffusion is not the limiting factor in these preparations so that the critical pressure, and its temperature coefficient, must be the property of the intracellular respiratory systems concerned. The finding that the logarithm of the critical partial pressure can be represented as a linear function of the reciprocal of the absolute temperature is consistent with this view. Temperature exercises a more pronounced effect on the critical oxygen pressure of the pacemaking process, than it does on the over-all velocity of that process as indicated by the normal frequency of the heart. It is concluded that the critical pressure is a characteristic of the chemical systems in the pace-making cells of the heart.