Phosphate uptake by isolated rabbit heart

¹ Department of Physiology, Stritch School of Mediiine, Loyola University; and Department of Physiology, University of Illinois College of Medicine, Chicago, Illinois

Isolated rabbit hearts were perfused with Locke Ringer's fluid to which P³² was added. Cellular uptake of P³² was determined by analyzing ventricles from which a rapidly exchangeable extracellular P³² fraction had been removed. In studies on concentration dependence, a plot of phosphate uptake during 1 hr of perfusion against perfusate phosphate concentration revealed that the early phase of uptake follows saturation kinetics. In studies on time course with high external phosphate, i.e., at a level where the uptake mechanism is apparently saturated, relative specific activity (RSA) of total acid-soluble phosphate attained a plateau in 2 hr. RSA is the ratio of specific activities of a tissue fraction to perfusate inorganic phosphate (IP). The plateau was reached at a level corresponding to 10% of the equilibrium value, and it appears that early uptake of phosphate may be confined to a single, rapidly exchanging compartment. This pool appears to contain high-energy and inorganic phosphate since 90% of tissue P³² was found in adenosine polyphosphate, phosphocreatine, and IP fractions. Perfusion with low phosphate led to a decrease in tissue IP with relatively little variation in organic phosphates. This depletion of IP seemed to be accompanied by the emergence of a second compartment as revealed by the RSA data.

Note:
With the Technical Assistance of Mitsuko Eto