Phosphate uptake by isolated rabbit heart.

Abstract

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.