Compartmental analysis of potassium efflux from growth-oriented heart cells

Radioisotopic flux studies were initiated with a new preparation of growth-oriented heart cells to determine the contribution of heterogeneous cell types and the limitations of extracellular diffusion in quantitating the passive movement of potassium ions. The efflux of potassium-42 from contractile preparations, which contain two populations of cells, cardiac muscle and fibroblastlike, could be resolved into two components similar to that described for naturally occurring preparations of cardiac muscle. Compartmental analysis of the efflux data, using analog and digital computational methods, resolved the tracer kinetics into a slow compartment (k=0.015 min-1) associated with fibroblastlike cells and a fast compartment (k=0.067 min-1) associated with the cardiac muscle cells. The rate constants derived from compartmental analysis were independent of tracer equilibration and preparation dry weight. Analytical measurements of the preparations provided a quantitative basis for determing the transmembrane potassium fluxes from the tracer kinetics. Cardiac muscle cells stimulated at a rate of 150 min-1 in the presence of 5.4 mM external potassium were found to have a potassium efflux of 15.7 pmoles cm-2sec-1 whereas the value obtained for the fibroblastlike cells was 1.88 pmoles cm-2sec-1. Diffusional limitations of 42K efflux were analyzed for several important variables which can affect isotopic reflux, namely, transmembrane flux, cell volume-to-surface area and cell packing fraction.