Kinetic analyses of calcium movements in cell cultures. 3. Effects of calcium and parathyroid hormone in kidney cells

Calcium compartments and fluxes were measured by kinetic analyses in kidney cell suspensions in a three-compartment closed system. The fast phase influx and compartment size increase linearly with the medium calcium and the half-time of exchange is only 1.3 min which suggests that the fast component is extracellular. The slow phase compartment rises linearly from 0.1 to 0.5 mmole calcium/kg cell water when the medium calcium is raised from 0.02 to 2.5 mM. The slow phase calcium influx exhibits the pattern of saturation kinetics with a V_max of 0.065 µµmole cm^-2 sec^-1 and a K_m of 0.3 mM indicating that it is a carrier-mediated transport process. PTH has no effect on the fast phase of calcium influx, but increases both calcium influx and the calcium pool size of the slow component. The maximum effect is obtained at medium calcium concentration of 1.3 mM. Below 0.3 mM extracellular calcium, the effects of the hormone cannot be demonstrated. PTH increases the V_max of calcium influx from 0.065 to 0.128 µµmole cm^-2 sec^-1 while the K_m rises from 0.3 to 1.15 mM. These findings suggest that PTH increases the translocation of the calcium-carrier complex across the membrane and not the carrier concentration or its binding affinity for calcium.