Vanadate-induced movements of Ca2+ and K+ in human red blood cells

Fresh human red blood cells become highly labeled with 45Ca2+ when exposed to 0.5 mM vanadate. The effect of vanadate requires its penetration into the cell, and is attributed to the inhibition of the outwardly directed Ca2+-pumping ATPase which would otherwise "mask" the uptake of 45Ca2+. Since the inhibition of the CA2+ pump by vanadate is not complete, a transmembrane Ca2+-Ca2+ exchange can be detected. The influx leg of the exchange is inhibited by verapamil, quinidine, and Co2+. This, as well as additional (kinetic) evidence, indicates that the influx of Ca2+ is a carrier-mediated process. Experiments in which the transmembrane K+ gradient has been abolished or decreased with ionophores, or by increasing the K+ concentration in the medium, suggest that the K+ gradient may play a role in the influx of Ca2+. The vanadate-induced accumulation of Ca2+ by red cells promotes a massive efflux of K+, indicating the activation of a Ca2+-sensitive K+-channel. The results indicate the occurrence of a slow cycling of Ca2+ across the red cell membrane. The influx leg of the cycle occurs through a verapamil-sensitive channel, and is possibly driven by the discharge of the transmembrane K+ gradient. The efflux leg of the cycle consists of the Ca2+-pumping ATPase.