Studies with Verapamil and Nifedipine Provide Evidence for the Presence in the Liver Cell Plasma Membrane of Two Types of Ca2+ Inflow Transporter Which Are Dissimilar to Potential-operated Ca2+ Channels

The addition of 500 microM verapamil or nifedipine to isolated hepatocytes incubated in the presence of 1.3 mM Ca2+ caused 20% inhibition of Ca2+ inflow as measured by the initial rate of 45Ca2+ exchange. No stimulation of 45Ca2+ exchange was observed in the presence of the Ca2+ agonist CGP 28392. An increase in the concentration of extracellular K+ from 6 to 60 mM (to depolarize the plasma membrane) increased the initial rate of 45Ca2+ exchange by 30%. In the presence of 60 mM K+, 400 microM verapamil inhibited the initiate rate of 45Ca2+ exchange by 50%. Verapamil and nifedipine completely inhibited vasopressin-induced Ca2+ inflow as determined by measurement of the initial rate of 45Ca2+ exchange and of glycogen phosphorylase a activity. This effect of verapamil was completely reversed by increasing the extracellular concentration of Ca2+. The concentrations of Ca2+ antagonist which gave 50% inhibition of vasopressin- or K+-stimulated Ca2+ inflow were in the range 50-100 microM, about 50-fold greater than the concentration which gave 50% inhibition of the beating of electrically-stimulated myocardial muscle cells. In the absence of vasopressin, verapamil caused a transient increase in glycogen phosphorylase a activity by a process which is largely independent of Ca2+. It is concluded that verapamil and nifedipine inhibit the transport of Ca2+ across the hepatocyte plasma membrane through a putative Ca2+ transporter which is activated by vasopressin and which differs in nature from potential-operated Ca2+ channels in excitable cells and from the Ca2+ transporter present in hepatocytes in the absence of hormone.