Studies with verapamil and nifedipine provide evidence for the presence in the liver cell plasma membrane of two types of Ca 2+ inflow transporter which are dissimilar to potential-operated Ca 2+ channels

Abstract

The addition of 500 μM verapamil or nifedipine to isolated hepatocytes incubated in the presence of 1.3 mM Ca 2+ caused 20% inhibition of Ca 2+ inflow as measured by the initial rate of 45Ca 2+ exchange. No stimulation of 45Ca 2+ exchange was observed in the presence of the Ca 2+ 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 45Ca 2+ exchange by 30%. In the presence of 60 mM K +, 400 μM verapamil inhibited the initiate rate of 45Ca 2+ exchange by 50%. Verapamil and nifedipine completely inhibited vasopressin-induced Ca 2+ inflow as determined by measurement of the initial rate of 45Ca 2+ exchange and of glycogen phosphorylase a activity. This effect of verapamil was completely reversed by increasing the extracellular concentration of Ca 2+. The concentrations of Ca 2+ antagonist which gave 50% inhibition of vasopressin- or K +-stimulated Ca 2+ inflow were in the range 50–100 μM, 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 Ca 2+. It is concluded that verapamil and nifedipine inhibit the transport of Ca 2+ across the hepatocyte plasma membrane through a putative Ca 2+ transporter which is activated by vasopressin and which differs in nature from potential-operated Ca 2+ channels in excitable cells and from the Ca 2+ transporter present in hepatocytes in the absence of hormone.