Effects of vasopressin and La³⁺ on plasma-membrane Ca²⁺ inflow and Ca²⁺ disposition in isolated hepatocytes. Evidence that vasopressin inhibits Ca²⁺ disposition

Vasopressin caused a 40% inhibition of ⁴⁵Ca uptake after the addition of 0.1 mM-⁴⁵Ca²⁺ to Ca²⁺-deprived hepatocytes. At 1.3 mM-⁴⁵Ca²⁺, vasopressin and ionophore A23187 each caused a 10% inhibition of ⁴⁵Ca²⁺ uptake, whereas La³⁺ increased the rate of ⁴⁵Ca²⁺ uptake by Ca²⁺-deprived cells. Under steady-state conditions at 1.3 mM extracellular Ca²⁺ (Ca²⁺₀), vasopressin and La³⁺ each increased the rate of ⁴⁵Ca²⁺ exchange. The concentrations of vasopressin that gave half-maximal stimulation of ⁴⁵Ca²⁺ exchange and glycogen phosphorylase activity were similar. At 0.1 mM-Ca²⁺₀, La³⁺ increased, but vasopressin did not alter, the rate of ⁴⁵Ca²⁺ exchange. The results of experiments performed with EGTA or A23187 or by subcellular fractionation indicate that the Ca²⁺ taken up by hepatocytes in the presence of La³⁺ is located within the cell. The addition of 1.3 mM-Ca²⁺₀ to Ca²⁺-deprived cells caused increases of approx. 50% in the concentration of free Ca²⁺ in the cytoplasm ([Ca²⁺](i)) and in glycogen phosphorylase activity. Much larger increases in these parameters were observed in the presence of vasopressin or ionophore A23187. In contrast with vasopressin, La³⁺ did not cause a detectable increase in glycogen phosphorylase activity or in [Ca²⁺](i). It is concluded that an increase in plasma membrane Ca²⁺ inflow does not by itself increase [Ca²⁺](i), and hence that the ability of vasopressin to maintain increased [Ca²⁺](i) over a period of time is dependent on inhibition of the intracellular removal of Ca²⁺.