The liver cell plasma membrane Ca²⁺ inflow systems exhibit a broad specificity for divalent metal ions

1. The inflow of Mn²⁺ across the plasma membranes of isolated hepatocytes was monitored by measuring the quenching of the fluorescence of intracellular quin2, by atomic absorption spectroscopy and by the uptake of ⁵⁴Mn²⁺. The inflow of other divalent metal ions was measured using quin2. 2. Under ionic conditions which resembled those present in the cytoplasmic space, Mn²⁺, Zn²⁺, Co²⁺, Ni²⁺ and Cd²⁺ each quenched the fluorescence of a solution of Ca²⁺-quin2. 3. The addition of Mn²⁺, Zn²⁺, Co²⁺, Ni²⁺ or Cd²⁺ to cells loaded with quin2 caused a time-dependent decrease in the fluorescence of intracellular quin2. Plots of the rate of decrease in fluorescence as a function of the concentration of Mn²⁺ reached a plateau at 100 μM-Mn²⁺. 4. The rate of decrease in fluorescence induced by Mn²⁺ was stimulated by 20% in the presence of vasopressin. The effect of vasopressin was completely inhibited by 200 μM-verapamil. Adrenaline, angiotensin II and glucagon also stimulated the rate of decrease in the fluorescence of intracellular quin2 induced by Mn²⁺. 5. The rate of decrease in fluorescence induced by Zn²⁺, Co²⁺, Ni²⁺ or Cd²⁺ was stimulated by between 20 and 190% in the presence of vasopressin or angiotensin II. 6. The rates of uptake of Mn²⁺ measured by atomic absorption spectroscopy or by using ⁵⁴Mn²⁺ were inhibited by about 20% by 1.3 mM-Ca²⁺ and stimulated by 30% by vasopressin. 7. Plots of Mn²⁺ uptake, measured by atomic absorption spectroscopy or with ⁵⁴Mn²⁺, as a function of the extracellular concentration of Mn²⁺ were biphasic over the range 0.05-1.0 mM added Mn²⁺ and did not reach a plateau at 1.0 mM-Mn²⁺. 8. It is concluded that (i) hepatocytes possess both a basal and a receptor-activated divalent cation inflow system, each of which has a broad specificity for metal ions, and (ii) the receptor-activated divalent cation system is the receptor-operated Ca²⁺ channel.