Mechanism of Li inhibition of vsopressin-sensitive adenylate cyclase in cultured renal epithelial cells

We investigated the mechanism for lithium-induced inhibition of vasopressin-stimulated adenosine 3',5'-cyclic monophosphate (cAMP) production in the renal epithelial cell line LLC-PK₁. In LLC-PK₁ membranes lithium caused direct inhibition of hormone-stimulated adenylate cyclase activity by competing with magnesium. Fifty percent inhibition occurred at 20 mM lithium. The maximum transport activity (V(max)) but not the activation constant (K(a)) for activation by vasopressin was altered. Activation by GTP and its nonhydrolyzable analogues was also inhibited by lithium. Furthermore, kinetic studies revealed that the lag phase in the activation of adenylate cyclase by 5'-guanylimidotriphosphate [Gpp(NH)p] was prolonged from 1 to 3 min, suggesting an effect of lithium on magnesium-dependent activation of the stimulatory GTP binding protein G(s). The function of the corresponding inhibitory of GTP-binding protein G(i), as assessed by GTP inhibition of vasopressin-stimulated adenylate cyclase activity in the presence and absence of pertussis toxin pretreatment, was unaffected. Intact LLC-PK₁ cells incubated in 10 mM lithium (approximate urinary concentration in lithium-treated patients) attained an intracellular lithium concentration of 17 mM, which led to a 40% reduction in cAMP formation. Magnesium loading of intact cells with the ionophore A23187 reversed the inhibitory effect of lithium. It is concluded that lithium directly inhibits the activation of vasopressin-sensitive adenylate cyclase in renal epithelia by competing wtih magnesium for activation of G(s). This direct effect on G(s) activation accounts for the inhibitory effect of lithium on cAMP production in the intact cell.