Polyarginine, polylysine, and protamine mimic the effects of high extracellular calcium concentrations on dispersed bovine parathyroid cells

Abstract

We investigated the effects of the basic peptides polyarginine, protamine, and polylysine on dispersed bovine parathyroid cells. All three peptides produced a dose-dependent inhibition of dopamine-stimulated cAMP accumulation, with half-maximal inhibition at 4 × 10⁻⁸, 1.5 × 10⁻⁷, 3 × 10⁻⁷, and 2 × 10⁻⁶ M, respectively, for polyarginine, protamine, and two preparations of polylysine of molecular weights 10,200 and 3800. The inhibition of cAMP accumulation was reversible and was blocked by preincubating the cells overnight with 0.5 μg/ml of pertussis toxin. The same peptides also inhibited PTH release at similar concentrations, markedly stimulated the accumulation of inositol phosphates at two- to threefold higher concentrations, and produced transient increases in the cytosolic Ca²⁺ concentration (Ca_i) in fura-2-loaded parathyroid cells. The polylysine-evoked spike in Ca_i persisted despite the removal of extracellular Ca²⁺, indicating that it arose from intracellular Ca²⁺ stores. Exposure of the cells to elevated extracellular magnesium (Mg²⁺) concentrations elicited a similar spike in Ca_i but blocked the Ca_i transient in response to subsequent addition of polylysine, or vice versa. Thus, Mg²⁺ and polylysine mobilize Ca²⁺ from the same intracellular store(s). These results indicate that highly basic peptides closely mimic the effects of polyvalent cations on parathyroid function, suggesting that both agents may regulate parathyroid function via similar biochemical pathways.