Early effects of parathyroid hormone on membrane potential of rat osteoblasts in culture: Role of cAMP and Ca2+

Microelectrodes were used to investigate the possible involvement of cAMP and Ca2+ ions in the parathyroid hormone's, bPTH(1–34), effect on the membrane potential of rat osteoblasts in primary culture. Parathyroid hormone (10−7 M) depolarized cell membrane by 25.0 ± 6.1 mV (mean ± standard deviation, SD; n = 17). Blocking Ca2+ influx with the Ca channel blocker cobalt revealed two phases in the hormone effect: a rapid and slight membrane hyperpolarization followed by sustained depolarization. In addition, cobalt significantly (p < 0.01) decreased the magnitude of the PTH depolarizing action. The addition of dibutyryl-cAMP (10−3 M) to the perfusion solution also resulted in a biphasic effect. At a lower concentration (10−4 M), dibutyryl-cAMP produced only membrane hyperpolarization, suggesting a cAMP dose dependence of the opposite membrane potential changes. Forskolin (10−5 M) and the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (10−4 M) mimicked the depolarizing effect of PTH. IBMX at a low concentration (5 × 10−6 M) potentiated the depolarizing effect of PTH. Increases in [Ca2+]i using Ca2+ ionophore A23187 and intracellular injection of CaCl2 or inositol trisphosphate decreased the PTH depolarizing action, whereas intracellular injection of EGTA enhanced this effect. These results indicate that PTH evokes a biphasic change in rat osteoblast membrane potential that seems to be mediated by an increase in cAMP and modulated by intracellular calcium.