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 less than 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 x 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.
Fritsch
J
J
Laboratoire des Tissus Calcifiés, CNRS UA.583, Paris, France.
Edelman
A
A
Balsan
S
S
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002478:Cells, Cultured; D003035:Cobalt / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology*; D004533:Egtazic Acid / Q000494:pharmacology; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D008839:Microelectrodes; D010006:Osteoblasts / Q000502:physiology*; D010281:Parathyroid Hormone / Q000494:pharmacology / Q000502:physiology*; D010446:Peptide Fragments / Q000494:pharmacology; D051381:Rats; D015290:Second Messenger Systems
D007295:Inositol Phosphates; D010281:Parathyroid Hormone; D010446:Peptide Fragments; C025076:parathyroid hormone (1-34), bovine; D005576:Colforsin; D000001:Calcimycin; D003035:Cobalt; D004533:Egtazic Acid; D003994:Bucladesine; D015544:Inositol 1,4,5-Trisphosphate; D000242:Cyclic AMP; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine