Desensitization of calcium messenger system in parathyroid hormone- stimulated opossum kidney cells

We have studied the desensitization of the calcium message system to parathyroid hormone (PTH) by monitoring intracellular calcium concentration ([Ca²⁺](i)) in an opossum kidney cell line (OKP). PTH (10^-7 M) caused a transient increase in [Ca²⁺](i), with an average peak height of 48.7 ± 4.7% above baseline (n = 32). Cells stimulated with either 10^-7 or 10^-8 M PTH did not respond to a second challenge with a maximal dose (10^-7 M) of the hormone, whereas lower concentrations of PTH (10^-9 M and 10^-10 M) only partially desensitized the cells, since a [Ca²⁺](i) transient of smaller amplitude (12.7 ± 2.1 and 40.6 ± 6.2% above baseline, respectively) was observed with a second stimulation. Desensitization developed within 5 min of initial hormone exposure, when PTH receptor binding was not significantly decreased. Maximal reduction of PTH binding sites (37.0 ± 1.4%) was achieved only after 2 h. Partial desensitization was reproduced by 10^-9 M phorbol 12-myristate 13-acetate (PMA) but not by dibutyryladenosine 3',5'-cyclic monophosphate, and it was blocked by staurosporine. However, staurosporine had no effect on the complete desensitization induced by high doses of PTH. At 10^-9 M, PTH also caused a time-dependent desensitization of the adenosine 3',5'-cyclic monophosphate (cAMP) response, with maximal inhibition achieved after 2 h. PMA also decreased the cAMP response to PTH, but its inhibitory effect was less potent than that of 10^-9 M PTH. Therefore PTH induces a dose-dependent homologous desensitization of the Ca²⁺ message system in OKP cells, independent of receptor occupancy. Although protein kinase C plays a role in the loss of both cAMP and [Ca²⁺](i) responses, other postreceptor mechanisms are involved in the development of the absolute [Ca²⁺](i) refractoriness induced by high doses of PTH.