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.
|Journal||American Journal of Physiology - Endocrinology and Metabolism|
|Issue number||6 27-6|
|State||Published - 1993|
- adenosine 3',5'- cyclic monophosphate
- intracellular calcium
- protein kinase C
- proximal tubule cells