AGONIST-INDUCED DESENSITIZATION OF MUSCARINIC RECEPTOR-MEDIATED CALCIUM EFFLUX WITHOUT CONCOMITANT DESENSITIZATION OF PHOSPHOINOSITIDE HYDROLYSIS

  • 1 January 1985
    • journal article
    • research article
    • Vol. 27  (3) , 325-332
Abstract
Phosphoinositide hydrolysis does not appear to desensitize in 1321N1 astrocytoma cells. The evidence for this is that the rate of accumulation of [3H]inositol 1-phosphate is linear for up to 90 min in the presence of carbachol, pretreatment of cells with 100 .mu.M carbachol for 75 min does not diminish the subsequent ability of carbachol to increase [3H]inositol 1-phosphate accumulation and the production of all of the [3H]inositol phosphates including the polyphosphoinositide metabolites [3H]inositol bis- and trisphosphates continues for up to 75 min in the presence of carbachol and declines rapidly when the muscarinic receptor antagonist atropine is added. Only when cells are treated with carbachol for 2.5 h or longer is there a reduction in carbachol-stimulated phosphoinositide hydrolysis: this is associated with a decrease in muscarinic receptor number. There does appear to be desensitization of hormone-stimulated Ca2+ mobilization in 1321N1 cells, because treatment of these cells with carbachol for 75 min leads to loss of the subsequent ability of carbachol to stimulate unidirectional 45Ca2+ efflux. Histamine-stimulated 45Ca2+ efflux also is lost in cells pretreated with carbachol, indicating that the desensitization is heterologous. Apparently, desensitization of hormone-stimulated, unidirectional 45Ca2+ efflux cannot be accounted for by a loss of receptor-mediated phosphoinositide hydrolysis. If phosphoinositide hydrolysis or inositol trisphosphate formation are signals for Ca mobilization, the site at which the Ca response desensitizes must be distal to the initial receptor-mediated activation of phospholipase C.

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