ACUTE DESENSITIZATION OF PHOSPHOLIPASE C-COUPLED MUSCARINIC M3 RECEPTORS BUT NOT GONADOTROPIN-RELEASING-HORMONE RECEPTORS COEXPRESSED IN ALPHA-T3-1 CELLS - IMPLICATIONS FOR MECHANISMS OF RAPID DESENSITIZATION

In the present study we have expressed the muscarinic M3 receptor in a n immortalized mouse pituitary cell line (alpha T3-1), which expresses an endogenous gonadotropin-releasing hormone (GnRH) receptor, to exam ine potential differences in acute receptor regulation. Both of these receptors couple to the activation of phosphoinositide-specific phosph olipase C (PLC) in these cells and we demonstrate that, despite expres sion in the same cell background, acute desensitization is a feature o f muscarinic M3 receptors but not of GnRH receptors. We show that, whe n the concentrations of GnRH and methacholine are matched to give appr oximately equivalent maximal elevations of Ins(1,4,5)P-3, the GnRH rec eptor is able to sustain PLC activity at the initial rate, whereas the muscarinic M3 receptor cannot. Thus PLC-activating G-protein-coupled receptors are able to undergo rapid desensitization in this cell line, indicating that the desensitization profile is receptor-specific rath er than cell-specific. This argues strongly that post-receptor regulat ory features do not have a prominent role in mediating rapid desensiti zation in these cells. Furthermore GnRH receptor-mediated PLC activity is sustained despite a marked and persistent depletion in the steady- state level of PtdIns(4,5)P-2. In contrast, activation of muscarinic r eceptors is not sustained despite only a transient decrease in PtdIns( 4,5)P-2 concentration. Thus, whereas the contribution of PtdIns(4,5)P- 2 depletion to the temporal profile of receptor-mediated PLC signallin g has been difficult to assess, the present results demonstrate that t his is unlikely to be of importance in these cells. We suggest that un ique structural features of the GnRH receptor result in a lack of appr opriate regulatory phospho-acceptor sites and that the absence of agon ist-dependent phosphorylation might underlie the lack of acute regulat ion.