CHRONIC OXYTOCIN PRETREATMENT INHIBITS ADENYLYL-CYCLASE ACTIVITY IN CULTURED RAT MYOMETRIAL CELLS

To determine whether chronic oxytocin pretreatment inhibits adenylyl c yclase, we compared adenylyl cyclase activity in membranes prepared fr om cultured, immortalized rat myometrial cells that were untreated or pretreated for 24 h with oxytocin. Chronic oxytocin pretreatment (1 x 10(-5) M for 24 h) attenuated basal, guanosine triphosphate (1 x 10(-5 ) M)-, isoproterenol (1 x 10(-4) M)-, forskolin (1 x 10(-5) M)-, MnCl2 (20 mM)- or NaF (1 x 10(-2) M)-stimulated adenylyl cyclase activity b y 27 +/- 5% to 39 +/- 11% (n = 6, p < 0.05). Oxytocin pretreatment for 2 h (n = 5) did not produce a significant effect. To understand the m echanism by which oxytocin pretreatment decreased activity of the aden ylyl cyclase pathway, we compared effects of pretreatment with either oxytocin or phenylephrine on adenylyl cyclase activity and determined the effects of Ci inhibition and protein kinase C (PKC) depletion. Chr onic (24 h) phenylephrine pretreatment (1 x 10(-4) M) had effects simi lar to those of oxytocin pretreatment (1 x 10(-5) M). PKC depletion wi th phorbol 12-myristate 13-acetate (1 x 10(-6) M, 41 h) prevented atte nuation of adenylyl cyclase activity by oxytocin pretreatment (1 x 10( -5) M for 24 h). Inhibition of G(i) by pertussis toxin pretreatment (1 .25 mu g/ml, 41 h) had no significant effect. These findings suggest t hat chronic oxytocin pretreatment desensitizes the adenylyl cyclase pa thway by a cross-regulatory mechanism that involves activation of G(q) and PKC.