LONG-TERM EFFECT OF FORSKOLIN ON THE ACTIVATION OF ADENYLYL-CYCLASE IN ASTROCYTES

Long-term (48-h) forskolin treatment of rat astroglial cells led to a slight decrease (30-40%) in the response to isoproterenol, vasoactive- intestinal peptide, guanyl 5'-(beta gamma-imido)diphosphate, guanosine 5'-O-(3-thiotriphosphate) [GTP(S)], and AIF(4)(-) in crude membrane f ractions, In contrast, the acute stimulatory effect of forskolin was i ncreased by 1.25-1.5-fold. These two opposite effects of forskolin wer e mediated by a cyclic AMP-dependent mechanism, No changes in G(s) alp ha, G(i) alpha, or G beta protein levels could be determined by immuno blotting using specific antisera. No significant differences were obse rved in the ability of G proteins extracted from control and forskolin -treated cells to reconstitute a full adenylyl cyclase activity in mem branes from S49 cyc(-) cells, lacking G(s) alpha protein. G(s) alpha p roteins were detected in two pools of membranes, one in the heavy sucr ose fractions and the other in light sucrose fractions. Forskolin trea tment of the cells shifted G(s) alpha protein toward the light-density membranes. We did not find any significant change in the distribution of adenylyl cyclase. In contrast to the decreased stimulation of aden ylyl cyclase activity by agonists acting via G(s) alpha, observed in t he crude membrane fraction, the responses of adenylyl cyclase to forsk olin as well as to GTP(S) were increased in the purified plasma membra ne fractions. These results may indicate that sensitization of the cat alyst appears to be the dominant component in the astroglial cell resp onse to long-term treatment by forskolin.