Effect of forskolin on the spontaneous maturation and cyclic AMP content of rat oocyte--cumulus complexes

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Abstract
Forskolin (0-100 .mu.M), a reversible stimulator of the catalytic subunit of adenylate cyclase, induced dose-response increases in the % germinal vesicle (GV) (ID50 = 2.68 .mu.M, where ID50 is the forskolin dose which maintained meiotic arrest at the germinal vesicle stage, determined cytogenetically, of 50% cultured oocytes), and the cAMP content (determined by RIA [radioimmunoassay]) of cumulus-enclosed oocytes and cumulus masses. A significant positive correlation was established between the amount of cAMP within the cumulus mass and that in the corresponding enclosed oocyte (r = 0.78). In contrast, neither the % GV nor the cAMP content of cumulus-free oocytes was affected by the drug. The arresting action of forskolin upon cumulus-enclosed oocytes was dependent upon the presence of adherent cumulus cells, and was transient and fully reversible. The gradual decrease in the % GV of cumulus-enclosed oocytes cultured from 4 to 12 h in 25 .mu.M-forskolin (84, 54 and 22% GV at 4, 8 and 12 h, respectively) was accompanied by a drastic fall in intra-oocyte cAMP at 8 h (41.2 .+-. 2.5 and 1.1 .+-. 0.6 fmol/oocyte at 4 and 8 h, respectively), while the cumulus cell cAMP content remained constant (135.3 .+-. 14.7 and 145.9 .+-. 28.7 fmol/cumulus at 4 and 8 h, respectively). Moreover, heterologous metabolic coupling, assessed by determination of the fraction of radiolabelled uridine marker that was transferred from the cumulus mass to the oocyte, significantly decreased. These results show that cumulus cell cAMP is transferred to the rat oocyte where it appears to play a pivotal role in the regulation of meiosis and that the rat oolemma does not appear to possess active catalytic subunits of adenylate cyclase in an amount adequate to stimulate measurably cAMP synthesis.
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