Possible involvement of A(1) receptors in the inhibition of gonadotropin secretion induced by adenosine in rat hemipituitaries in vitro

We investigated the participation of A(1) or A(2) receptors in the gonadotr ope and their role in the regulation of LH and FSH secretion in adult rat h emipituitary preparations, using adenosine analogues. A dose-dependent inhi bition of LH and FSH secretion was observed after the administration of gra ded doses of the R-isomer of phenylisopropyladenosine (R-PIA; 1 nM, 10 nM, 100 nM, 1 mu M and 10 mu M). The effect of R-PIA (10 nM) was blocked by the addition of 8-cyclopentyltheophylline (CPT), a selective A(1) adenosine re ceptor antagonist, at the dose of 1 mu M. The addition of an A(2) receptor- specific agonist, 5-N-methylcarboxamidoadenosine (MECA), at the doses of 1 nM to 1 mu M had no significant effect on LH or FSK secretion, suggesting t he absence of this receptor subtype in the gonadotrope. However, a sharp in hibition of the basal secretion of these gonadotropins was observed after t he administration of 10 mu M MECA. This effect mimicked the inhibition indu ced by R-PIA, supporting the hypothesis of the presence of Al receptors in the gonadotrope. R-PIA (1 nM to 1 mu M) also inhibited the secretion of LK and FSH induced by phospholipase C (0.5 IU/ml) in a dose-dependent manner. These results suggest the presence of A(1) receptors and the absence of A(2 ) receptors in the gonadotrope. It is possible that the inhibition of LH an d FSH secretion resulting from the activation of A(1) receptors may have oc curred independently of the increase in membrane phosphoinositide synthesis .