BETA-ADRENOCEPTORS MEDIATE THE CATECHOLAMINE-INDUCED STIMULATION OF OXYTOCIN SECRETION FROM CULTURED BOVINE GRANULOSA-CELLS

Bovine granulosa cells were treated in culture with alpha- and beta-adrenoceptor ligands to determine the receptor subtype mediating their response to catecholamines. The secretion of oxytocin by granulosa cells in serum-free medium was measured on the fourth day of culture (during the period of acquisition of a luteal phenotype). Cultures were performed in the presence of 0.5 mm ascorbic acid, which increased hormone output and potentiated the response to catecholamines. The effects of adrenaline and noradrenaline on oxytocin secretion were concentration-dependent; maximum stimulation was over 700% with adrenalin (EC50 92 nm) and 500% with noradrenalin (EC50 87 nm). The response to noradrenaline (10(-6) m) and adrenaline (10(-6) M) could be blocked by propranolol but not by phentolamine, suggesting that beta- rather than alpha-adrenoceptors were involved. Blockade by metoprolol and practolol (beta-1-adrenoceptor antagonists) was poor and dobutamine (beta-1-agonist) was weakly stimulatory. A concentration-dependent stimulatory response (EC50 200 nm) was obtained with salbutamol (beta-2-adrenoceptor agonist) and stimulation by adrenaline or salbutamol could be blocked by a selective beta-2-adrenoceptor antagonist (ICI 118,551). It is concluded that, during luteinization, the long-term response of bovine granulosa cells to stimulation induced by catecholamines is mediated through beta- rather than alpha-adrenoceptors. Although the beta-2-subtype is probably involved, the similar potencies of adrenaline and noradrenaline are uncharacteristic Of beta-2-adrenoceptors and may be peculiar to the long-term response shown by these cells.