Stimulatory effect of phenylephrine on the secretion of β-endorphin from rat adrenal medulla in vitro

In an attempt to investigate the role of α1-adrenoceptors in the regulation of opioid secretion from adrenal gland, phenylephrine was employed to investigate the effect on secretion of β-endorphin-like immunoreactivity (BER) from adrenal medulla of rat in vitro. Phenylephrine enhanced the BER from isolated adrenal medulla in a concentration-dependent manner and this action was abolished by the antagonists of α1-adrenoceptors, prazosin and tamsulosin. Investigations of signal pathway further support that an activation of α1-adrenoceptors is responsible for the stimulatory effect of phenylephrine on BER secretion from adrenal medulla. In the presence of U73312, the specific inhibitor of phospholipase C (PLC), phenylephrine-induced change of BER was reduced in a concentration-dependent manner but it was not affected by U73343, the negative control of U73312. Moreover, chelerythrine and GF 109203X diminished the action of phenylephrine at concentration sufficient to inhibit protein kinase C (PKC). In conclusion, our results suggest that an activation of α1-adrenoceptors in adrenal medulla by phenylephrine may enhance the secretion of opioids from adrenal gland of rat via signals of PLC–PKC pathway.