Clonidine releases immunoreactive beta-endorphin from rat para distalis.

Clonidine (10(-6), 10(-7) M) evokes the release of beta endorphin-like immunoreactivity (beta-END-LI) from cell cultures of anterior (pars distalis) but not neurointermediate (pars nervosa plus pars intermedia) lobe of the rat pituitary. This drug-induced secretion is blocked by alpha-adrenergic (phenoxybenzamine, yohimbine; 10(-5) M) but not beta-adrenergic (propranolol, 10(-5) M) antagonism. Gel filtration (Sephadex G-50) reveals that beta-END-LI released from anterior lobe cells consists of 2 major forms of immunoreactivity which coelute with beta-lipotropin or beta-endorphin standards. Conversely, beta-END-LI released spontaneously from neurointermediate lobe cells almost entirely corresponds to beta-endorphin. The data show that alpha-adrenergic stimulation by clonidine releases beta-END-LI selectively from cells of anterior but not neurointermediate lobe in vitro and suggests that the clonidine-induced release of pituitary beta-END-LI we have observed in vivo occurs in part by direct action on the corticotrophs of the pars distalis.