Evidence for independent secretion of beta-endorphin immunoreactivity from rat pars distalis in vivo

The distinctive chromatographic patterns of beta-endorphin-like immunoreactivity (beta END-LI) released from rat pars distalis (PD) or pars intermedia in vitro as well as the selective inhibitory effects of dexamethasone on PD were used to determine which lobe secretes beta END-LI into plasma after clonidine administration in vivo. Gel chromatography (Sephadex G-50) indicates that cultured PD cells released two major immunoreactive species which coelute with beta-lipotropin (beta LPH) or beta END standards. Conversely, virtually all of the beta END-LI secreted by neurointermediate lobe [pars intermedia plus pars nervosa (NIL)] cells in vitro resembled beta END in size, while none was detected which cochromatographed with beta LPH. Incubation of PD cells with clonidine (10(-6) M) evoked a 2-fold increase in beta END-LI release, while the drug had no effect on beta END-LI released from cultured NIL cells. Dexamethasone (10(-7) M) inhibited the clinidine-induced release of beta END-LI from PD cells, whereas it did not influence the stimulated release of beta END-LI from NIL by isoproterenol (10(-6) M). In vivo administration of clonidine (0.5 mg/kg, ip, for 15 min) increased total plasma beta END-LI from 0.75 +/- 0.16 to 1.35 +/- 0.18 ng/ml; 85% of this rise corresponded to beta LPH as determined by gel chromatography. Prior administration of dexamethasone (60 micrograms/kg, ip, for 4 h) completely prevented the clonidine-induced release of beta END-LI in vivo. These results demonstrate that clonidine probably acts selectively on the PD in vivo to release pituitary beta END-LI, and further, that PD can release beta END-LI independently of the pars intermedia.