DIRECT EFFECTS OF GROWTH-HORMONE (GH)-RELEASING HEXAPEPTIDE (GHRP-6) AND GH-RELEASING FACTOR (GRF) ON GH SECRETION FROM CULTURED PORCINE SOMATOTROPES

Growth hormone (GH)-releasing hexapeptide (GHRP-6) belongs to the expa nding family of synthetic GH secretagogues (GHSs). Previous studies ha ve shown that non-peptidyl GHRP-6 analogues stimulate GH release in vi vo in pigs, and interact synergistically with GH-releasing factor (GRF ), but its direct effects on porcine somatotropes have not been addres sed hitherto. In the present study, we have evaluated the response of cultured porcine pituitary cells to GHRP-6, and its interaction with G RF and somatostatin (SRIF). Secretory response of somatotropes was ass essed by using two distinct techniques. GH released by monolayer cell cultures was evaluated by enzyme immunoassay, whereas that secreted by individual somatotropes was measured by immunodensitometry using a ce ll blotting assay. Our results demonstrate that both GHRP-6 and GRF st imulated GH release from monolayer cultures at doses equal to or above 10(-9) M. Use of cell immunoblot assay demonstrated that, like GRF, t he hexapeptide acts directly upon porcine somatotropes to exert its ac tion. Moreover, regardless of the technique applied, combined administ ration of GHRP-6 (10(-6) or 10(-9) M) and GRF (10(-8) M) resulted in a n additive, but not synergistic, stimulatory GH response. Finally, SRI F (10(-7) M) inhibited the stimulatory effect of GHRP-6 alone or in co mbination with GRF. These results indicate that GHRP-6 directly and ef fectively stimulates GH secretion from porcine somatotropes in vitro, and acts additively when coadministered with GRF. Therefore, the syner gistic stimulatory effect of GHSs and GRF reported in vivo in this spe cies might require additional factors that are lacking in the ill vitr o situation.