GROWTH-HORMONE (GH)-RELEASING PEPTIDE AND GH RELEASING HORMONE STIMULATE GH RELEASE FROM SUBPOPULATIONS OF SOMATOTROPHS IN RATS

The synthetic hexapeptide GH-releasing peptide (His-D-Trp-Ala-Trp-D-Ph e-Lys-NH2; GHRP-6) and GH releasing hormone (GHRH) are both potent sti mulators of GH release in rats. Using reverse hemolytic plaque assay ( RHPA), we have compared the effects of human GHRH and GHRP-6 on GH rel ease from the dispersed individual cells of rat anterior pituitary. In a single RHPA, we quantified the percentage of plaque forming cells ( % PFC) and their mean plaque area (MPA) after 30 min-incubation, and c alculated a total secretion index (TSI) by multiplying % PFC and MPA. 10 nM GHRH and 100 nM GHRP-6 each caused a significant increase in % P FC (%) (GHRH 39.15, GHRP-6 29.4, vs vehicle 24.3, P<0.01), MPA(x10(-2) mu m(2)) (GHRH 124.04, GHRP-6 94.80, vs Vehicle 44.57, P<0.01) and TS I (x 10(-2)) (GHRH 54.46, GHRP-6 32.87, VS vehicle 10.84, P<0.01). Sim ultaneous addition of both secretagogues caused a further increase in GH release (% PFC 46.4, MPA 142.55, TSI 69.82, P<0.01 vs vehicle), alt hough the effect was additive but not synergistic. Somatostatin analog , SMS201-995 (SMS) partially suppressed all parameters in GH secretion after stimulation by GHRH and/or GHRP-6. A double RHPA was then perfo rmed to test whether all somatotrophs respond equally to GHRH and GHRP -6 or some cells formed plaques only by either GHRH or GHRP-6. There w ere somatotrophs responsive to only GHRH (23.3% vs control 6.2%, P<0.0 1), those responsive to only GHRP-6 (11.9% vs control 6.1%, P<0.01), a nd those responsive to both GHRH and GHRP-6 (7.8% vs control 0.2%, P<0 .01). These results confirmed the previous findings that GHRP-6 and GH RH directly but independently stimulate GH release from the pituitary cells, and further suggest the presence of at least three functionally distinct somatotroph subpopulations concerning the responsiveness to GHRP-6 and GHRH in rats.