The study shows that the peptide GHRP‑6 makes the pituitary release growth hormone quickly, but the effect fades fast and the cells need about an hour to become fully responsive again. GHRP‑6 works through a different receptor than the usual growth‑hormone‑releasing hormone (GHRH), and using both together can give a bigger hormone surge that isn’t completely blocked by somatostatin.

The hexapeptide His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6) and GH-releasing factor (GHRH) produced a rapid release of GH upon perifusion of dispersed rat pituitary cells. In contrast to the native hormone GHRH, GHRP-6 elicited a response of short duration. When perifusion of each secretagogue was continued until the cells no longer released GH, a challenge by the alternative secretagogue immediately resulted in a secondary release of GH. These results are consistent with each secretagogue causing desensitization of discrete receptor-linked second messenger pathways. Cells which were perifused for 1 min with GHRP-6 required continued perifusion with culture medium alone for 60 min before they completely regained responsiveness to a subsequent challenge with GHRP-6. Somatostatin (SRIF) was able to inhibit the action of either secretagogue completely. However, when both GHRH and GHRP-6 were perifused together, SRIF attenuated but did not block GH secretion. These perifusion data add support to conclusions derived from static cell culture studies, that GHRH and GHRP-6 act through different receptor sites and that through discrete signalling pathways their individual effects on GH release are amplified.