This is an interesting study done in cattle. Apparently cattle get one 2 hour feeding a day.
So one hour before the meal GHRH by itself or GHRP-6 by itself worked better then when administered by itself one hour post-meal. That is what we would expect.
In addition one hour pre-meal the GHRH + GHRP-6 produced a larger pulse of GH together. This we would also expect.
What is surprising and interesting is that taking GHRP-6 + GHRH together one hour post-meal produced a pulse of GH basically equivalent to the pre-meal pulse. In other words the synergy of the two peptides over came the meal refractory effect (where either one administered alone was unable).
Perhaps a similar effect takes place in humans... i.e. even when the stomach is full (1 hour post-meal) GHRH+GHRP-6 creates an undiminished GH pulse.
GH-releasing peptide-6 overcomes refractoriness of somatotropes to GHRH after feeding, C D McMahon, Journal of Endocrinology (2001) 170, 235–241
After a meal, somatotropes are temporarily refractory to growth hormone-releasing hormone (GHRH), the principal hormone that stimulates secretion of growth hormone (GH). Refractoriness is particularly evident when free access to feed is restricted to a 2-h period each day. GH-releasing peptide-6 (GHRP-6), a synthetic peptide, also stimulates secretion of GH from somatotropes. Because GHRH and GHRP-6 act via different receptors, we hypothesized that GHRP-6 would increase GHRH-induced secretion of GH after feeding. Initially, we determined that intravenous injection of GHRP-6 at 1, 3 and 10 ug/kg body weight (BW) stimulated secretion of GH in a dose-dependent manner.
Next, we determined that GHRP-6- and GHRH-induced secretion of GH was lower 1 h after feeding (22.5 and 20 ng/ml respectively) than 1 h before feeding (53.5 and 64.5 ng/ml respectively; pooled (S.E.M.=8.5).
However, a combination of GHRP-6 at 3 ug/kg BW and GHRH at 0.2 ug/kg BW synergistically induced an equal and massive release of GH before and after feeding that was fivefold greater than GHRH induced release of GH after feeding.
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