No Desensitization of the Growth Hormone (GH) Response between GH-Releasing Peptide-2 and GH-Releasing Factor in Calves

We investigated the desensitization caused by growth hormone (GH)-releasing peptide-2 (D-Ala-D-βNal-Ala-Trp-D-Phe-Lys-NH₂; GHRP-2) in vivo using calves. The GHRP-2 (12.5μg·kgBW^-1·h^-1) or GH-releasing factor (GRF; .125 μg·kgBW^-1·h^-1) were infused for 180 min into four calves, each followed by either GHRP-2 (12.5 μg/kg BW) or GRF (.125 μg/kg BW) injection after 60 min. All treatments were given to all of the calves. The continuous infusion of GHRP-2 did not attenuate the GH secretion caused by a GRF bolus, and vice versa. Continuous exposure to GHRP-2 attenuated the GH secretion caused by a GHRP-2 bolus. In an additional experiment, two repetitive injections of GHRP-2 (12.5 μg/kg BW) at 1-, 2-, 3-, or 4-h intervals were administered. Attenuation of the GH response caused by GHRP-2 was maintained until 4 h. These results suggest that GHRP-2 is not cross-desensitized with GRF and that the mechanisms by which they cause GH release may be different in calves.