Growth hormone-releasing peptides (GHRPs) are known to release growth
hormone (GH) in vivo and in vitro by a direct action on receptors in a
nterior pituitary cells. Measurement of second messengers released fol
lowing somatotroph stimulation suggests the existence of more than one
GHRP receptor subtype in the hypothalamic-pituitary system. Furthermo
re, hexarelin, a hexapeptide of the GHRP family and a potent GH secret
agogue, is reported to increase left ventricular ejection fraction, su
ggesting the expression of specific myocardial GHRP binding sites. In
order to confirm such a hypothesis, a photoactivatable derivative of h
exarelin, Tyr-p-benzoyl phenylalanine-Ala-hexarelin, was developed. A
putative GHRP receptor with an apparent relative molecular mass of 57
000 was specifically labelled and characterized in human, bovine and p
orcine anterior pituitary membranes using this hexarelin derivative. T
he existence of myocardial binding sites was also demonstrated using t
he same approach. The differential binding affinity of GHRP analogues
to cardiac tissue raises the possibility of the existence of distinct
GHRP receptor subtypes in the pituitary and the cardiovascular system,
for which physiological roles have yet to be determined.