Whether the
growth hormone (GH) /
Insulin-like
growth factor 1(IGF-I) axis exerts cardioprotective effects remains controversial; and the underlying mechanism(s) for such actions are unclear. Here we tested the
hypothesis that growth-
hormone releasing
hormone (GHRH) directly activates cellular reparative mechanisms within the injured heart, in a GH / IGF-I independent fashion. Following experimental
myocardial infarction (MI), rats were randomly assigned to receive, during a 4 week period, either
placebo (n=14), rat
recombinant GH (rrGH, n=8) or JI-38 (n=8; 50 μg / Kg / day), a potent GHRH-
agonist. JI-38 did not elevate serum levels of GH or IGF-I, but markedly attenuated the degree of cardiac functional decline and remodeling after injury. In contrast, GH administration markedly elevated
body weight, heart weight, circulating GH and IGF-I, but did not offset the decline in
cardiac structure and function. Whereas, both JI-38 and GH augmented levels of cardiac
precursor cell proliferation, only JI-38 increased anti-
apoptotic gene expression. The
receptor for GHRH was detectable on myocytes supporting direct activation of cardiac
signal transduction. Collectively, these findings demonstrate that within the heart GHRH-agonists can activate
cardiac repair following MI, suggesting the existence of a potential signaling pathway based on GHRH in the heart. The phenotypic profile of the response to a potent GHRH
agonist has therapeutic implications.