Abstract

Using a high-throughput functional screen, the atypical L-type Ca²⁺ channel blocker diltiazem was discovered to be an agonist at the human ghrelin (GHSR1a) receptor. In cellular proliferation, Ca²⁺ mobilization, and bioluminescence resonance energy transfer (BRET-2) assays, diltiazem was a partial agonist at GHSR1a receptors, with 50 to 80% relative efficacy compared with the GHSR1a peptide agonist GHRP-6, and high nanomolar to low micromolar potency, depending upon the assay. Seven of the known primary metabolites of diltiazem were synthesized, and three of them (M_A, M₁, and M₂) were more efficacious and/or more potent than diltiazem at GHSR1a receptors, with a rank order of agonist activity of M₂ > M₁ > M_A > diltiazem, whereas M₄ and M₆ metabolites displayed weak agonist activity, and the M₈ and M₉ metabolites were inactive. Binding affinities of diltiazem and these metabolites to GHSR1a receptors followed a similar rank order. In vivo tests showed that diltiazem and M₂ each stimulated growth hormone release in male Sprague-Dawley neonatal rats, although to a lesser degree than GHRP-6. Thus, diltiazem and chemical analogs of diltiazem represent a new class of GHSR1a receptor agonists. The possible contributions of GHSR1a receptor activation to the clinical actions of diltiazem are discussed in the context of the known beneficial cardiovascular effects of ghrelin.