Abstract 387: Prolactin-Releasing Peptide, the Novel Endogenous Ligand of the Orphan Receptor GPR10, Regulates Cardiac Contractility
The hypothalamic prolactin-releasing peptide (PrRP), which is the endogenous ligand of the orphan receptor GPR10, plays an important role in the regulation of the central stress response, sleep regulation and feeding behavior. Although specific binding sites have been found in the rat myocardium, the functional importance of PrRP in the regulation of cardiovascular system is still unknown. The objective of the present study was to characterize the direct cardiac effects of PrRP as well as the underlying signaling mechanisms. In the isolated perfused, paced rat heart preparation (male 7-week-old, Sprague-Dawley rats, n=138), intracoronary infusion of PrRP-31 (1–30 nmol/L) for 15 min induced a dose-dependent, slowly developing positive inotropic effect, with a maximal increase in developed tension at the dose of 10 nmol/L (13.6%±3.1, P< 0.001). Next we tested if various counter-regulatory mechanisms could modify the effect of PrRP. The phosphodiesterase inhibitor IBMX (10 μmol/L) failed to alter the inotropic response to PrRP (10.7±1.9%, P=N.S.) excluding the involvement of cAMP-dependent mechanisms. In contrast, the specific protein kinase C-alfa (PKC-alpha) inhibitor Ro32– 0432 (100 nmol/L) significantly increased the inotropic response to PrRP (21.6±3.4%, P<0.05) as well as the phosphorylation of phospholamban at Ser-16 (1.5±0.09 vs. 1.0±0.18; P<0.001) as measured by Western blotting. Moreover, the protein phosphatase (PP1/PP2A) inhibitor calyculin A (CyA) augmented the effect of PrRP on contractility (20.7 ±1.7%, P<0.001) similarly to PKC-alpha inhibition, whereas the PP2A inhibitor okadaic acid (10 nmol/L) had no effect (7.8±1.0 %, P=N.S.). Furthermore, long-term (120 min) administration of PrRP resulted in a greater increase in contractility (20.3±3.1 %) as compared to short-term infusion (15 min; P<0.05). Our results provide the first evidence for the functional significance of PrRP in the heart. The positive inotropic effect of PrRP is mediated via cAMP-independent mechanisms, which are suppressed by enhanced activity of PKC-alpha and PP1. We suggest that PrRP may play an important role in the long-term regulation of myocardial contractility.