Inotropic and lusitropic effects of ghrelin and their modulation by the endocardial endothelium, NO, prostaglandins, GHS-R1a and KCa channels

Contractile effects of ghrelin (10−9 to 10−6 M) were tested in rat papillary muscles of normal (n = 50) and hypertrophic (n = 16) right ventricles (RV). RV hypertrophy was induced by pulmonary hypertension using monocrotaline. In normal muscles, ghrelin was added either alone (n = 9) or after pre-treatment with indomethacin (cycloxygenase inhibitor, 10−5 M; n = 10), l-nitro-l-arginin (NO synthase inhibitor, 10−4 M; n = 9), d-Lys(3)-GHRP-6 (GHS-R1a antagonist; 10−4 M; n = 8) or apamin + charybdotoxin (KCa channels blockers; 10−6 M, n = 7), as well as after damaging the endocardial endothelium (n = 7). In hypertrophic muscles, ghrelin was added either alone (n = 9) or after pre-treatment with apamin + charybdotoxin (10−6 M, n = 7). Ghrelin concentration-dependently decreased active tension (AT) and maximal velocity of tension rise (negative inotropic effect), as well as, maximal velocity of tension decay (negative lusitropic effect) and time to AT (onset of relaxation). These effects were maximal at 10−6 M, similar in normal and hypertrophic muscles and were significantly altered only by apamin + charybdotoxin, indomethacin and l-nitro-l-arginin. Apamin + charybdotoxin attenuated the negative inotropic effect, while indomethacin and l-nitro-l-arginin, respectively, blunted and exacerbated the premature onset of relaxation. In conclusion, ghrelin induces negative inotropic and lusitropic effects and an earlier onset of relaxation in normal and hypertrophic myocardium, which are independent of GHS-R1a, since they were not affected by d-Lys(3)-GHRP-6. The negative inotropic effect is partly mediated by KCa channels, while the earlier onset of relaxation is modulated by prostaglandins and NO.