Ca²⁺-induced Ca²⁺ release involved in positive inotropic effect mediated by CGRP in ventricular myocytes

To investigate the effects and mechanisms of calcitonin gene-related peptide (CGRP) on ventricular contractility, ventricular myocytes isolated from adult rat and mouse hearts were exposed to CGRP. Myocyte contractility was assessed by a video edge motion detector, and the intracellular [Ca²⁺] transients were measured by a spectroflurophotometer in fura 2-loaded myocytes. CGRP exerted a potent concentration-dependent (10 pM-10 nM, EC₅₀ = 44.1 pM) positive inotropism on rat ventricular myocytes. CGRP (1 nM) increased cell shortening during contraction by 140 ± 40% above baselines and increased maximum velocity of contraction and relaxation by 98 and 106%, respectively. CGRP failed to produce any response in the presence of the CGRP₁ receptor antagonist. CGRP induced similar inotropic response in mouse ventricular myocytes. CGRP increased the amplitude of [Ca²⁺] transients of ventricular myocytes by 120 ± 25% above baseline and shortened the time of half-maximum myoplasmic Ca²⁺ clearance by 30 ± 5%. Increase in intracellular Ca²⁺ mobilization by CGRP was dependent on Ca²⁺ influx through the activation of the L-type Ca²⁺ channel, because nifedipine blocked the CGRP-induced increase in [Ca²⁺] transients. Furthermore, CGRP failed to increase [Ca²⁺] transients after the inhibition of protein kinase A in ventricular myocytes. These data indicate that stimulation of mammalian ventricular myocardial CGRP₁ receptors enhances [Ca²⁺] transients through the activation of protein kinase A, which in turn activates voltage-dependent L-type Ca²⁺ channels. These events lead to Ca²⁺-induced intracellular Ca²⁺ release and enhanced myocyte contraction and facilitated relaxation.