RhoA/ROCK Signaling and Pleiotropic α1A-Adrenergic Receptor Regulation of Cardiac Contractility

Aims To determine the mechanisms by which the α1A-adrenergic receptor (AR) regulates cardiac contractility. Background We reported previously that transgenic mice with cardiac-restricted α1A-AR overexpression (α1A-TG) exhibit enhanced contractility but not hypertrophy, despite evidence implicating this Gαq/11-coupled receptor in hypertrophy. Methods Contractility, calcium (Ca2+) kinetics and sensitivity, and contractile proteins were examined in cardiomyocytes, isolated hearts and skinned fibers from α1A-TG mice (170-fold overexpression) and their non-TG littermates (NTL) before and after α1A-AR agonist stimulation and blockade, angiotensin II (AngII), and Rho kinase (ROCK) inhibition. Results Hypercontractility without hypertrophy with α1A-AR overexpression is shown to result from increased intracellular Ca2+ release in response to agonist, augmenting the systolic amplitude of the intracellular Ca2+ concentration [Ca2+]i transient without changing resting [Ca2+]i. In the absence of agonist, however, α1A-AR overexpression reduced contractility despite unchanged [Ca2+]i. This hypocontractility is not due to heterologous desensitization: the contractile response to AngII, acting via its Gαq/11-coupled receptor, was unaltered. Rather, the hypocontractility is a pleiotropic signaling effect of the α1A-AR in the absence of agonist, inhibiting RhoA/ROCK activity, resulting in hypophosphorylation of both myosin phosphatase targeting subunit 1 (MYPT1) and cardiac myosin light chain 2 (cMLC2), reducing the Ca2+ sensitivity of the contractile machinery: all these effects were rapidly reversed by selective α1A-AR blockade. Critically, ROCK inhibition in normal hearts of NTLs without α1A-AR overexpression caused hypophosphorylation of both MYPT1 and cMLC2, and rapidly reduced basal contractility. Conclusions We report for the first time pleiotropic α1A-AR signaling and the physiological role of RhoA/ROCK signaling in maintaining contractility in the normal heart.