RhoA/ROCK Signaling and Pleiotropic α(1A)-Adrenergic Receptor Regulation of Cardiac Contractility

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