Chronic receptor-mediated activation of Gi/o proteins alters basal t-tubular and sarcolemmal L-type Ca2+ channel activity through phosphatases in heart failure

Kashihara T, Nakada T, Shimojo H, Horiuchi-Hirose M, Gomi S, Shibazaki T, Sheng X, Hirose M, Hongo M, Yamada M. Chronic receptor-mediated activation of G(i/o) proteins alters basal t-tubular and sarcolemmal L-type Ca2+ channel activity through phosphatases in heart failure. Am J Physiol Heart Circ Physiol 302: H1645-H1654, 2012. First published February 3, 2012; doi: 10.1152/ajpheart.00589.2011.-L-type Ca2+ channels (LTCCs) play an essential role in the excitation-contraction coupling of ventricular myocytes. We previously found that t-tubular (TT) LTCC current density was halved by the activation of protein phosphatase (PP) 1 and/or PP2A, whereas surface sarcolemmal (SS) LTCC current density was increased by the inhibition of PP1 and/or PP2A activity in failing ventricular myocytes of mice chronically treated with isoproterenol (ISO mice). In the present study, we examined the possible involvement of inhibitory heterotrimeric G proteins (G(i/o)) in these abnormalities by chronically administrating pertussis toxin (PTX) to ISO mice (ISO + PTX mice). Compared with ISO mice, ISO + PTX mice exhibited significantly higher fractional shortening of the left ventricle. The expression level of G alpha(i2) proteins was not altered by the treatment of mice with ISO and/or PTX. ISO + PTX myocytes had normal TT and SS LTCC current densities because they had higher and lower availability and/or open probability of TT and SS LTCCs than ISO myocytes, respectively. A selective PKA inhibitor, H-89, did not affect LTCC current densities in ISO + PTX myocytes. A selective PP2A inhibitor, fostriecin, did not affect SS or TT current density in control or ISO + PTX myocytes but significantly increased TT but not SS LTCC current density in ISO myocytes. These results indicate that chronic receptor-mediated activation of G(i/o) in vivo decreases basal TT LTCC activity by activating PP2A and increases basal SS LTCC activity by inhibiting PP1 without modulating PKA in heart failure.