Expression of active p21-activated kinase-1 induces Ca flux modification with altered regulatory protein phosphorylation in cardiac myocytes

Sheehan KA, Ke Y, Wolska BM, Solaro RJ. Expression of active p21-activated kinase-1 induces Ca -flux modification with altered regulatory protein phosphorylation in cardiac myocytes. Am J Physiol Cell Physiol 296: C47–C58, 2009. First published October 15, 2008; doi:10.1152/ajpcell.00012.2008.—p21-Activated kinase-1 (Pak1) is a serine-threonine kinase that associates with and activates protein phosphatase 2A in adult ventricular myocytes and, thereby, induces increased Ca sensitivity of skinned-fiber tension development mediated by dephosphorylation of myofilament proteins (Ke Y, Wang L, Pyle WG, de Tombe PP, Solaro RJ. Circ Res 94: 194–200, 2004). We test the hypothesis that activation of Pak1 also moderates cardiac contractility through regulation of intracellular Ca fluxes. We found no difference in fieldstimulated intracellular Ca concentration ([Ca ]i) transient amplitude and extent of cell shortening between myocytes expressing constitutively active Pak1 (CA-Pak1) and controls expressing LacZ; however, time to peak shortening was significantly faster and rate of [Ca ]i decay and time of relengthening were slower. Neither caffeine-releasable sarcoplasmic reticulum (SR) Ca content nor fractional release was different in CA-Pak1 myocytes compared with controls. Isoproterenol application revealed a significantly blunted increase in [Ca ]i transient amplitude, as well as a slowed rate of [Ca ]i decay, increased SR Ca content, and increased cell shortening, in CA-Pak1 myocytes. We found no significant change in phospholamban phosphorylation at Ser or Thr in CA-Pak1 myocytes. Analysis of cardiac troponin I revealed a significant reduction in phosphorylated species that are primarily attributable to Ser in CA-Pak1 myocytes. Nonstimulated, spontaneous SR Ca release sparks were significantly smaller in amplitude in CA-Pak1 than LacZ myocytes. Propagation of spontaneous Ca waves resulting from SR Ca overload was significantly slower in CA-Pak1 myocytes. Our data indicate that CA-Pak1 expression has significant effects on ventricular myocyte contractility through altered myofilament Ca sensitivity and modification of the [Ca ]i transient.