Modulation of ion channel activity is a fundamental mechanism for heart function including cardiac pacemaker activity. Pathways regulating the phosphorylation of ion channel in cardiac cells are relatively well understood; however, the counterbalance mechanism by dephosphorylation of these proteins remains unclear. Here we report a novel role of P21 activated kinase-1 (Pak1)-mediated signaling in blunting isoproterenol (ISO)-induced enhancement of L-type Ca2+ current (ICaL) and delayed rectifier potassium current (IK) in sino-atrial node (SAN) pacemaker cells. (Ke et al., 2007) We demonstrated that there is abundant expression of endogenous Pak1 in cardiac pacemaker cells. Over-expressing Pak1 in these cells by adenovirus expressing constitutively active Pak1 (Ad-Pak1) blunts β-adrenoceptor agonist ISO-induced up-regulation of ICaL and IK. ISO (100 nM) increased the amplitudes of ICaL by ~57 % and IK by ~410 % in Ad-LacZ infected cells, whereas the corresponding increases were only ~13 % and ~70 % in the Ad-Pak1 infected cells. Moreover, the IK decay time constant was reduced (by 39 ± 8 %, Fig. 4G-I). However, the effect of ISO on the rate of IK decay in the Ad-Pak1 group was much less than that of the control group, and indeed not statistically significant (17 ± 9 %). Such a difference in the response to ISO between the two groups indicates that the effect of ISO on ICaL and IK was significantly attenuated in cells infected with Ad-Pak1. This effect of active Pak1 mainly presented when ICaL and IK activity was enhanced by β-stimulation. Such effect can be reversed by inhibition of PP2A. Furthermore, L-type Ca2+ channels (alpha 1C) associate with Pak1 in SAN tissue and PP2A co-immunoprecipitates with endogenous Pak1 in sinoatrial node (SAN) tissue and expression of constitutively active Pak1 blunts the ISO-induced chronotropic action on pacemaker activity of intact SAN preparations. Thus, our data demonstrate that a Pak1 signaling pathway exists in cardiac pacemaker cells and this pathway may play an important role in the regulation of ion channel activity in the heart.
University of Manchester (2007) Proc Physiol Soc 8, SA20
Research Symposium: A novel role of P21 activated kinase-1 (Pak1)-mediated signaling in regulation of cardiac pacemaker channels
M. Lei1, Y. Ke2, T. P. Collins3, S. Rakovic3, P. A. Mattick3, M. Yamasaki3, M. S. Brodie2, D. A. Terrar3, J. R. Solaro2
1. School of Medicine, University of Manchester, Manchester, United Kingdom. 2. Department of Physiology and Biophysics and Center for Cardiovascular Research, University of Illinois at Chicago, Chicago, IL, USA. 3. Department of Pharmacology, University of Oxford , Oxford, United Kingdom.
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