Date Published: March 1, 2018
Publisher: American Physiological Society
Author(s): Simon M. Bryant, Cherrie H. T. Kong, Mark B. Cannell, Clive H. Orchard, Andrew F. James.
β2-Adrenoceptors and L-type Ca2+ current (ICa) redistribute from the t-tubules to the surface membrane of ventricular myocytes from failing hearts. The present study investigated the role of changes in caveolin-3 and PKA signaling, both of which have previously been implicated in this redistribution. ICa was recorded using the whole cell patch-clamp technique from ventricular myocytes isolated from the hearts of rats that had undergone either coronary artery ligation (CAL) or equivalent sham operation 18 wk earlier. ICa distribution between the surface and t-tubule membranes was determined using formamide-induced detubulation (DT). In sham myocytes, β2-adrenoceptor stimulation increased ICa in intact but not DT myocytes; however, forskolin (to increase cAMP directly) and H-89 (to inhibit PKA) increased and decreased, respectively, ICa at both the surface and t-tubule membranes. C3SD peptide (which decreases binding to caveolin-3) inhibited ICa in intact but not DT myocytes but had no effect in the presence of H-89. In contrast, in CAL myocytes, β2-adrenoceptor stimulation increased ICa in both intact and DT myocytes, but C3SD had no effect on ICa; forskolin and H-89 had similar effects as in sham myocytes. These data show the redistribution of β2-adrenoceptor activity and ICa in CAL myocytes and suggest constitutive stimulation of ICa by PKA in sham myocytes via concurrent caveolin-3-dependent (at the t-tubules) and caveolin-3-independent mechanisms, with the former being lost in CAL myocytes.
L-type Ca2+ current (ICa) plays a key role in excitation-contraction (EC) coupling in cardiac ventricular myocytes: activation of L-type Ca2+ channels (LTCCs) during the action potential causes influx of Ca2+ that triggers Ca2+ release via ryanodine receptors (RyRs) in the adjacent sarcoplasmic reticulum (SR) membrane (2, 8). Previous work has shown that the function of many of the key proteins involved in EC coupling, including LTCCs and RyRs, occurs predominantly at the t-tubules: invaginations of the surface membrane that enable near-synchronous SR Ca2+ release, and thus contraction, throughout the cell (18, 21, 28). The mechanism for the localization of ICa at the t-tubules is less clear, although it has been suggested that the caveolar protein caveolin-3 (Cav-3) plays a role in the localization of ICa, possibly via a mechanism involving cAMP/PKA signaling pathways (1, 5, 9, 24).
This study presents two novel findings regarding the regulation of ICa in heart failure. First, stimulation of ICa by β2-adrenoceptors, but not by adenylyl cyclase/PKA, is localized to the t-tubules in sham myocytes and redistributes to the cell surface after CAL. Second, it demonstrates constitutive stimulation of ICa by PKA in sham myocytes that is mediated both via Cav-3-dependent (at the t-tubules) and Cav-3-independent mechanisms, whereas in CAL myocytes, constitutive regulation by Cav-3 is lost, although that via PKA remains at both sites. Thus, the present study advances previous findings from our laboratory that Cav-3 plays a role in the regulation of ICa at the t-tubule by PKA and β2-adrenoceptors in normal myocytes (5, 9) and that ICa is redistributed from the t-tubules to the surface sarcolemma in CAL-induced heart failure (6). Interestingly, although constitutive PKA-dependent stimulation of ICa at the cell surface appeared to be the same in both sham and CAL myocytes, constitutive stimulation of t-tubular ICa appeared to increase in CAL myocytes, helping to maintain t-tubular ICa. Figure 7 shows schematic diagrams illustrating the regulation of ICa by β2-adrenoceptors, Cav-3, and PKA in normal cells (Fig. 7A) and in heart failure (Fig. 7B).
This work was funded by British Heart Foundation Grants PG/10/91/28644, PG/14/65/31055, and RG/12/10/29802.
No conflicts of interest, financial or otherwise, are declared by the authors.
S.M.B. performed experiments; S.M.B. analyzed data; S.M.B., C.H.K., M.B.C., C.H.O., and A.F.J. interpreted results of experiments; S.M.B. prepared figures; S.M.B., C.H.K., C.H.O., and A.F.J. drafted manuscript; S.M.B., C.H.K., M.B.C., C.H.O., and A.F.J. edited and revised manuscript; S.M.B., C.H.K., M.B.C., C.H.O., and A.F.J. approved final version of manuscript; C.H.O. and A.F.J. conceived and designed research.