Regulation of cardiac L-type Ca2+ current in Na+-Ca2+ exchanger knockout mice: functional coupling of the Ca2+ channel and the Na+-Ca2+ exchanger.
L-type Ca2+ current (I(Ca)) is reduced in myocytes from cardiac-specific Na+-Ca2+ exchanger (NCX) knockout (KO) mice. This is an important adaptation to prevent Ca2+ overload in the absence of NCX. However, Ca2+ channel expression is unchanged, suggesting that regulatory processes reduce I(Ca). We tested the hypothesis that an elevation in local Ca2+ reduces I(Ca) in KO myocytes. In patch-clamped myocytes from NCX KO mice, peak I(Ca) was reduced by 50%, and inactivation kinetics were accelerated as compared to wild-type (WT) myocytes. To assess the effects of cytosolic Ca2+ concentration on I(Ca), we used Ba2+ instead of Ca2+ as the charge carrier and simultaneously depleted sarcoplasmic reticular Ca2+ with thapsigargin and ryanodine. Under these conditions, we observed no significant difference in Ba2+ current between WT and KO myocytes. Also, dialysis with the fast Ca2+ chelator BAPTA eliminated differences in both I(Ca) amplitude and decay kinetics between KO and WT myocytes. We conclude that, in NCX KO myocytes, Ca2+-dependent inactivation of I(Ca) reduces I(Ca) amplitude and accelerates current decay kinetics. We hypothesize that the elevated subsarcolemmal Ca2+ that results from the absence of NCX activity inactivates some L-type Ca2+ channels. Modulation of subsarcolemmal Ca2+ by the Na+-Ca2+ exchanger may be an important regulator of excitation-contraction coupling.
Pott
Christian
C
Department of Physiology, David Geffen School of Medicine at the University of California, Los Angeles, California 90095, USA.
Yip
Mey
M
Goldhaber
Joshua I
JI
Philipson
Kenneth D
KD
D000818:Animals; D002118:Calcium / Q000502:physiology*; D020746:Calcium Channels, L-Type / Q000502:physiology*; D066298:In Vitro Techniques; D015640:Ion Channel Gating; D051379:Mice; D018345:Mice, Knockout; D032383:Myocytes, Cardiac / Q000502:physiology*; D018408:Patch-Clamp Techniques; D012508:Sarcolemma / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000235:genetics / Q000502:physiology*
D020746:Calcium Channels, L-Type; D019831:Sodium-Calcium Exchanger; D002118:Calcium