Steady-state coupling of plasma membrane calcium entry to extrusion revealed by novel L-type calcium channel block.
The L-type Ca2+ channel (Ca(v)1.2) is the main pathway for trans-sarcolemmal (SL) Ca2+ influx in cardiac myocytes. To maintain Ca2+ homeostasis, chronic SL Ca(2+)-influx must be matched by chronic SL efflux. In this study we tested the hypothesis that chronic downregulation of SL Ca2+ entry regulates SL extrusion. We studied mRNA and Ca2+ handling responses to chronic down-regulation of Ca2+ channel current induced by over-expression of the small GTPase Rem. Rem lowered net SL diastolic Ca2+ entry, and reduced the twitch Ca2+ amplitude. Rem also significantly slowed Ca2+ transient decay kinetics (p < 10(-3)). Rem reduced NCX1.1 protein level and function. To measure Na-Ca2+ exchange (NCX) function and sarcoplasmic reticulum (SR) store load we perfused Ca(2+)-free bath for 25s followed by rapid application of 50 mM caffeine. In control, caffeine transient relaxations were described by a bi-exponential decay with a fast phase that was 10 mM Ni(2+)-sensitive. Rem significantly slowed caffeine-induced relaxation time course (Rem versus control, p < 10(-6)). To test whether extrusion slowing was mediated by insufficient basal Ca2+ for allosteric NCX activation we measured the effect of increasing bath Ca2+ from 1.8 to 6 mM on caffeine-induced relaxation kinetics. 6 mM Ca2+ did not alter kinetics of control cells, but in Rem-over-expressed cells 6 mM Ca2+ sped kinetics. We conclude that chronic block of Ca(v)1.2 channel-mediated SL entry alters NCX expression, and coincidentally controls SR Ca loading and SL Ca2+ efflux.
Lester
William C
WC
Department of Physiology, MS-508, University of Kentucky College of Medicine, 800 Rose Street, Lexington, KY 40536-0298, USA.
Schroder
Elizabeth A
EA
Burgess
Don E
DE
Yozwiak
Doug
D
Andres
Douglas A
DA
Satin
Jonathan
J
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D015536:Down-Regulation; D051379:Mice; D020559:Monomeric GTP-Binding Proteins / Q000378:metabolism*; D032383:Myocytes, Cardiac / Q000378:metabolism*; D012508:Sarcolemma / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*
C514350:CACNA1C protein, mouse; D020746:Calcium Channels, L-Type; C499849:NCX1 protein, mouse; C108135:Rem protein, mouse; D019831:Sodium-Calcium Exchanger; D020559:Monomeric GTP-Binding Proteins; D002118:Calcium