Steady-state coupling of plasma membrane calcium entry to extrusion revealed by novel L-type calcium channel block

摘要

Summary The L-type Ca2+ channel (Cav1.2) is the main pathway for trans-sarcolemmal (SL) Ca2+ influx in cardiac myocytes. To maintain Ca2+ homeostasis, chronic SL Ca2+-influx must be matched by chronic SL efflux. In this study we tested the hypothesis that chronic down-regulation 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桟a2+ exchange (NCX) function and sarcoplasmic reticulum (SR) store load we perfused Ca2+-free bath for 25 s 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 Ni2+-senstive. 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 6mM on caffeine-induced relaxation kinetics. 6mM 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 Cav1.2 channel-mediated SL entry alters NCX expression, and coincidentally controls SR Ca loading and SL Ca2+ efflux. ?2008 Elsevier Ltd. All rights reserved#1 Dollars 1001N200306651Dollars 1200