Effects of Na+/Ca2+ exchanger downregulation on contractility and [Ca2+]i transients in adult rat myocytes

Postmyocardial infarction (MI) rat myocytes demonstrated depressed Na+/Ca2+ exchange (NCX1) activity, altered contractility, and intracellular Ca2+ concentration ([Ca2+](i)) transients. We investigated whether NCX1 downregulation in normal myocytes resulted in contractility changes observed in MI myocytes. Myocytes infected with adenovirus expressing antisense (AS) oligonucleotides to NCX1 had 30% less NCX1 at 3 days and 66% less NCX1 at 6 days. The half-time of relaxation from caffeine-induced contracture was twice as long in ASNCX1 myocytes. Sarcoplasmic reticulum (SR) Ca2+-ATPase abundance, SR Ca2+ uptake, resting membrane potential, action potential amplitude and duration, L-type Ca2+ current density and cell size were not affected by ASNCX1 treatment. At extracellular Ca2+ concentration ([Ca2+](o)) of 5 mM, ASNCX1 myocytes had significantly lower contraction and [Ca2+](i) transient amplitudes and SR Ca2+ contents than control myocytes. At 0.6 mM [Ca2+](o), contraction and [Ca2+](i) transient amplitudes and SR Ca2+ contents were significantly higher in ASNCX1 myocytes. At 1.8 mM [Ca2+](o), contraction and [Ca2+](i) transient amplitudes were not different between control and ASNCX1 myocytes. This pattern of contractile and [Ca2+](i) transient abnormalities in ASNCX1 myocytes mimics that observed in rat MI myocytes. We conclude that downregulation of NCX1 in adult rat myocytes resulted in decreases in both Ca2+ influx and efflux during a twitch. We suggest that depressed NCX1 activity may partly account for the contractile abnormalities after MI.