Altered Na/Ca exchange distribution and activity in ventricular myocytes from failing hearts

In mammalian cardiac ventricular myocytes, Ca efflux via Na/Ca exchange (NCX) occurs predominantly at t-tubules. Heart failure is associated with disrupted t-tubule structure, but its effect on t-tubule function is less clear. We therefore investigated t-tubular NCX activity in ventricular myocytes isolated from rat hearts ~18 weeks after coronary artery ligation (CAL) or corresponding Sham operation. NCX current (INCX) and L-type Ca current (ICa) were recorded using the whole-cell voltage-clamp technique in intact and detubulated (DT) myocytes; intracellular ([Ca]i) was monitored simultaneously using fluo-4. INCX was activated and measured during application of caffeine to release Ca from sarcoplasmic reticulum (SR). Whole-cell INCX was not significantly different in Sham and CAL myocytes, and occurred predominantly in the t-tubules in Sham myocytes. CAL was associated with redistribution of INCX and ICa away from the t-tubules to the cell surface, and an increase in t-tubular INCX/ICa density from 0.12 in Sham to 0.30 in CAL myocytes. The decrease in t-tubular INCX in CAL myocytes was accompanied by an increase in the fraction of Ca sequestered by SR. However, SR Ca content was not significantly different in Sham, Sham DT and CAL myocytes, but was significantly increased by DT of CAL myocytes. In Sham myocytes there was hysteresis between INCX and [Ca]i, which was absent in DT Sham, but present in CAL and DT CAL, myocytes. These data suggest altered distribution of NCX in CAL myocytes.