Positive inotropic effect of ceramide in adult ventricular myocytes:: mechanisms dissociated from its reduction in Ca2+ influx

Ceramide, a sphingolipid metabolite produced by activation of sphingomyelinase, has been previously shown to reduce L-type Ca2+ channel current (I-Ca,I- L) in adult rat ventricular myocytes; however, its effect on contractile function is unknown. In this study, we investigated the effects of ceramide on excitation-contraction coupling in adult ventricular myocytes and on left ventricular (LV) function in isolated hearts. Surprisingly, in patch-clamped myocytes, ceramide increased contraction concomitant with reductions in I-Ca,I- L. In intact myocytes, ceramide increased cell shortening (CS) concurrently with enhancing maximum rates of shortening and relaxation and the duration of contraction. Ceramide also increased the amplitudes of postrest potentiated (PRP) contraction. In fura-PE3-loaded myocytes, ceramide increased systolic Ca2+ and the magnitude and maximum rates of the rising and declining phases of Ca2+ transients. Ceramide-elicited decreases in magnitudes of PRP relative to steady-state contraction and the Ca2+ transient suggest an increased fractional Ca2+ release from the sarcoplasmic reticulum (SR). However, ceramide slightly reduced the caffeine-induced Ca2+ transient and had no significant effect on the amplitude of the PRP-elicited Ca2+ transient. Additionally, the ceramide-induced upward shift in the relationship of contraction and the Ca2+ transient and increase in the Ca2+ responsiveness of CS suggest an increase in myofilament Ca2+ sensitivity. In isolated hearts, ceramide increased LV developed pressure and maximum rates of contraction and relaxation at balloon volumes of 30-50 mul. In summary, regardless of decreasing I-Ca,I- L, ceramide elicits distinct positive inotropic and lusitropic effects, resulting probably from enhanced SR Ca2+ release and uptake, and increased Ca2+ sensitivity of ventricular myocytes.