ENHANCED NA-CA2+ EXCHANGE ACTIVITY IN CARDIOMYOPATHIC SYRIAN-HAMSTER()

The signaling of contraction by Ca2+ in the Syrian hamster (BIG 14.6) heart in the late stage of the cardiomyopathy (220 to 300 days old) wa s compared with that in age-matched healthy hamster hearts. Membrane c urrent and cell shortening or intracellular Ca2+ transients were measu red simultaneously in isolated whole-cell-clamped myocytes. The densit y of the L-type Ca2+ current was smaller in myopathic than in normal m yocytes (2.13+/-0.3 versus 3.21+/-0.4 pA/pF at 0 mV, P<.05). In both c ontrol and myopathic myocytes, the L-type Ca2+ current gated the relea se of Ca2+ and activation of contraction. In myopathic myocytes, activ ation of contraction also activated a slowly inactivating inward curre nt of 1.73+/-0.2 pA/pF. The Na+-Ca2+ exchanger generated this current (I-Na-Ca), because it was suppressed by rapid replacement of Na+ with Li+ and depletion of the intracellular Ca2+ pool by caffeine. I-Na-Ca, activated by rapid application of caffeine, was not significantly dif ferent in both groups (3.7+/-0.5 pA/pF in control hearts versus 3.9+/- 0.5 pA/pF in cardiomyopathic hearts). The activation of the inward exc hanger current in myopathic myocytes coincided with a significant prol ongation of contraction and the intracellular Ca2+ transient and a del ay in the onset of relaxation. These results suggest that the enhanced activity of the Na+-Ca2+ exchanger may be related to compromised sequ estration of Ca2+ in these animals.