Altered E-C coupling in rat ventricular myocytes from failing hearts 6 wk after MI

Excitation-contraction (E-C) coupling was investigated in rat hearts 6 wk after induction of myocardial infarction (MI) by ligation of the left coronary artery. Heart weight was increased by 74% and left ventricular end-diastolic pressure was 23 ± 2 mmHg in MI compared with 8 ± 2 mmHg in sham-operated controls (Sham, P < 0.001). Cell shortening was measured in voltage-clamped myocytes at 36°C. In solutions where Cs⁺ had been replaced by K⁺, the voltage dependence of contraction was sigmoidal between -20 and +100 mV in Sham and MI cells. Verapamil (20 μM) blocked L-type Ca²⁺ current and reduced contraction in Sham cells by ~50% (P < 0.01) but did not decrease contraction significantly in MI cells at test potentials above +10 mV. Verapamil-insensitive contractions were blocked by Ni²⁺ (5 mM). Na⁺/Ca²⁺ exchange current was doubled in MI compared with Sham cells at test potentials between -20 and +80 mV (P < 0.05), whereas mRNA and protein expression increased by 30-40%. Finally, voltage dependence of contraction was bell shaped in Na⁺-free solutions, but contraction was significantly increased in MI cells over a wider voltage range (P < 0.05). The insensitivity to Ca²⁺ channel block in MI cells may result from an increased contribution of the Na⁺/Ca^- exchanger to triggering of E-C coupling. These results suggest significant changes in E-C coupling in the hypertrophy and failure that develop in response to extensive MI.