Hypothermia increases the gain of excitation-contraction coupling in guinea pig ventricular myocytes

Components of excitation-contraction (EC)-coupling were compared at 37 degrees C and 22 degrees C to determine whether hypothermia altered the gain of EC coupling in guinea pig ventricular myocytes. Ca(2+) concentration (fura-2) and cell shortening (edge detector) were measured simultaneously. Hypothermia increased fractional shortening (8.3 +/- 1.7 vs. 2.6 +/- 0.3% at 37 degrees C), Ca(2+) transients (157 +/- 33 vs. 35 +/- 5 nM at 37 degrees C), and diastolic Ca(2+) (100 +/- 9 vs. 60 +/- 6 nM at 37 degrees C) in field-stimulated myocytes (2 Hz). In experiments with high-resistance microelectrodes, the increase in contractions and Ca(2+) transients was accompanied by a twofold increase in action potential duration (APD). When voltage-clamp steps eliminated changes in APD, cooling still increased contractions and Ca(2+) transients. Hypothermia increased sarcoplasmic reticulum (SR) Ca(2+) stores (83 +/- 17 at 37 degrees C to 212 +/- 50 nM, assessed with caffeine) and increased fractional SR Ca(2+) release twofold. In contrast, peak Ca(2+) current was much smaller at 22 degrees C than at 37 degrees C (1.3 +/- 0.4 and 3.5 +/- 0.7 pA/pF, respectively). In cells dialyzed with sodium-free pipette solutions to inhibit Ca(2+) influx via reverse-mode Na(+)/Ca(2+) exchange, hypothermia still increased contractions, Ca(2+) transients, SR stores, and fractional release but decreased the amplitude of Ca(2+) current. The rate of SR Ca(2+) release per unit Ca(2+) current, a measure of EC-coupling gain, was increased sixfold by hypothermia. This increase in gain occurred regardless of whether cells were dialyzed with sodium-free solutions. Thus an increase in EC-coupling gain contributes importantly to positive inotropic effects of hypothermia in the heart.