1. Regulation of unloaded cell shortening and relaxation by sarcolemmal Na+-Ca2+ exchange was investigated in rat ventricular myocytes. Contraction of single cells at 22+/-1-degrees-C was measured simultaneously with membrane current and voltage using the whole-cell voltage clamp technique in combination with a video edge-detection device. 2. The extent of mechanical activation (cell shortening amplitude) was strongly dependent on diastolic membrane potential over the voltage range -140 to -50 mV. This voltage sensitivity of contraction was abolished completely when a recently described inhibitory peptide of the cardiac Na+-Ca2+ exchanger (XIP, 2 x 10(5) M) was present in the recording pipette, demonstrating that in rat ventricular cells Na+-Ca2+ exchange is modulated by diastolic membrane potential. 3. Possible influences of Na+-Ca2+ exchange on contraction were studied from a holding potential of -80 mV. Depolarizations (-50 to +60 mV) resulted in a bell-shaped shortening-voltage (S-V) relationship. These contractions were suppressed completely by either Cd2+ (10(-4) M) or verapamil (10(-5) M), but remained unchanged during superfusion with tetrodotoxin (TTX, 1.5 x 10(-5) M), when [Na+]o was reduced from 140 to 10 mM by substitution with either Li+ or Cs+ ions or when pipette Na+ was varied between 8 and 13 mM. XIP (2 x 10(-5) M) increased the magnitude and duration of twitch contractions, but had no effect on the shape of the S-V relationship. Thus, the Ca2+ current but not the Na+ current or Ca2+ influx due to reversed Na+-Ca2+ exchange can release Ca2+ from the sarcoplasmic reticulum (SR) under these experimental conditions. 4. The effect of the rate of repolarization on cell shortening was studied under voltage clamp by applying ramp waveforms immediately following the depolarizations which activated contraction. Although slowing of the rate of repolarization had no effect on the first contraction following a train of conditioning depolarizations, a positive inotropic effect developed thereafter. 5. Caffeine (10 mM) was applied to determine whether Na+-Ca2+ exchange and/or Ca2+ sequestration/buffering by the sarcoplasmic reticulum were primarily responsible for these inotropic effects. In the presence of caffeine the positive inotropic effect developed fully during the first test depolarization. Changes in the rate of repolarization had much less effect on shortening in cells dialysed intracellularly with XIP (2 x 10(-5) M). In combination, these results suggest that the changes in the inotropic effects resulting from changes in rate of repolarization may be due to altered loading and release of Ca2+ from the SR. 6. We conclude that the dependence of Na+-Ca2+ exchange activity on the diastolic membrane potential and the waveform (e.g. rate) of repolarization are very important parameters in modulating contractility in rat ventricle; and that release of Ca2+ from intracellular stores depends primarily on Ca2+ influx through sarcolemmal Ca2+ channels.
