TENSION-VOLTAGE RELATIONS OF SINGLE MYOCYTES REFLECT CA RELEASE TRIGGERED BY NA CA EXCHANGE AT 35-DEGREES-C BUT NOT 23-DEGREES-C/

Contractile tension in response to 200-ms voltage-clamp pulses was mea sured in isolated guinea pig ventricular cells conditioned to constant Ca load. At 23 degrees C, the tension-voltage relation was bell shape d, decaying from a maximum at +20 mV to zero at +100 mV, but at 35 deg rees C it was sigmoidal, with similar twitch tensions at +20 and +100 mV. Tension at 35 degrees C and +100 mV was reduced by ryanodine or ca ffeine and abolished by removal of Ca just before the test pulse. At 3 5 degrees C and +100 mV, twitch tension increased markedly as the Na c oncentration in the patch pipette ([Na](p)) was varied between 0 and 2 0 mM. Cd (300 mu M) blocked tension at all potentials at 23 degrees C, but tension remained in the presence of Cd at 35 degrees C (29% of co ntrol at +2 mV and 100% of control at +100 mV). Cd-resistant tension b egan to relax during the damp pulse at all potentials (80 +/- 10 ms at +2 mV and 140 +/- 12 ms at +100 mV). Ni (3.6 mM) both reduced and slo wed tension transients at all potentials. The results suggest that fas t contractions due to sarcoplasmic reticulum Ca release can be trigger ed by Ca influx through either Ca current (I-Ca) or Na/Ca exchange and that those triggered through exchange are much more temperature sensi tive than those triggered by I-Ca.