1. Unloaded cell shortening and membrane currents were examined in isolated guinea-pig ventricular myocytes at 37 degrees C using video edge detection and single-electrode voltage clamp. 2. Inward Na+ currents were eliminated by lidocaine, tetrodotoxin, replacement of extracellular Na+ with choline chloride or sucrose, or by voltage inactivation of Na+ channels. In the absence of Na+ current, the threshold for contraction was approximately -50 or -55 mV. 3. Verapamil (5 mu M) and nifedipine (2 mu M) failed to inhibit contractions at negative membrane potentials when positive conditioning pulses were used to maintain intracellular Ca2+ stores via Na+-Ca2+ exchange. In contrast, 200 mu M Ni2+ inhibited these contractions 4. Contractions were abolished when the extracellular solution was nominally Ca2+ free. However, contractions were restored by as little as 50 mu M extracellular Ca2+. 5. Ryanodine (30 nM) completely abolished contractions initiated by depolarizing steps from -65 to -40 mV, but had minimal effects on contractions initiated by depolarizing steps from -40 to +5 mV. Subtraction of contraction-voltage relations determined in the presence of ryanodine from control relations revealed a ryanodine-sensitive component of contraction. This component activated at -55 mV and reached a plateau near -25 mV. 6. The amplitudes of contractions initated by depolarizing steps from -40 mV were directly proportional to the magnitude of Ca2+ current (I-Ca). In contrast, contractions initiated by steps from either -55 to -65 mV were not proportional to I-Ca. These contractions appeared at potentials negative to the threshold for L-type Ca2+ current, increased to a plateau at more positive potentials and did not decrease at potentials at which I-Ca decreased. 7. Subtraction of the contraction-voltage relationship determined from a membrane potential of -40 mV from that at -55 mV revealed a component of contraction with a negative activation threshold whose amplitude was not proportional to inward current. The shape of this relationship was virtually identical to that of the ryanodine-sensitive component of contraction. 8. This study identifies a component of contraction associated with Ca2+ release from sarcoplasmic reticulum (SR) which can be separated from other mechanisms of contraction on the basis of membrane potential. Our observations suggest that this voltage-dependent release mechanism is a true trigger mechanism which activates a portion of cardiac contraction which is attributable to SR Ca2+ release.
