Voltage-dependent Ca2+ release from the SR of feline ventricular myocytes is explained by Ca2+-induced Ca2+ release

1. Direct voltage-gated (voltage-dependent Ca2+ release, VDCR) and Ca2+ influx-gated (Ca2+ induced Ca2+ release, CICR) sarcoplasmic reticulum (SR) Ca2+ release were studied in feline ventricular myocytes. The voltage-contraction relationship predicted by the VDCR hypothesis is sigmoidal with large contractions at potentials near the Ca2+ equilibrium potential (E-Ca). The relationship predicted by the CICR hypothesis is bell-shaped with no contraction at E-Ca. 2. The voltage dependence of contraction was measured in ventricular myocytes at physiological temperature (37 degrees C), resting membrane potential and physiological [K+]. Experiments were performed with cyclic adenosine 3',5'-monophosphate (cAMP) in the pipette or in the presence of the beta-adrenergic agonist isoproterenol (isoprenaline; ISO). 3. The voltage-contraction relationship was bell-shaped in Na+-free solutions (tu eliminate the Na+ current and Na+-Ca2+ exchange, NCX) but the relationship was broader than the L-type Ca2+ current (I-Ca,I-L)-voltage relationship. 4. Contractions induced with voltage steps from normal resting potentials to -40 mV are thought to represent VDCR rather than CICR. We found that cAMP and ISO shifted the voltage dependence of I-Ca,I-L activation to more negative potentials so that I-Ca,I-L was always present with steps to -40 mV. I-Ca,I-L at -40 mV inactivated when the holding potential was decreased (V-1/2 = -57.8 +/- 0.49 mV). 5. ISO increased inward current, SR Ca2+ load and contraction in physiological [Na+] and a broad bell-shaped voltage-contraction relationship was observed. Inhibition of reverse-mode NCX, decreasing I-Ca,I-L and decreasing SR Ca2+ loading all decreased contractions at strongly positive potentials near E-Ca. 6. The voltage-contraction relationship in 200 mu M cadmium (Cd2+) was bell-shaped, supporting a role of I-Ca,I-L rather than VDCR. 7. All results could be accounted for by the CICR hypothesis, and many results exclude the VDCR hypothesis.