V. Piacentino et al., Voltage-dependent Ca2+ release from the SR of feline ventricular myocytes is explained by Ca2+-induced Ca2+ release, J PHYSL LON, 523(3), 2000, pp. 533-548
1. Direct voltage-gated (voltage-dependent Ca2+ release, VDCR) and Ca2+ inf
lux-gated (Ca2+ induced Ca2+ release, CICR) sarcoplasmic reticulum (SR) Ca2
+ release were studied in feline ventricular myocytes. The voltage-contract
ion relationship predicted by the VDCR hypothesis is sigmoidal with large c
ontractions at potentials near the Ca2+ equilibrium potential (E-Ca). The r
elationship predicted by the CICR hypothesis is bell-shaped with no contrac
tion at E-Ca.
2. The voltage dependence of contraction was measured in ventricular myocyt
es 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-adre
nergic agonist isoproterenol (isoprenaline; ISO).
3. The voltage-contraction relationship was bell-shaped in Na+-free solutio
ns (tu eliminate the Na+ current and Na+-Ca2+ exchange, NCX) but the relati
onship was broader than the L-type Ca2+ current (I-Ca,I-L)-voltage relation
ship.
4. Contractions induced with voltage steps from normal resting potentials t
o -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 nega
tive 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 physiologi
cal [Na+] and a broad bell-shaped voltage-contraction relationship was obse
rved. Inhibition of reverse-mode NCX, decreasing I-Ca,I-L and decreasing SR
Ca2+ loading all decreased contractions at strongly positive potentials ne
ar 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 resu
lts exclude the VDCR hypothesis.