Dihydropyridine receptors are primarily functional L-type calcium channels in rabbit ventricular myocytes

We measured [³H]PN200-110 binding and patch-clamp currents in rabbit ventricular myocytes to determine if there is a disparity between the density of dihydropyridine-specific receptors and functional L-type calcium channels, as has been reported for skeletal muscle. The dihydropyridine receptor density was 74.7±4.2 fmol/mg protein (mean±SEM, K(d)=1.73±0.29 nM, n=6) in ventricular homogenates and 147±6 fmol/mg protein (K(d)=1.15±0.16 nM, n=4) in myocytes. Ventricular homogenates contained 121±9 mg protein/g wet wt (n=7). These values were used to calculate a dihydropyridine receptor density of 12.9 dihydropyridine sites/μm² for ventricular homogenates and 14.8 dihydropyridine sites/μm² for myocytes. The number of functional L-type calcium channels (N) was calculated from measurements of whole-cell current (I), single-channel current (i), and open probability (p(o)), where N=I/(ixp(o)). We measured sodium current through calcium channels (I(ns)) to avoid calcium-induced inactivation. Whole-cell (I(ns)) and single-channel (i(ns) and p(o)) measurements were obtained under similar ionic conditions at a test potential of -20 mV. In six cells, the peak I(ns) was ~105 pA/pF. The single-channel conductance was 40.8 ± 2.6 pS (n=12), and i(ns) at -20 mV was 1.96 pA. The mean p(o) at -20 mV was 0.030±0.002 in 16 patches in which only a single channel was evident. The calculated density of functional L-type calcium channels was ~18 channels/μm². Thus, in rabbit cardiac muscle, the number of L-type calcium channels (18 channels/μm²) is of similar magnitude to the density of specific dihydropyridine receptors (13-15 dihydropyridine sites/μm²). We conclude that the majority of dihydropyridine receptors in cardiac muscle are functional L-type calcium channels. For reasons discussed, this conclusion may also be valid for skeletal muscle.