DIHYDROPYRIDINE RECEPTORS ARE PRIMARILY FUNCTIONAL L-TYPE CALCIUM CHANNELS IN RABBIT VENTRICULAR MYOCYTES

We measured [H-3]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/mu-m2 for ventricular homogenates and 14.8 dihydropyridine sites/mu-m2 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 approximately 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 approximately 18 channels/mu-m2. Thus, in rabbit cardiac muscle, the number of L-type calcium channels (18 channels/mu-m2) is of similar magnitude to the density of specific dihydropyridine receptors (13-15 dihydropyridine sites/mu-m2). 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.