Comparison of the number of dihydropyridine receptors with the number of functional L-type calcium channels in embryonic heart.

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Abstract
We compared the density of dihydropyridine (DHP) receptor sites with the density of functional L-type calcium channels in ventricular myocytes from chick heart at embryonic day 11. DHP receptors were quantified by using the DHP antagonist (+)-[3H]PN200-110 and by competition binding with the agonist Bay K 8644. The number of agonist and antagonist binding sites per ventricle was similar (250 +/- 15 and 244 +/- 8 fmol, respectively; mean +/- SEM). The mean number of myocytes per ventricle was 8.57 +/- 0.65 x 10(6), as determined by histological methods. From these data, the number of DHP receptors was calculated to be approximately 17,000 per myocyte or 25 to 26 DHP receptors per square micron of cell membrane, based on a mean myocyte membrane capacitance of 6.7 pF and a specific membrane capacitance of 1 microF/cm2. We next determined the number of functional L-type calcium channels by nonstationary fluctuation analysis with whole-cell patch clamp. The mean number of functional L-type channels per cell was 291 +/- 49 and 131 +/- 10 with Ca2+ and Ba2+ as the charge carriers, which yielded a channel density of 0.50 +/- 0.08 and 0.28 +/- 0.02 per square micron of cell membrane, respectively. From these data, the density of DHP receptor binding sites was determined to be from 50 to 100 times the density of functional L-type calcium channels. The function of the "excess" DHP receptors and the determination of whether the proportion of functional channels increases with development require further investigation.