DIHYDROPYRIDINE AND RYANODINE BINDING IN VENTRICLES FROM RAT, TROUT, DOGFISH AND HAGFISH

In the adult mammalian heart, the majority of Ca2+ required for contra ction is released from the sarcoplasmic reticulum (SR) via the Ca2+-re lease channel or ryanodine receptor (RyR), Such release is dependent u pon a relatively small influx of Ca2+ entering the cell across the sar colemma (SL) by means of the L-type Ca2+ channel or the dihydropyridin e receptor (DHPR),In lower vertebrates, there is indirect evidence sug gesting that Ca2+ influx across the SL may be sufficient to support co ntraction in the absence of Ca2+ release from the SR, This apparent di fference in myocardial excitation-contraction (E-C) coupling was inves tigated further by determining DHPR and RyR densities in ventricular h omogenate preparations from rat, trout, dogfish and hagfish, DHPR B-ma x values (means +/- S.E.M.) were highest in rat (0.30+/-0.01 pmol mg(- 1)), lower in trout (0.16+/-0.01 pmol mg(-1)) and dogfish (0.27+/-0.03 pmol mg(-1)), and slightly above the level of detection in hagfish (0 .03+/-0.01 pmol mg(-1)). The DHPR dissociation constants (K-d) of 40-7 0 pmol l(-1) in these three species were of similar magnitude, RyR bin ding revealed both high- and low-affinity sites in all species. RyR B- max for the high-affinity site was greatest in the rat (0.68 pmol mg(- 1)), lower in trout (0.19 pmol mg(-1)) and dogfish (0.07 pmol mg(-1)) and lowest in hagfish (0.01 pmol mg(-1)). The RyR K-d1 values for the high-affinity sites were comparable in all preparations (range 12-87 n mol l(-1)). The quantitative expression of RyRs in these species is co nsistent with the relative amount of SR present as indicated in physio logical experiments and electron micrographs, Taking into consideratio n myocyte morphology of teleost and elasmobranch species, the data are consistent with a greater reliance on Ca2+ influx across the SL durin g E-C coupling in lower vertebrates, although a functional role for Ca 2+ release from the SR in the more active species awaits further inves tigation.