In the adult mammalian heart, the majority of Ca2+ required for contraction is released from the sarcoplasmic reticulum (SR) via the Ca2+-release channel or ryanodine receptor (RyR). Such release is dependent upon a relatively small influx of Ca2+ entering the cell across the sarcolemma (SL) by means of the L-type Ca2+ channel or the dihydropyridine receptor (DHPR). In lower vertebrates, there is indirect evidence suggesting that Ca2+ influx across the SL may be sufficient to support contraction in the absence of Ca2+ release from the SR. This apparent difference in myocardial excitation-contraction (E-C) coupling was investigated further by determining DHPR and RyR densities in ventricular homogenate preparations from rat, trout, dogfish and hagfish. DHPR Bmax 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 (Kd) of 40-70 pmoll-1 in these three species were of similar magnitude. RyR binding revealed both high- and low-affinity sites in all species. RyR Bmax 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 Kd1 values for the high-affinity sites were comparable in all preparations (range 12-87 nmoll-1). The quantitative expression of RyRs in these species is consistent with the relative amount of SR present as indicated in physiological experiments and electron micrographs. Taking into consideration myocyte morphology of teleost and elasmobranch species, the data are consistent with a greater reliance on Ca2+ influx across the SL during E-C coupling in lower vertebrates, although a functional role for Ca2+ release from the SR in the more active species await further investigation.

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