Ratio of Dihydropyridine to Ryanodine Receptors in Mammalian and Frog Twitch Muscles in Relation to the Mechanical Hypothesis of Excitation-Contraction Coupling

Abstract

An indirect argument in favour of the mechanical hypothesis of excitation-contraction (EC) coupling [Schneider & Chandler (1973) Nature242, 244-246] is the fixed stoichiometry between the voltage-driven dihydropyridine receptor (DHP-R) on the transverse tubule (TT) and the ryanodine-sensitive Ca2+ channel (RyR) of sarcoplasmic reticulum (SR) terminal cisternae (TC), based mainly on the structural arrangement described by Block et al. at junctional triads of toadfish swimbladder muscle [Block, B., et al. (1988) J. Cell Biol.107, 2587-2600]. We calculated the overall DHP-R/RyR ratios for a wide variety of rabbit twitch muscles, as compared to fast- and slow-twitch muscles of the rat, and to frog sartorius, following quantification of high-affinity binding sites for tritiated PN200-110 and ryanodine. To this purpose, we carried out our binding measurements on both skeletal muscle membranes detached from myofibrils by extensive extraction with low-ionic strength medium,and on the myofibrillar residue containing a proportion of binding sites. Our results expressed per gram of muscle allow the following main conclusions: i) The overall DHP-R/RyR ratio is approximately 2 in both fast-twitch and slow-twitch muscle of the rabbit, implying that most, if not all, Ca2+-channels at junctional triads are stoichiometrically associated with the voltage sensor; in agreement with Block′s model; ii) There is a major inconsistency in these values, rather than in the absolute values of RyR sites, on side by side comparison of fast-twitch and of slow-twitch muscles of the rabbit and rat, in that the DHP-R/RyR ratio is invariably lower for the rat, i.e.,only about 1.2. Since the absolute values of DHP-R sites, according to our calculations, agree well with those obtained for the same muscles by Lamb & Walsh [(1987) J. Physiol. 393, 595-617], it seems improbable that they were underestimated. On the assumption that there is a fixed stoichiometry between DHP-R and RyR at the junctional contact area between TT and TC, the relatively large excess of RyR sites in the rat, expressed per g. muscle, should be attributed to the presence of a proportion of Ca2+-channels that are located outside this area, i.e.,are extrajunctional [Dulhunty, A., et al. (1992) Proc. R. Soc. Lond. (B) 247, 69-75]. We also find similarities in DHP-R/RyR ratios between twitch muscles of the rat and the frog; iii) Our findings lend themselves to speculate that the presence of extrajunctional Ca2+-channels, which are natural candidates for Ca2+-dependent modulation of depolarization-induced Ca2+-release, is a species-specific feature, independent of the class of Vertebrates to which animals belong and of the particular triadic pattern of the muscle.