pThe plant alkaloid ryanodine, known to interact selectively with the intracellular Ca(2+)-release channel in skeletal and cardiac muscles, has been repeatedly reported to affect smooth-muscle contractile functions that are consistent with its intracellular action at the Ca(2+)-release channel sites. Direct evidence for the binding of [3H]ryanodine to smooth-muscle membranes is sparse. Following our recent detailed characterization of functional effects of ryanodine and a preliminary report on the presence of [3H]ryanodine binding sites in rat vas deferens smooth muscle, we now report in this study a detailed characterization of binding of [3H]ryanodine to smooth muscle at the subcellular-membrane level. The ryanodine receptor in rat vas deferens muscle layer is primarily of smooth-muscle origin and is localized at the subcellular membrane site that is consistent with its role as a Ca(2+)-release channel in the sarcoplasmic reticulum (SR). Ryanodine binding to its receptor is Ca(2+)-dependent, with half-maximal binding occurring within the physiologically relevant cytosolic Ca2+ concentration. It is also sensitive to many factors, including change in Mg2+ concentration, ionic strength and osmolarity across the membrane vesicles. Agents known to inhibit (Ruthenium Red, Mg2+) or enhance (caffeine, Na+, K+) the Ca(2+)-induced Ca2+ release also inhibit or enhance the binding of ryanodine. Quantitative differences in ryanodine receptors exist among smooth muscles and do not seem to parallel their SR contents. Results from the present study indicate both the need and the basis for future investigations of the functional role of the ryanodine receptor in different smooth muscles./p
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