EVIDENCE FOR A CA2-GATED RYANODINE-SENSITIVE CA2+ RELEASE CHANNEL IN VISCERAL SMOOTH-MUSCLE()

Although a role for the ryanodine receptor (RyR) in Ca2+ signaling in smooth muscle has been inferred, direct information on the biochemical and functional properties of the receptor has been largely lacking. S tudies were thus carried out to purify and characterize the RyR in sto mach smooth muscle cells from the toad Bufo marinus. Intracellular Ca2 + measurements with the Ca2+-sensitive fluorescent indicator fura-2 un der voltage clamp indicated the presence of a caffeine- and ryanodine- sensitive internal store for Ca2+ in these cells. The (CHAPS)-solubili zed, [H-3]ryanodine-labeled RyR of toad smooth muscle was partially pu rified from microsomal membranes by rate density centrifugation as a 3 0-S protein complex. SDS/PAGE indicated the comigration of a high mole cular weight polypeptide with the peak attributed to 30-S RyR, which h ad a mobility similar to the cardiac RyR and on immunoblots cross-reac ted with a monoclonal antibody to the canine cardiac RyR. Following pl anar lipid bilayer reconstitution of 30-S stomach muscle RyR fractions , single-channel currents (830 pS with 250 mM K+ as the permeant ion) were observed that were activated by Ca2+ and modified by ryanodine. I n vesicle-Ca-45(2+) efflux measurements, the toad channel was activate d to a greater extent at 100-1000 muM than 1-10 muM Ca2+. These result s suggest that toad stomach muscle contains a ryanodine-sensitive Ca2 release channel with properties similar but not identical to those of the mammalian skeletal and cardiac Ca2+-release channels.