Characterization of [3H]Ryanodine Binding Sites in Mammalian Lung

The ryanodine-sensitive calcium channels, also called ryanodine receptors, are intracellular Ca2+-release channels that have been shown to bind the neutral plant alkaloid ryanodine with nanomolar affinity. The activity of the skeletal muscle (RyR1), cardiac muscle (RyR2), and brain (RyR3) ryanodine receptor isoforms have been shown to be highly regulated by physiological factors including pH, temperature, and ionic strength; endogenous compounds including Ca2+, Mg2+, and adenosine triphosphate (ATP); and pharmacological agents including caffeine, ruthenium red, and neomycin. RyR3 is reportedly expressed in diverse tissues including lung; however, specific [3H]ryanodine binding sites in mammalian lung tissue have not been characterized. In this study, hamster lung ryanodine binding proteins were shown to specifically bind [3H]ryanodine with an affinity similar to that of RyR isoforms found in other tissues and this binding was shown to be sensitive to Ca2+ concentration, stimulation by caffeine and spermine, and inhibition by Mg2+, ruthenium red, and neomycin. The solubilized, intact ryanodine binding protein from hamster lung demonstrated approximately the same 30S sedimentation coefficient as RyR1 and RyR2, but a putative ryanodine receptor subunit from hamster lung was not found to cross-react with antibodies specific for the three known isoforms. We conclude that the hamster lung ryanodine binding protein demonstrates sedimentation and binding characteristics that are similar to those of the known RyR isoforms, but may exhibit antigenic dissimilarity from the typical RyR isoforms found in muscle and brain.