Studies in smooth muscle bundles have shown the presence of functionally different compartments of Ca2+ store, one (Sα) sensitive to both caffeine and inositol 1,4,5-trisphosphate (IP3), and the other (Sβ) sensitive only to IP3, Ca2+ release in isolated single smooth muscle cells from guinea pig taenia caeci was studied to see if both compartments exist within a cell. Responses to caffeine and carbachol were consistently observed but were abolished after treatment with ryanodine, while intracellular application of IP3 induced Ca2+ release after the treatment, albeit smaller in size than control. Thus Sα and Sβ coexist in a single smooth muscle cell and agonist-induced Ca2+ release requires whole store to be loaded with Ca2+.
