In cultured human myometrial cells application of caffeine (1–30 mM) did not result in an elevation of intracellular Ca2+ ([Ca2+]i). Caffeine was found to reversibly inhibit both spontaneous and agonist-induced repetitive rises in [Ca2+]i possibly as a consequence of its ability to interfere with the binding of inositol trisphosphate (IP3) to the receptor on the sarcoplasmic reticulum. Brief applications of ryanodine (1–10μM) were observed to elevate [Ca2+]i and repeated exposures to ryanodine could elicit Ca2+ transients of similar magnitude. Ryanodine was also observed to mobilise Ca2+ in cells bathed in nominally Ca2+-free solution. These observations suggest the presence of a novel type of ryanodine-sensitive Ca2+-induced Ca2+ release (R-CICR) system in human myometrial cells.
