Characteristics of Ca2+ release for activation of K+ current and contractile system in some smooth muscles

Characteristics of Ca2+ release from stores were investigated in strips from ileum and portal vein and in isolated myocytes from ileum and urinary bladder of the guinea pig with use of caffeine and 9-methyl-7-bromoeudistomin D (MBED), a potent releaser of Ca2+ from skeletal muscle sarcoplasmic reticulum. In skinned strips, 1-30 mM caffeine elicited a transient contraction, but 10-300 microM MBED did not. Pretreatment with 100 microM MBED did not affect the subsequent caffeine-induced contraction. In single cells loaded with indo 1-acetoxymethyl ester, 10 mM caffeine increased cytoplasmic Ca2+ concentration, whereas 100 microM MBED elicited a small or no increase. Under whole cell clamp, spontaneous transient outward currents through Ca(2+)-dependent K+ (BK) channels were first enhanced and then suppressed by 30 microM MBED or 5 mM caffeine. The amplitude of Ca(2+)-dependent transient K+ current on depolarization was reduced by MBED and caffeine (50% inhibitory concentrations = 20 microM and 1 mM, respectively). Other currents and single BK channel activity were not significantly affected by MBED. The Ca2+ release from stores responsible for BK channel activation may be resolved from that for the activation of the contractile system by MBED in these smooth muscle cells.