AJP - Cell Physiology, Vol 271, Issue 3 C772-C782, Copyright © 1996 by American Physiological Society

ARTICLES

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

Y. Imaizumi, S. Henmi, Y. Uyama, K. Atsuki, Y. Torii, Y. Ohizumi and M. Watanabe
Department of Chemical Pharmacology, Faculty of Pharmaceutical Sciences, Nagoya City University, Japan.

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.

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