Mechanisms of the Bat ' - induced Contraction in Smooth Muscle Cells of the Rabbit Mesenteric Artery

The mechanism ofthe Bat+-induced contraction was investigated using intact and saponin-treated skinned smooth muscle (skinned muscle) strips of the rabbit mesenteric artery . After depletion of Ca21 stored in the caffeinesensitive site, >0 .65 mM Ba2+ evoked contraction in muscle strips depolarized with 128 mM K+ in Cat+-free solution in a dose-dependent fashion, and the ED50 values for Ca2' and Ba2+ were 0.5 mM and 1 .2 mM in intact muscle strips, respectively . Nisoldipine (10 nM) blocked the contraction evoked by high K+ or 10 jM norepinephrine (NE) in the presence of 2.6 mM Ba2+ , but did not block the contraction evoked in the presence of 2 .6 mM Ca2+ . These results may indicate that Ba2+ permeates the voltage-dependent Ca21 channel . In skinned muscle strips, the ED50 values for Ca2' and Ba2+ were 0 .34 and 90,uM, respectively, as estimated from the pCaand pBa-tension relationships . Calmodulin enhanced and trifluoperazine inhibited the Ba2+ and Ca2'-induced contractions. After the application of Ba2+ or Ca21 with ATP,S in rigor solution, myosin light chain (MLC) was irreversibly thiophosphorylated, as estimated from the Ba2+or Ca2'-independent contraction . Furthermore, both divalent cations phosphorylated MLC, as measured using two-dimensional gel electrophoresis, to the extent expected from the amplitudes of the contraction evoked by these cations . Thus, Ba2+ is capable of activating the contractile proteins as Ca2' does . The amount of Ca21 or Ba2+ stored in cells was estimated from the caffeine response evoked in Ca2'-free solution in intact and skinned muscle strips . After the application of 0 .3 jM Ca21 or 0.1 mM Ba2+ for 60 s to skinned muscle strips after the depletion of Ca21 stored in cells, caffeine produced a contraction only upon pretreatment with Ca2+ but not with Ba2+ . When Ba2+ was applied successively just after the application of Ca2+ , the subsequently evoked caffeine-induced contraction was much smaller than that evoked by pretreatment with Ca2' alone . The above results indicate that (a) Ba2+ permeates the voltage-dependent Ca21 channel but may not permeate the receptoroperated Ca21 channel, (b) it releases Ca21 from store sites but is not accumuAddress reprint requests to Dr . Shinji Satoh, Dept . of Pharmacology, Faculty of Medicine, Kyushu University 60, Fukuoka 812, Japan . J . GEN . PHYSIOL . © The Rockefeller University Press 0022-1295/87/02/0215/23 $1 .00 215 Volume 89 February 1987 215-237 on Jne 1, 2017 D ow nladed fom Published February 1, 1987