title: Mechanisms of the Ba2+-induced contraction in smooth muscle cells of the rabbit mesenteric artery.
abstract: The mechanism of the Ba2+-induced contraction was investigated using intact and saponin-treated skinned smooth muscle (skinned muscle) strips of the rabbit mesenteric artery. After depletion of Ca2+ stored in the caffeine-sensitive site, greater than 0.65 mM Ba2+ evoked contraction in muscle strips depolarized with 128 mM K+ in Ca2+-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 microM 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 Ca2+ channel. In skinned muscle strips, the ED50 values for Ca2+ and Ba2+ were 0.34 and 90 microM, respectively, as estimated from the pCa- and pBa-tension relationships. Calmodulin enhanced and trifluoperazine inhibited the Ba2+- and Ca2+-induced contractions. After the application of Ba2+ or Ca2+ with ATP gamma 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 Ca2+ 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 microM Ca2+ or 0.1 mM Ba2+ for 60 s to skinned muscle strips after the depletion of Ca2+ 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 Ba2+ permeates the voltage-dependent Ca2+ channel but may not permeate the receptor-operated Ca2+ channel, it releases Ca2+ from store sites but is not accumulated into the store site, and it directly activates the contractile proteins via formation of a Ba2+-calmodulin complex.
mesh_terms: D000818:Animals; D001464:Barium / Q000494:pharmacology*; D001696:Biomechanical Phenomena; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D003285:Contractile Proteins / Q000378:metabolism; D008297:Male; D008638:Mesenteric Arteries / Q000187:drug effects*; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D009543:Nifedipine / Q000031:analogs & derivatives / Q000494:pharmacology; D015737:Nisoldipine; D011188:Potassium / Q000494:pharmacology; D011817:Rabbits
chemical_list: D003285:Contractile Proteins; D001464:Barium; D002110:Caffeine; D015737:Nisoldipine; D009543:Nifedipine; D011188:Potassium; D002118:Calcium