Ba2+ induces contraction in swine carotid artery by mobilizing intracellular Ca2+

Recent data suggest that Ba2+ activates the smooth muscle contractile apparatus. However, in vitro studies indicate that Ba2+ activates Ca2+ calmodulin-dependent enzymes only at very high concentrations. These observations might suggest that Ba2+ activates the contractile apparatus by a mechanism independent of myosin phosphorylation. We tested this hypothesis using intact and Triton X-100 skinned swine carotid medial strips. In intact tissues, Ba2+ stimulated dose-dependent contractions in the absence of extracellular Ca2+. However, in skinned tissues, Ba2+ induced much lower stress development at concentrations more than three orders of magnitude greater than needed for Ca2+-induced contraction. Ba2+ also failed to maintain stress in skinned tissues previously contracted with 4 microM Ca2+. The Ba2+-induced contraction in intact strips was associated with gradual increases in myosin phosphorylation from a basal level of approximately 5% to a sustained level of 30.0 +/- 0.8%. Partial intracellular calcium depletion with 100 microM histamine, 25 mM caffeine, and 5 mM ethyleneglycol-bis(beta-aminoethyl-ether)-N,N'-tetraacetic acid (EGTA) failed to abolish Ba2+-induced contractions. However, repeated contractions with Ba2+ in the absence of extracellular Ca2+ led to decreases in stress development. These observations suggest that Ba2+ induces stress development by mobilizing intracellular Ca2+ that was not released by histamine and caffeine.