AJP - Cell Physiology, Vol 252, Issue 4 C378-C384, Copyright © 1987 by American Physiological Society
Ba2+ induces contraction in swine carotid artery by mobilizing intracellular Ca2+
C. M. Hai and R. A. Murphy
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.
