Stimulatory effects of Ba2+ on contractile activity in the smooth muscle of the rat portal vein.

The effects on mechanical activity in rat portal vein of adding Ba2+, Sr2+ or Mg2+ (0.3--10 mM) to a tris-buffered solution (Na-tris) with 2.5 mM Ca2+ was investigated and compared to the effects of addition of Ca2+. Ba2+ induced a continuous tetanical activity and increased integrated force from threshold (0.3 mM) to sevenfold (10 mM). Addition of Sr2+ and Ca2+ had only minor effects on mechanical activity, whereas Mg2+ in increasing concentrations exerted an inhibitory effect. Contractures were elicited in K+-high tris solution with 2.5 mM Ca2+. The amplitudes of the contractures were not affected by added Mg2+, whereas Ca2+, Sr2+, and Ba2+ increased contracture force to some extent, the increase being highest for Ba2+ (twofold in 10 mM). Sucrose gap recordings of electrical and mechanical activity showed that added Ba2+ (0.3 mM) to Na-tris with 2.5 mM Ca2+ increased spike discharge and force/spike, the latter almost twofold. A corresponding addition of 0.3 mM Ca2+ had no effect. The effects of adding Sr2+ are very similar to that of added Ca2+, Mg2+ decreased spontaneous mechanical activity. Relaxation rates after K-contractures with 2.5 mM Ca2+, Ba2+, or Sr2+ were determined. No difference was found after K-tris with Ca2+ or Ba2+, whereas the veins relaxed faster after K-tris with Sr2+. We conclude tht Ba2+ acts as a constrictor of the smooth muscle in rat portal vein partly by initiating an increased spike discharge, and partly by increasing the entry or release of Ca2+ per spike, whereas the removal of Ca2+ is unaffected.