The action of Ba++ on membrane potential (Em) and resistance (Rm) of frog (R. pipiens) sartorius fibers was studied. In normal Cl- Ringer's, Ba++ (<9 mM) did not depolarize or induce contractions, but increased Rm slightly above the control value of 3.8 ± 0.6 KΩ-cm2. In Cl--free Ringer's (methane sulfonate) Rm was 28.8 ± 2.8 KΩ-cm2, and low concentrations of Ba++ (0.05–5.0 mM) depolarized and induced spontaneous contractions (fibrillation), even in tetrodotoxin. To stop disturbance of the microelectrodes, contractions were prevented by using two Cl--free solutions: (a) twice hypertonic with sucrose (230 mM), or (b) high K+ (83 mM) partially replacing Na+. In the hypertonic solution, the fiber diameters decreased, Em increased slightly, and Rm decreased to 9.0 ± 0.6 KΩ-cm2 (perhaps due to swelling of sarcotubules). Ba++ (0.5 mM) rapidly increased Rm to 31.3 ± 3.8, decreased Em (e.g., to -30 mv), and induced spontaneous "action potentials;" Sr++ had no effect. In the high K+ solution, the fibers were nearly completely depolarized, and Rm was decreased markedly to 1.5 ± 0.2 KΩ-cm2; Ba++ increased Rm to 6.7 ± 0.5 KΩ-cm2. The Ba++ actions usually began within 0.5 min and reached a maximum within 5 min. Addition of SO4=, to precipitate the Ba++, rapidly reversed the increase in Rm. Ba++ must act by decreasing K+ conductance (gK). In Cl- Ringer's, the high gCl/gK ratio masked the effect of Ba++ on gK. Thus, small concentrations of Ba++ specifically and rapidly decrease gK.

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