Decreased K+ Conductance Produced by Ba++ in Frog Sartorius Fibers

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Abstract
The action of Ba++ on membrane potential (Em) and resistance (Rm) of frog (Rava pipiens) sartorius fibers was studied. In normal Cl- Ringer''s, Ba++ (<9 m[image]) did not depolarize or induce contractions, but increased Rm slightly above the control value of 3.8[plus or minus] 0.6 K[OMEGA]-cm2. In Cl-free Ringer''s (methane sulfonate) Rm was 28.8 [plus or minus] 2.8 K[OMEGA]-cm2, and low concentrations of Ba++ (0.05-5.0 m[image]) depolarized and induced spontaneous contractions (fibrillation), even in tetrodotoxin. To stop disturbance of the microelectrodes, contractions were prevented by using two Cl--free solutions: twice hypertonic with sucrose (230 m[image]), or high K+ (83 m[image]) partially replacing Na+. In the hypertonic solution, the fiber diameters decreased, Em increased slightly, and Rm decreased to 9.0 [plus or minus] 0.6 K[OMEGA] -cm2 (perhaps due to swelling of sarcotubules). Ba.++ (0.5 m[image]) rapidly increased Rm to 31.3 [plus or minus] 3.8, decreased Em (e.g., to [long dash]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 [plus or minus] 0.2 K[OMEGA]-cm2; Ba++ increased Rm to 6.7 [plus or minus] 0.5 K -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.