Decreased K+ conductance produced by Ba++ in frog sartorius fibers

Abstract

The action of Ba(++) on membrane potential (E(m)) and resistance (R(m)) of frog (R. pipiens) sartorius fibers was studied. In normal Cl(-) Ringer's, Ba(++) (<9 mM) did not depolarize or induce contractions, but increased R(m) slightly above the control value of 3.8 +/- 0.6 K ohm-cm(2). In Cl(-)-free Ringer's (methane sulfonate) R(m) was 28.8 +/- 2.8 K ohm-cm(2), 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, E(m) increased slightly, and R(m) decreased to 9.0 +/- 0.6 K ohm-cm(2) (perhaps due to swelling of sarcotubules). Ba(++) (0.5 mM) rapidly increased R(m) to 31.3 +/- 3.8, decreased E(m) (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 R(m) was decreased markedly to 1.5 +/- 0.2 K ohm-cm(2); Ba(++) increased R(m) to 6.7 +/- 0.5 K ohm-cm(2). The Ba(++) actions usually began within 0.5 min and reached a maximum within 5 min. Addition of SO(4) (=), to precipitate the Ba(++), rapidly reversed the increase in R(m). Ba(++) must act by decreasing K(+) conductance (g(K)). In Cl(-) Ringer's, the high g(Cl)/g(K) ratio masked the effect of Ba(++) on g(K). Thus, small concentrations of Ba(++) specifically and rapidly decrease g(K).