Substitute anions and the chloride conductance of frog muscle: effects of chlorate and bromate on steady-state values and kinetics.

Voltage-clamp experiments have been used to study the effects of external nitrate, chlorate and bromate on the chloride conductance of sarcolemma of Xenopus laevis. Nitrate reduces inward current (chloride efflux), but less potently than does thiocyanate [Vaughan (1987) Pflügers Arch 410:153-158] and does not affect conductance kinetics. As its concentration is increased the blocking effect of nitrate saturates; at a nitrate mole fraction of 0.6 the anion conductance is reduced to about 50% and further increases in nitrate concentration are without significant effect. Nitrate's influences are not voltage-dependent. Chlorate is a much less potent blocker than is nitrate, and its effects are voltage-dependent. With small hyperpolarizations, currents are sometimes seen to be larger than the control, but the degree of block (or the conversion of augmentation to reduction) increases with the size of the voltage step. Anomalous mole-fraction effects are observed in the range 0.4-0.6 mol/mol, in that in some cells the reduction of conductance is noticeably greater in the lower than in the higher concentration of the replacement ion. In the presence of chlorate, relaxation rates are significantly increased, and this influence is not anomalously dependent on the mole fraction. Similar effects are observed in bromate. The effect on kinetics is not pH-dependent. The main series of experiments was conducted at pH 5, but the same influence on kinetics was observed at pH 9. Using point voltage-clamp experiments, chlorate and thiocyanate were both seen to lower the contraction threshold voltage, but thiocyanate has no influence on conductance kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)