Sensitivity to H, Li and Mg ions of the slow inward sodium current in frog atrial fibres.

Membrane currents have beenmeasured in frog atrial fibres under voltage clamp conditions (double sucrose gap method); the effects on these currents of (i) an increase in [H]_o, (ii) the complete substitution of external Na by Li, (iii) the addition of Mg have been studied.The TTX-sensitive rapid inward current is decreased in acid Ringer (pH 5.6) as compared to values in normal Ringer (pH 7.8); it is unchanged in Li-Ringer and increased in Mg-Ringer. The slow inward calcium current (measured in TTX, Na-free solution) is decreased in acid Ringer and in Mg-Ringer and is unchanged in Li-Ringer. Increases or decreases affecting the rapid sodium current and the calcium current never exceed 40% and are generally smaller. On the other hand the slow inward sodium current (measured in TTX, Ca-free solution) is always suppressed in the three conditions studied.The participation of Na ions to the slow inward current in so called normal Ringer, that is in Mg-free Ringer is discussed. Indirect evidences are presented in favour of such a participation. After addition of Mg 1.8 mm to Ringer the slow inward current is carried only by Ca ions. Since the slow inward sodium current increases markedly in Mg-free Ca-free Ringer, it is concluded that the slow Na conductance is controlled by both Ca and Mg ions in addition to being strongly pH-dependent. It is also suggested that Na and Ca ions participating to the slow inward current do not penetrate necessarily through the same channel.