THE NEGATIVE INOTROPIC EFFECT OF RAISED EXTRACELLULAR POTASSIUM AND CAESIUM IONS ON ISOLATED FROG ATRIAL TRABECULAE

The exposure of frog atrial trabeculae to Ringer solution containing an elevated K + concentration, produces a depolarization of the membrane and a reduction of both the duration of the action potential and the strength of the heart beat. In voltage‐clamped preparations, the effect of perfusion with K + ‐rich Ringer solution is threefold. First, a sustained inward current develops at the holding potential (‐80 mV). Secondly, the contractions evoked by depolarizing clamp pulses are reduced: this effect which is greater upon the tonic phase of the contraction than the early phasic tension, is also seen to follow the addition of Cs + ions to the bathing fluid; at equal concentrations K + ions are the more effective. Thirdly, when measured with an ion‐sensitive micro‐electrode in ventricular trabeculae, the intracellular Na + ion activity ( a i Na ) declines with a time course similar to the development of the negative inotropic effect. This suggests that the actions of raised [K + ] o or [Cs + ] o upon tension may be secondary to an effect on the movement of Na + ions across the cell membrane, which by reducing a i Na may affect tension by way of the Na‐Ca exchange.