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

¹ Department of Physiology, University of Leicester, University Road, Leicester LE1 7RH

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 (aⁱ_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 aⁱ_Na may affect tension by way of the Na-Ca exchange.

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