The effect of moderate elevation in extracellular potassium concentration (up to 12 mM) on contraction of cat ventricular muscle was examined. Isometric force development was recorded from eight excised trabeculae and from six coronary-perfused in situ papillary muscle preparations. Contraction in the steady state was variably affected, sometimes decreasing monotonically, sometimes remaining unchanged, with increasing potassium level. In 11 of these 14 preparations, the steady state was preceded by a transient period in which the contraction was augmented. In addition, eight excised trabeculae were used in an experimental arrangement designed to distinguish between inotropic effects caused by potassium-induced alterations in the action potential and other, more direct, effects of this ion on contraction. The negative inotropic effect is attributable to a potassium-induced reduction in the amplitude and/or duration of the action potential plateau. The positive inotropic effect was found in experimental arrangements where effects of the potassium-rich medium on action potential time-course were effectively "buffered." The positive inotropic effect thus depends on the presence of the elevated potassium concentration and can occur independently of effects on the action potential time-course.

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