Heart rate and paired ventricular stimulation: effects on contractile force and K+ exchange

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Abstract
In open-chest dogs the ventricles were driven at various rates by evenly spaced stimuli (Sl)or paired stimuli (S1+ S2). The rate constant of k42 washout from the heart was determined during constant-flow coronary per-fuslon. Cardiac K balance was determined from coronary blood flow and arteriovenous K difference. The linear increase of efflux of cardiac K and the increase of contractile force with increasing heart rate were confirmed. Paired stimulation markedly increases contractile force, but does not affect the efflux or balance of cardiac K. Therefore, more frequent, evenly spaced beats (increased heart rate) increase both efflux of K and contractile force in parallel manner, but extra beats introduced soon after the refractory period do not contribute measurably to K efflux, although they increase contractile force. It appears that the excitation cycle, rather than the system that controls contractile force, is the major determinant of potassium flux in the ventricle.