Effect of a low sodium medium, ouabain and noradrenaline on relaxation of the myocardium of the perfused rabbit heart

Random stimulation of the perfused heart allows relationships between the rate of contractions (dP/dtmax), the size of contraction (Pmax) and the rate of relaxation (dP/dtmin) of contractions of varying intensity to be studied. The present study concerns these relationships during perfusion with ouabain, a low sodium medium and noradrenaline. Isolated rabbit hearts were perfused with Tyrode solution (O2 95%, CO2 5%, 36.4 degrees C), the isovolumic contractions of the left ventricle were recorded and the right ventricle was stimulated at random for 30 s (pulse width 10 ms, voltage double the threshold value). Perfusion was then switched over to perfusion with ouabain solution (10(-6) mol.l-1), with noradrenaline solution (10(-6) mol.l-1) or with low sodium solution (with 31% of the normal Na concentration). When spontaneous contraction size had attained a stable level, random stimulation was repeated. During random stimulation, dP/dtmin was directly proportional to Pmax (dP/dtmin = k1.P max) and to dP/dtmax (dP/dtmin = k2.dP/dtmax). Ouabain and low sodium did not change k1 or k2 and noradrenaline did not change k2. The increase in k1 during noradrenaline perfusion corresponds to shorter duration of contraction. It was found that dP/dtmax, which corresponds to the sarcoplasmic calcium concentration at the outset of activation, was the main factor determining the relaxation rate during ouabain, noradrenaline and low sodium perfusion.