Effects of calcium, sodium and potassium ions on contractility of isolated atria and their responses to noradrenaline.

1. Rabbit left atrial preparations driven electrically at different rates were used for studies on inotropic effects of cations, drugs and coupled pacing. Sino-atrial node-atrial preparations were used for investigating the chronotropic effect of noradrenaline.2. The contractile tension-driving rate relationship was moved upwards by an elevation of [Ca(++)]o, coupled pacing and noradrenaline. In preparations exposed to Na(+)-poor and K(+)-free solutions the contractile strength at low driving rates (6 to 30 c/min) was markedly enhanced, but at high rates (120 to 240 c/min) it was not influenced. The contractile strength was reduced at low [Ca(++)]o and at high [K(+)]o.3. The positive inotropic effect of noradrenaline was markedly inhibited by a reduction of [Ca(++)]o and to some extent by a reduction of [K(+)]o. The noradrenaline-inotropy was not appreciably affected by an elevation of [Ca(++)]o and a reduction of [Na(+)]o.4. Cardiac excitability studied in preparations driven at high rates was enhanced by noradrenaline, a reduction of [K(+)]o and an elevation of [Ca(++)]o, but was reduced at low [Ca(++)]o, low [Na(+)]o and high [K(+)]o.5. The positive chronotropic response to noradrenaline was enhanced at high [Ca(++)]o, low [Na(+)]o and low [K(+)]o, but was reduced in solutions deficient in Ca(++) or rich in K(+).6. Inotropic effects of the ions and of coupled pacing were compared with those of ouabain. It is suggested that characteristic changes in the tension-rate curve seen in Na(+)-poor, K(+)-free and ouabain-containing solutions are correlated with an inhibition of active processes in the cardiac cell membrane, which affect ionic movements across it. It seems likely that mechanisms mediating adrenergic responses of the contractile tissue and the S-A node are associated with [Ca(++)]o.