1. The mechanism by which noradrenaline stimulates ouabain-sensitive rubidium uptake in guinea-pig myocardial tissue has been studied.
2. The stimulatory action of low concentrations of noradrenaline was reversed by high doses of propranolol (10−5 mol/l) in atrial tissue, but was not reversed by α- or β-, or combined α- and β-adrenoceptor antagonists in ventricular tissue.
3. Rubidium uptake was also found to increase with increasing extracellular potassium concentration ([K]o). The percentage values of stimulation by noradrenaline decreased with increasing [K]o.
4. Noradrenaline had no effect on the rate of ATP splitting by an isolated membrane preparation of Na,K-ATPase.
5. It is proposed that noradrenaline stimulates active cation transport by either (a) an effect secondary to increased passive efflux of K ions, or (b) an action at a novel adrenergic receptor, distinct from the ATPase enzyme itself.