Effect of isoprenaline on active Na transport in sheep cardiac Purkinje fibres.

The effect of isoprenaline (ISO; 5.10(-8) to 10(-6) M) on active Na transport was studied in depolarized sheep cardiac Purkinje fibres. Membrane current (I) and intracellular Na activity were measured simultaneously during enhanced Na pumping in voltage clamped preparations. ISO stimulated enhanced active Na transport but did not affect membrane current or intracellular Na concentration (ciNa) in the steady state under the chosen experimental conditions. The stimulatory effect of ISO was mediated by beta 1-adrenoceptors via a cAMP dependent pathway. The effect depended on the extracellular K concentration (coK) and was inhibited by external Ba ions. Complementary experiments on isolated sheep Purkinje cells revealed no ISO induced alteration of the Na pump current. The mechanism of the ISO induced stimulation of enhanced Na pumping in sheep Purkinje fibres probably involves an augmented K efflux. A direct effect on the pump molecule seems unlikely.