Characterization of a beta-adrenergically inhibited K+ current in rat cardiac ventricular cells

1. The electrophysiological properties and β-adrenergic regulation of a non-inactivating K+ current were studied using the whole-cell patch-clamp technique (22 ± 2°C) in adult rat ventricular cells. 2. In the presence of 4-aminopyridine, an inhibitor of the rapidly inactivating current, the depolarization-activated current consisted only of a slowly decaying outward current (I(K)). The presence of a non-inactivating current (I(ss)) was revealed when analysing inactivation curves. 3. I(K) and I(ss) were both sensitive to 50 mM tetraethylammonium and 10 mM 4-aminopyridine inhibition. I(K) was totally blocked by 100 μM clofilium, while I(ss) was not inhibited but rather enhanced by this class III anti-arrhythmic agent. 4. Unlike I(K), I(ss) was only slightly decreased by depolarizing prepulses and it did not show time-dependent inactivation when measured during 500 ms depolarizations. 5. I(ss) was decreased by the β-adrenergic agonist, isoprenaline (1 μM). Forskolin (10 μM) mimicked the effects of isoprenaline. The non-specific β-adrenergic antagonist, propranolol (3 μM), and a specific β1-adrenergic antagonist, CGP 20712A (0.3 μM), both prevented the effects of isoprenaline. Cell perfusion with 100 μM PKI6-22, a peptide inhibitor of the cyclic AMP-dependent protein kinase, reduced or abolished the effects of isoprenaline. 6. The dose-response curve for the inhibition of I(ss) by isoprenaline was positioned to the left of that for the calcium current. The threshold dose and the dose giving 50% of the maximal effect were, respectively, 0.1 and 0.21 nM for I(ss) and 1 and 4.3 nM for I(Ca). 7. In view of the high sensitivity of I(ss) to isoprenaline, its possible physiological effect was evaluated on action potential duration during β-adrenergic stimulation. At 1 nM, a concentration that did not increase I(Ca), isoprenaline induced a significant prolongation of action potential duration as a consequence of I(ss) inhibition. With 1 μM isoprenaline, the action potential was further prolonged, due largely to an evoked increase in I(Ca). 8. In conclusion, a K+ current displaying a weak voltage-dependent inactivation is present in rat ventricular cells. It is inhibited by stimulation of β1-adrenergic receptors and is highly sensitive to phosphorylation by protein kinase A. This current may play an important role in the neuromodulation of excitation-contraction coupling.