Mechanism of receptor‐mediated modulation of the delayed outward potassium current in guinea‐pig ventricular myocytes.

Abstract

1. Receptor‐mediated modulation of the delayed outward potassium current (IK) was investigated in guinea‐pig single ventricular cells by using whole‐cell voltage clamp and intracellular dialysis. 2. Isoprenaline increased IK in a dose‐dependent manner with a half‐maximum dose of 1.8 X 10(‐8) M. Isoprenaline (10(‐6) M) maximally increased IK by a factor of 2.85. This effect did not depend on the concentration of intracellular Ca2+ [( Ca2+]i). 3. External application of 10(‐5) M‐forskolin and internal application of 5 X 10(‐5) M‐cyclic AMP or 5 X 10(‐6) M of the catalytic subunit of cyclic AMP‐dependent protein kinase (PKA) also increased IK about 3‐fold. The effect of isoprenaline on IK was masked by previous application of cyclic AMP. 4. All the above phosphorylating agents increased the amplitude of IK without a significant change in the current kinetics. 5. In the presence of 10(‐5) M‐forskolin, an additional application of 10(‐8) M‐12‐O‐tetradecanoylphorbol‐13‐acetate, an activator of protein kinase C (PKC), produced a further increase in IK, suggesting that the active sites of PKA and PKC on the IK channel are different. 6. Acetylcholine (10(‐6) M) suppressed IK when the current was previously enhanced by 2 X 10(‐8) M‐isoprenaline, but had little effect in the absence of isoprenaline. 7. We conclude that beta‐adrenergic modulation of IK is mediated by cyclic AMP‐dependent phosphorylation but not by an increase in [Ca2+]i, that PKA and PKC enhance IK independently, and that acetylcholine antagonizes beta‐adrenergic stimulation of IK most probably by inhibiting adenylate cyclase.