Effect of ambasilide, a new class III agent, on plateau currents in isolated guinea pig ventricular myocytes: block of delayed outward potassium current.

The actions of ambasilide (LU-47110) on the action potential and membrane currents of isolated guinea pig ventricular myocytes were studied using voltage clamp techniques. Ambasilide (1 microM) prolonged the action potential (APD) at 20, 50 and 90% repolarization by 11.2 +/- 4.3, 13.8 +/- 3.9 and 13.6 +/- 3.7%, respectively, compared to control (n = 10). APD prolongation was attributed to the block of delayed rectifier outward current (Ik) in a concentration-dependent fashion (0.01-10 microM). The effects on the APD and Ik were both partially reversed after perfusion with drug-free Tyrode's solution. The block of Ik by ambasilide was compared to that by E-4031 (5 microM), a putative selective blocker of that fast, inwardly rectifying component of Ik identified in guinea pig ventricle. E-4031 produced about 65% block of Ik for pulse durations between 80 and 420 msec, but the block decreased as the pulse duration increased further, the block accounting for 34 +/- 5% of Ik at 6.3 sec. In contrast, the percentage of reduction of Ik by 10 microM ambasilide did not produce a consistent magnitude of block over a similar range of short depolarizations, but rather progressively decreased Ik as the pusle duration lengthened. Block at the end of a 2-sec pulse was about 48 +/- 8%, more block than could be attributed to an E-4031-sensitive current block alone. Whereas E-4031 (5 microM) shifted the activation curve of Ik 10 mV toward positive potentials and decreased the slope factor, k, by about 4 mV, ambasilide (5 microM) had no effect on these parameters.(ABSTRACT TRUNCATED AT 250 WORDS)