David A. Lathrop

The role of the delayed rectifier component IKs in dog ventricular muscle and Purkinje fibre repolarization

The relative contributions of the rapid and slow components of the delayed rectifier potassium current (IKr and IKs, respectively) to dog cardiac action potential configuration were compared in ventricular myocytes and in multicellular right ventricular papillary muscle and Purkinje fibre preparations. Whole‐cell patch‐clamp techniques, conventional microelectrode and in vivo ECG measurements were made at 37°C. Action potential duration (APD) was minimally increased (less than 7%) by chromanol 293B (10 μM) and L‐735,821 (100 nM), selective blockers of IKs, over a range of pacing cycle lengths (300–5000 ms) in both dog right ventricular papillary muscles and Purkinje fibre strands. D‐Sotalol (30 μM) and E‐4031 (1 μM), selective blockers of IKr, in the same preparations markedly (20–80%) lengthened APD in a reverse frequency‐dependent manner. In vivo ECG recordings in intact anaesthetized dogs indicated no significant chromanol 293B (1 mg kg−1 i.v.) effect on the QTc interval (332.9 ± 16.1 ms before versus 330.5 ± 11.2 ms, n= 6, after chromanol 293B), while D‐sotalol (1 mg kg−1 i.v.) significantly increased the QTc interval (323.9 ± 7.3 ms before versus 346.5 ± 6.4 ms, n= 5, after D‐sotalol, P < 0.05). The current density estimated during the normal ventricular muscle action potential (i.e. after a 200 ms square pulse to +30 mV or during a 250 ms long ‘action potential‐like’ test pulse) indicates that substantially more current is conducted through IKr channels than through IKs channels. However, if the duration of the square test pulse or the ‘action potential‐like’ test pulse was lengthened to 500 ms the relative contribution of IKs significantly increased. When APD was pharmacologically prolonged in papillary muscle (1 μM E‐4031 and 1 μg ml−1 veratrine), 100 nM L‐735,821 and 10 μM chromanol 293B lengthened repolarization substantially by 14.4 ± 3.4 and 18.0 ± 3.4% (n= 8), respectively. We conclude that in this study IKs plays little role in normal dog ventricular muscle and Purkinje fibre action potential repolarization and that IKr is the major source of outward current responsible for initiation of final action potential repolarization. Thus, when APD is abnormally increased, the role of IKs in final repolarization increases to provide an important safety mechanism that reduces arrhythmia risk.

Pharmacological block of the slow component of the outward delayed rectifier current (IKs) fails to lengthen rabbit ventricular muscle QTc and action potential duration

The effects of IKs block by chromanol 293B and L‐735,821 on rabbit QT‐interval, action potential duration (APD), and membrane current were compared to those of E‐4031, a recognized IKr blocker. Measurements were made in rabbit Langendorff‐perfused whole hearts, isolated papillary muscle, and single isolated ventricular myocytes. Neither chromanol 293B (10u2003μM) nor L‐735,821 (100u2003nM) had a significant effect on QTc interval in Langendorff‐perfused hearts. E‐4031 (100u2003nM), on the other hand, significantly increased QTc interval (35.6±3.9%, n=8, P<0.05). Similarly both chromanol 293B (10u2003μM) and L‐735,821 (100u2003nM) produced little increase in papillary muscle APD (less than 7%) while pacing at cycle lengths between 300 and 5000u2003ms. In contrast, E‐4031 (100u2003nM) markedly increased (30u2003–u200360%) APD in a reverse frequency‐dependent manner. In ventricular myocytes, the same concentrations of chromanol 293B (10u2003μM), L‐735,821 (100u2003nM) and E‐4031 (1u2003μM) markedly or totally blocked IKs and IKr, respectively. IKs tail currents activated slowly (at +30u2003mV, τ=888.1±48.2u2003ms, n=21) and deactivated rapidly (at −40u2003mV, τ=157.1±4.7u2003ms, n=22), while IKr tail currents activated rapidly (at +30u2003mV, τ=35.5±3.1u2003ms, n=26) and deactivated slowly (at −40u2003mV, τ1=641.5±29.0u2003ms, τ2=6531±343, n=35). IKr was estimated to contribute substantially more to total current density during normal ventricular muscle action potentials (i.e., after a 150u2003ms square pulse to +30u2003mV) than does IKs. These findings indicate that block of IKs is not likely to provide antiarrhythmic benefit by lengthening normal ventricular muscle QTc, APD, and refractoriness over a wide range of frequencies.

Cardiac effects of non-depolarizing neuromuscular blocking agents pancuronium, vecuronium, and rocuronium in isolated rat atria

Pancuronium, vecuronium, and rocuronium produce different cardiac effects. Using spontaneously beating right and electrically stimulated left rat atria, while measuring developed force, effective refractory period, and heart rate, we determined and compared the concentration-dependent cardiac effects of the compounds. The preparations were exposed to five progressively increasing concentrations of these compounds (10(-9), 10(-8), 10(-7), 10(-6), and 10(-5) mol/L). Pancuronium increased heart rate; vecuronium and rocuronium produced positive inotropic effects; and vecuronium shortened refractoriness. These effects may be the result of a blockade of the M2 muscarinic receptors. However, the concentrations required to produce changes were higher than those observed in patients under neuromuscular blockade.