Drug-induced prolongation of the QT interval, in humans, is characterised by the block of the rapidly activating voltage-dependent potassium (I_Kr) channel, which is the product of the ether-a-go-go (ERG) gene. Compounds that cause QT prolongation are currently assessed in vitro using hERG cell lines and isolated Purkinje fibre. These methods are not ideal because working on Purkinje fibres gives results very slowly and only one ion channel type can be measured in the hERG cell line. In contrast, myocytes allow the measurement of action potentials (AP) and their underlying ion channels. The aim of this study was to assess and compare the effect of known hERG channel blockers on I_Kr and AP duration in guinea-pig myocytes.

Adult guinea-pigs were humanely killed and ventricular myocytes were isolated by enzymatic digestion. The cells were continuously perfused with Tyrode solution at 33-35 °C containing (mM): 135 NaCl; 6 KCl; 0.33 NaH₂PO₄; 5 sodium pyruvate; 10 glucose; 10 Hepes; 1 MgCl₂; 2 CaCl₂; pH 7.4. Recordings were made using the whole-cell patch-clamp technique. The pipette solution contained (mM): 140 KCl; 10 Hepes; 1 MgCl₂; 5 BAPTA; 5 ATP; 0.1 ADP; pH 7.2. APs were elicited at a frequency of 0.2 Hz under current clamp. APD₉₀ values were measured from the upstroke to 90 % repolarisation. pIC₅₀ values were determined from plots of percentage increase of APD₉₀ against inhibitor concentration. Voltage clamp was used to study the effect of the drugs on I_Kr and calcium (I_Ca) channels.

The mean control APD₉₀ was 439.1 ± 20.1 ms (mean ± S.E.M.; n = 22). Dofetilide increased APD₉₀ in a concentration-dependent manner; at 1 mM the mean APD₉₀ was 854.1 ± 123.1 ms (n = 5). The pIC₅₀ was 7.3 ± 0.2 (n = 8). Dofetilide (1 mM) had no effect on I_Ca. Similarly, E-4031 increased APD₉₀ in a concentration-dependent manner; pIC₅₀ was 7.2 ± 0.02 (n = 4). The mean APD₉₀ at 1 mM E-4031 was 728.4 ± 41.3 ms (n = 3). In contrast, 10 mM cetirizine, inactive at the hERG channel, had little effect on AP duration or Kr channels. Mean APD₉₀ values in the absence and presence of the drug were 410.3 ± 48.8 and 455.5 ± 47.1 ms, respectively (n = 5).

The potency values measured thus far are comparable to other preparations such as the hERG cell line and Purkinje fibre. The compounds also exhibited similar effects to those observed in telemetry studies on man. In conclusion, these findings warrant further investigation into the possible use of myocytes for screening compounds with potential QT activity.

All procedures accord with current UK legislation.