1. Monophasic action potentials (MAPs) were recorded simultaneously from the endocardium and the epicardium in open-chested dogs (n = 16) during bolus intravenous injections of potassium chloride at 0.1 mmol/kg and 0.2 mmol/kg. MAP duration was measured at 90% repolarization. Local conduction was measured as the delay between the pacing artefact and upstroke of the MAP.
2. The animals were anaesthetized with α-chloralose and urethane. Heart block was created by atrioventricular nodal ablation with 0.1% (v/v) formalin. Infra-nodal pacing was established.
3. Bolus injections of potassium chloride at 0.1 mmol/kg shortened MAP duration. The effect was maximal at 15 s after the injection [epicardium: 171.5 ± 3.0 (control) and 148.9 ± 5.2 ms at 15 s (P < 0.001); endocardium: 178.7 ± 4.4 (control) and 165.6 ± 5.3 ms (P < 0.001)]. The shortening on the epicardium was significantly greater than on the endocardium (P < 0.05).
4. Local conduction time showed no significant change [epicardium: 37.8 ± 1.8 (control) and 41.2 ± 2.4 at 15 s; endocardium: 41.1 ± 1.6 (control) and 45.0 ± 3.4 at 15 s].
5. After β-adrenoceptor blockade, MAP duration shortened, exhibiting a maximum effect at 20 s after a bolus injection of potassium chloride at 0.1 mmol/kg [epicardium: 184.7 ± 7.1 (control) and 130.7 ± 9.0 ms at 20 s (P < 0.001); endocardium 192.0 ± 7.4 (control) and 148.6 ± 10.8 ms at 20 s (P < 0.001)]. The greater shortening observed on the epicardial surface did not reach statistical significance.
6. Local conduction time after β-adrenoceptor blockade was prolonged, reaching a maximum at 20 s [epicardium: 41.2 ± 4.8 (control) and 65.9 ± 8.6 ms at 20 s: endocardium: 47.5 ± 7.4 (control) and 87.3 ± 13.1 ms at 20 s].
7. Doubling of the dose of potassium chloride to 0.2 mmol/kg was usually terminal and often showed bizarre electrophysiological responses with striking regional differences.
8. Our results show that bolus injections of potassium chloride exert a differential influence on the time course of epicardial and endocardial repolarization by an effect on MAP duration rather than on local conduction time. After β-adrenoceptor blockade the effect of a bolus injection of potassium chloride on MAP duration was greater and conduction time was increased.
