This study examined whether cicletanine, an antihypertensive drug with cGMP phosphodiesterase inhibitory effect, could alleviate ventricular overdrive pacing-induced myocardial ischemia in chronically instrumented rabbits.
An electrode-catheter implanted into the right ventricle was used for pacing (500 bpm over 5 min) and for measuring intracavital ST-segment elevation and ventricular effective refractory period (VERP). PQ and QT intervals were measured in the chest-lead ECG, and dP/dtmax as well as left ventricular end-diastolic pressure (LVEDP) were recorded through a left intraventricular catheter. In separate groups, mean arterial blood pressure (MABP) was monitored from the right carotid artery. Experiments were performed on conscious rabbits after a week of convalescence. In anesthetized, open-chest rabbits, samples were taken from the left ventricle before and after drug treatment and/or overdrive pacing for determination of cGMP and cAMP contents by radioimmunoassay.
Intravenous cicletanine, 30 mg/kg body weight, did not change resting MABP, dP/dtmax, and LVEDP, but it did reduce heart rate and prolonged PQ and QT intervals and VERP. Overdrive pacing produced intracavital ST-segment elevation, increased LVEDP, and decreased dP/dtmax and MABP. Cicletanine administered 15 minutes before pacing significantly attenuated ST-segment elevation, increased LVEDP, and decreased dP/dtmax and MABP. In anesthetized animals, cicletanine itself slightly increased cardiac cGMP and cAMP contents. Overdrive pacing moderately increased cGMP and profoundly elevated cAMP, and in overpaced rabbits, cicletanine further increased cGMO and markedly attenuated cAMP content increased by overdrive pacing.
These results suggest that in correlation with alterations of cardiac cycle nudeotide contents, cicletanine protects the heart against pacing-induced myocardial ischemia.
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