Altered electrical response to caffeine exposure in hypertrophied rat myocardium

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Abstract
We investigated the electrophysiological effects of cardiac hypertrophy induced by different experimental models. Comparison of the action potentials of hypertrophied and control rat hearts reveals a pronounced prolongation of the action potential for all types of hypertrophy. This prolongation affects the entire repolarization phase of the action potential 8 days after severe aortic constriction, after 8 days of isoproterenol treatment (5 mg/kg per day), and 3 months after an aortocaval fistula. The electrical changes associated with myocardial hypertrophy induced by pressure overload (aortic constriction) were compared with those resulting from volume overload (aortocaval fistula). Our results show that action potential alterations depend on the nature, duration, and severity of the work load. Thus, pressure overload is more potent to induce these modifications. In the hearts subjected to pressure overload, action potential alterations appear more rapidly and are more marked for the same degree of hypertrophy than those of the volume-hypertrophied myocardium. Furthermore, such data also demonstrate that the early alteration of the action potential during the development of compensatory hypertrophy is a prolongation of the later phase of repolarization (phase 3), without prolongation of the other repolarization phases (1 and 2). This change appears 3 days after aortic constriction, 1 month after coronary artery ligation (in the healthy part of the left ventricle), and 1 month after an aortocaval fistula. In the rat heart, the ionic currents underlying this later phase of repolarization are known to be dependent on the increase of intracellular free calcium during activity. Since this elevation of myoplasmic calcium results from calcium release from the sarcoplasmic reticulum, we compared the effects of caffeine (a substance known to act on the sarcoplasmic reticulum) on the electrical events of hypertrophied and control hearts. Caffeine has a different qualitative effect in hypertrophied and control hearts. The results suggest that hypertrophy should be due to increased calcium release from the sarcoplasmic reticulum related to increased phosphoinositide hydrolysis and therefore that the prolongation of the action potential duration (phase 3) may be the result of an increase of the Na–Ca exchange current or of a specific suppression of the outward K+ current.Key words: aortic stenosis, isoproterenol treatment, coronary artery ligation, aortocaval fistula, action potential, caffeine.