Effects of Sequential Ischemia-reperfusion Cycles on Cyclic Nucleotide Phosphodiesterase Activity in Pig Heart

Abstract

Inhibition of cyclic nucleotide phosphodiesterase (PDE) activities is one of the prime causes of cAMP accumulation in ischemic myocardium [TP et al. 1994. JMCC 26: 486]. Cyclic nucleotides are among several mediators that may afford protection during myocardial ischemia. The purpose of the present investigation was to examine whether, and if so, how, successive ischemia-reperfusion cycles affect PDE activities and cAMP levels of pig myocardium. Regional myocardial ischemia was produced by occluding the anterior descending coronary artery half-way from its origin in anesthetised open-chest pigs. Reperfusion was effected by releasing the snare. Pig hearts were subjected to either 5, 10 or 15 min ischemia followed by variable durations (5, 15, 30, 120min) reperfusion and a subsequent ischemia-reperfusion of 15-15 min or 90-30 min. Serial drill biopsies were obtained from left ventricular myocardium during and following ischemia and were immediately frozen in liquid nitrogen. Specimens were analyzed for PDE activity and cAMP content. Severe reduction of perfusion flow caused progressive inhibition of myocardial PDE activities, with approx. 15, 35, 56 and 82% inhibition after 5, 10, 15 and 30 min ischemia, resp. Inhibition of PDEs was readily reversed by reperfusion. With short ischemic periods (up to 15 min tested), recovery was complete (100%) within 5 min. During a subsequent ischemia, inhibition of PDEs was attenuated. The extent of attenuation depended on both, the duration of the antecedent ischemia and the duration of reperfusion. The longer the antecedent ischemia lasted (5, 10 and 15min tested), the less inhibition occurred during a subsequent ischemia. The optimal durations of reperfusion were 15 or 30 min. Short (5 min) and long (120 min) reperfusions were less effective. Of the protocols tested, a single ischemia-reperfusion cycle of 15–30 min duration reduced the inhibition of PDE activities during a subsequent ischemia maximally (approx. 50% compared to control hearts). There was an inverse relationship between PDE activity and cAMP content. Accumulation of cAMP was marked during the first ischemia (biphasic increase with maxima at 5 and 30 min) and was blunted during the following ischemia. In hearts subjected to an antecedent 15–30 min ischemia-reperfusion cycle, the increase of cAMP during a subsequent ischemia was barely significant. PDE activities and cAMP content of non-ischemic myocardium remained constant throughout the occlusion-reperfusion cycles. The present findings show that temporary reductions of blood flow inhibited PDE activities of pig myocardium differentially. Whereas PDEs were strongly inhibited during the first ischemia, inhibition was attenuated during subsequent ischemic episodes, with reciprocal effects on cAMP levels. These findings support, but do not prove, a role of PDEs in myocardial protection during ischemia, with possibly dual effects. First, cAMP accumulating during the first ischemia may act as a signal and memory effect alike. Secondly, a preceding ischemia-reperfusion cycle may limit injury during a subsequent ischemia by preserving PDE activities, attenuating increase of cAMP and impeding cyclic nucleotide mediated signal transduction. This may mitigate stimulation of the myocardium at risk, like cardiac denervation or beta-adrenoceptor blocking agents. However, a preceding ischemia-reperfusion cycle may be more effective, as it attenuates the two main causes of cAMP accumulation, activation of adenylyl cyclase [TP et al. 1996. JMCC 28: 293] and inhibition of PDEs.