EFFECT OF ISCHEMIC PRECONDITIONING OF THE MYOCARDIUM ON CAMP

Reduction of cAMP has been implicated in the protection of ischemic pr econditioning (IP), but until now, this possibility has not been direc tly addressed. In this study, we found that in the in vivo rabbit hear t, 10 to 30 minutes of sustained regional ischemia was accompanied by a nearly twofold rise in cAMP levels. This increase in cAMP was attenu ated when sustained ischemia was preceded by IP induced with a single cycle of transient ischemia and reperfusion (TI/R) and prevented when ischemia was preceded by three cycles of TI/R. The mechanism of cAMP r eduction by IP does not involve activation of protein kinase C (PKC), since the PKC inhibitor polymyxin B (24 mg/kg) did not raise cAMP leve ls during sustained ischemia in IP hearts. Furthermore, this effect is also not mediated by reduced responsiveness of the beta- adrenergic e ffector pathway, since both nonischemic hearts and hearts subjected to three cycles of TI/R exhibited similar increases in cAMP in response to 5 mu g/kg isoproterenol. However, propranolol (0.75 mg/kg) abolishe d the rise in cAMP levels observed during sustained ischemia in contro l hearts but did not reduce cAMP levels further in IP hearts. These da ta indicate that the ischemia-induced rise in cAMP levels in control h earts was mediated by activation of the beta-adrenergic receptor. Take n together with data demonstrating that beta-adrenergic responsive-nes s was not affected by IP, these data support the conclusion that the l ack of elevation in cAMP levels observed during sustained ischemia in IP hearts is mediated by an attenuation of norepinephrine release. To examine whether the protection of IP against necrosis was mediated by the lack of elevation in cAMP levels, we determined whether the infarc t size-limiting effect of IP could be blocked by NKH477, an activator of adenylyl cyclase. Four groups of rabbits were subjected to 30 minut es of in vivo regional ischemia and 90 minutes of reperfusion. Control hearts (n=10) had 53.6+/-5.5% infarction of the area at risk. IP with three cycles of transient ischemia limited infarct size to 3.2+/-1.3% (n=13, P<.0001). NKH477 (45 mu g/kg) increased average cAMP levels in IP hearts during sustained ischemia to levels similar to those in unt reated control hearts. However, NKH477 did not block IP (50.2+/-7.7% o f the area at risk was infarcted in the control + NKH477 group [n = 10 ] versus 10.0+/-5.9% in the IP+NKH477 group [n=7], P<.05). Therefore, we conclude that although IP lowers cAMP levels during sustained ische mia, this effect is not necessary for its protection against necrosis, since raising cAMP does not block this protection of IP.