adenosine is an important mediator of ischemic preconditioning (IPC), its relative contribution to IPC remains unknown. Because adenosine is formed through the hydrolysis of ATP, the present study investigated the role of ATP and adenosine in IPC. Isolated and buffer-perfused rat hearts underwent IPC by three cycles of 5-min ischemia and 5-min reperfusion before 25 min of global ischemia. The rate-pressure product (RPP) 30 min after reperfusion was taken as an endpoint of functional protection. Interstitial fluid (ISF) adenine nucleotides and adenosine were measured by cardiac microdialysis techniques. Inhibition of IPC-induced recovery of RPP was partial by the adenosine receptor antagonist 8-(p-sulfophenyl)theophylline (SPT; 100 M) or by the structurally distinct P2Y purinoceptor antagonists suramin (300 M) or reactive blue (RB; 10 M) but was additive when SPT was given with suramin or RB. The P2X antagonist pyridoxalphosphate-6-azophenyl-2Ј,4Ј-disulfonic acid tetrasodium (50 M) had no effect on functional protection. The improved functional recovery was not significantly affected by an ecto-5Ј-nucleotidase inhibitor, ␣,␤-methylene adenosine diphosphate (AMP-CP; 100 M), alone but was inhibited by AMP-CP plus SPT, suramin, or RB. ISF ATP and adenosine increased temporarily by 10-fold during IPC. AMP-CP augmented the increase in ISF ATP associated with the decrease in ISF adenosine. There was a reciprocal correlation between the ISF concentration of ATP and adenosine in preconditioned hearts. In addition, there was a significant correlation between ISF adenosine and ATP and the inhibitory potency of SPT and suramin or RB against functional protection conferred by IPC. These results suggest that extracellular ATP and adenosine play a complementary role in IPC through P2Y purinoceptors and adenosine receptors, respectively. cardiac microdialysis ADENOSINE is known to play an important role in mediating ischemic preconditioning (IPC) in many species, including the rabbit, pig, dog, and human (33). However, the argument against the role of adenosine as the principal mediator for IPC in the rat heart has been provided by several investigators (26, 32). In addition, although ecto-5Ј-nucleotidase plays a major role in extracellular adenosine formation during IPC (16-18), the inhibitory effect of an ecto-5Ј-nucleotidase inhibitor, ␣,␤-methylene adenosine diphosphate (AMP-CP), on myocardial protection conferred by IPC is controversial (31). Thus there is circumstantial evidence suggesting that adenosine is not the sole mediator of IPC.Extracellular ATP is a local regulator of physiological functions in the cardiovascular system. ATP is released to the interstitial space from endothelial cells by mechanical and chemical stimuli such as bradykinin, acetylcholine, and serotonin (1, 42) and from sympathetic and parasympathetic perivascular nerves (3) and cardiomyocytes (9) in response to ischemia or hypoxia. ATP was indeed found in the coronary effluent of the isolated and perfused heart during hypoxia and postischemic reper...

These results suggest that myocardial protection by low-grade ischemic preconditioning and potassium cardioplegia are mediated through enhanced translocation of protein kinase C alpha to the membrane. It is therefore suggested that activation of the novel protein kinase C isoforms is necessary to potentiate myocardial protection under potassium cardioplegia.

Extracellular ATP plays an important role in ischemic preconditioning (IPC) through the activation of P(2y) purinoceptors. This study examined whether ATP-stimulated P(2y) purinoceptors are coupled to pertussis toxin (PTX)-insensitive G protein and whether activation of this pathway enhances myocardial protection afforded by IPC. The rat was treated with PTX for 48 h, and the heart was then isolated and buffer perfused. The heart underwent IPC by three cycles of 5-min ischemia and 5-min reperfusion before 25 min of global ischemia. Isovolumic left ventricular function was measured, and functional recovery at 30 min after reperfusion was taken as an end point of myocardial protection. PTX pretreatment partially inhibited functional protection by IPC. Treatment with 100 microM 8-(p-sulfophenyl) theophylline (SPT) during IPC had no further effect on PTX-induced inhibition of functional protection by IPC, whereas suramin (300 microM) or reactive blue (RB) (10 microM) completely abolished myocardial protection in the preconditioned heart pretreated with PTX. Supplementation with adenosine (30 microM), ATP (30 microM), or UTP (50 microM) significantly enhanced IPC-induced functional protection, although preconditioning with these nucleotides without IPC had no protective effect. Adenosine-enhanced IPC was inhibited by pretreatment with PTX and SPT but not by suramin or RB, whereas ATP-enhanced IPC was inhibited by suramin or RB in combination with PTX pretreatment. On the other hand, UTP-enhanced IPC was not affected by PTX pretreatment but was inhibited by suramin or RB. Adenosine supplemented IPC without PTX pretreatment and ATP supplemented IPC with PTX pretreatment were not affected by nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (100 microM). Although the protein kinase C inhibitor Ro318425 (0.3 microM) or tyrosine kinase inhibitor genistein (50 microM) had no significant effect on the functional protection afforded by adenosine-supplemented IPC without PTX pretreatment and ATP-supplemented IPC with PTX pretreatment, the combination of Ro318425 and genistein attenuated functional protection afforded by both the purinoceptor agonist-supplemented IPC. These results suggest the crucial involvement of PTX-sensitive and -insensitive G protein coupled purinoceptors in enhanced IPC by supplementation with adenosine, ATP, and UTP.