Positive inotropic effects by uridine triphosphate (UTP) and uridine diphosphate (UDP) via P2Y2 and P2Y6 receptors on cardiomyocytes and release of UTP in man during myocardial infarction

The aim of this study was to examine a possible role for extracellular pyrimidines as inotropic factors for the heart. First, nucleotide plasma levels were measured to evaluate whether UTP is released in patients with coronary heart disease. Then, inotropic effects of pyrimidines were examined in isolated mouse cardiomyocytes. Finally, expression of pyrimidine-selective receptors ( a subgroup of the P2 receptors) was studied in human and mouse heart, using real time polymerase chain reaction, Western blot, and immunohistochemistry. Venous plasma levels of UTP were increased (57%) in patients with myocardial infarction. In electrically stimulated cardiomyocytes the stable P2Y(2/4) agonist UTP gamma S increased contraction by 52%, similar to beta(1)-adrenergic stimulation with isoproterenol (65%). The P2Y(6)-agonist UDP gamma S also increased cardiomyocyte contraction (35%), an effect abolished by the P2Y(6)-blocker MRS2578. The phospholipase C inhibitor U73122 inhibited both the UDP beta S and the UTP gamma S-induced inotropic effect, indicating an IP3-mediated effect via P2Y(6) receptors. The P2Y(14) agonist UDP-glucose was without effect. Quantification of mRNA with real time polymerase chain reaction revealed P2Y(2) as the most abundant pyrimidine receptor expressed in cardiomyocytes from man. Presence of P2Y(6) receptor mRNA was detected in both species and confirmed at protein level with Western blot and immunohistochemistry in man. In conclusion, UTP levels are increased in humans during myocardial infarction, giving the first evidence for UTP release in man. UTP is a cardiac inotropic factor most likely by activation of P2Y(2) receptors in man. For the first time we demonstrate inotropic effects of UDP, mediated by P2Y(6) receptors via an IP3-dependent pathway. Thus, the extracellular pyrimidines ( UTP and UDP) could be important inotropic factors involved in the development of cardiac disease.