Microvascular oxygen tension in the rat mesentery

Extracellular purines and pyrimidines have major effects on cardiac rhythm and contraction. ATP/UTP are released during various physiopathological conditions, such as ischemia, and despite degradation by ectonucleotidases, their interstitial concentrations can markedly increase, a fact that is clearly associated with arrhythmia. In the present whole cell patch-clamp analysis on ventricular cardiomyocytes isolated from various mammalian species, ATP and UTP elicited a sustained, nonselective cationic current, I(ATP). UDP was ineffective, whereas 2'(3')-O-(4-benzoylbenzoyl)-ATP was active, suggesting that P2Y(2) receptors are involved. I(ATP) resulted from the binding of ATP(4-) to P2Y(2) purinoceptors. I(ATP) was maintained after ATP removal in the presence of guanosine 5'-gamma-thio]triphosphate and was inhibited by U-73122, a PLC inhibitor. Single-channel openings are rather infrequent under basal conditions. ATP markedly increased opening probability, an effect prevented by U-73122. Two main conductance levels of 14 and 23 pS were easily distinguished. Similarly, in fura-2-loaded cardiomyocytes, Mn(2+) quenching and Ba(2+) influx were significant only in the presence of ATP or UTP. Adult rat ventricular cardiomyocytes expressed transient receptor potential channel TRPC1, -3, -4, and -7 mRNA and the TRPC3 and TRPC7 proteins that coimmunoprecipitated. Finally, the anti-TRPC3 antibody added to the patch pipette solution inhibited I(ATP). In conclusion, activation of P2Y(2) receptors, via a G protein and stimulation of PLCbeta, induces the opening of heteromeric TRPC3/7 channels, leading to a sustained, nonspecific cationic current. Such a depolarizing current could induce cell automaticity and trigger the arrhythmic events during an early infarct when ATP/UTP release occurs. These results emphasize a new, potentially deleterious role of TRPC channel activation.