Pyridoxal 5′-Phosphate is an ATP-Receptor Antagonist in Freshly Isolated Rat Cardiomyocytes

Abstract

Although extracellular ATP is considered to exert a positive inotropic action on the myocardium through purinoceptors, very little information is available regarding interventions which may modify the actions of ATP on the heart. We report here that pyridoxal 5′-phosphate (PLP), an active form of vitamin B6, shows antagonism towards ATP-induced positive inotropic effect in isolated perfused rat hearts, ATP-induced increase in [Ca2+] in freshly isolated adult cardiomyocytes and ATP-binding in cardiac sarcolemma; ED50for PLP in each of these cases varied from 10–15μm . PLP (5–50μm) was observed to antagonize the positive inotropic effect of ATP but did not modify the action of isoproterenol in the isolated perfused heart. Preincubation of cardiomyocytes with 1–50μm PLP prevented the ATP-induced increase in [Ca2+]iin a concentration-dependent manner but showed no effect on the KCl-induced increase in [Ca2+]i. Creatine phosphate and Na2HPO4as well as vitamin B6-related compounds, such as pyridoxine, pyridoxal, 4-deoxypyridoxine and isonicotinic acid hydrazide showed no effect on the ATP-induced increase in [Ca2+]iin cardiomyocytes. Furthermore, different concentrations of PLP (1–50μm) were shown to inhibit the specific ATPγS binding at both the high and low affinity sites in the cardiac sarcolemmal membrane; adrenoceptor and Ca2+-channel inhibitors did not affect the ATP-binding. It is concluded that PLP may antagonize the actions of ATP on the heart in a selective manner and both pyridoxal and phosphate moieties are essential for its action. Furthermore, it is suggested that PLP may serve as a valuable tool for monitoring the role of purinoceptors in cellular function.