"arginine paradox" in cardiomyocytes of Sprague Dawley and spontaneously hypertensive rats: α₂-adrenoreceptor-mediated regulation of L-type Ca²⁺ currents by L-arginine

The "arginine paradox" in cardiomyocytes isolated from the left ventricle of Spraque Dawlay (SD) and spontaneously hypertensive rats (SHR) was studied. With 1 mM L-arginine in the bath, the addition of 5 mM L-arginine to incubation medium increased NO production and inhibited amplitude of L-type Ca²⁺ currents in SD cardiomyocytes. A variety of compounds, including the antagonist of α₂-adrenoceptors yohimbine and inhibitors of PI3 kinase (wortmanine), NO synthase (7NI), and cGMP-dependent protein kinase (KT5823), dramatically weakened the inhibitory effects of 5 mM L-arginine on Ca²⁺ currents. The agonist of α₂-adrenoceptors guanabenz acetate increased NO production and inhibited Ca²⁺ currents, while wortmanine, 7NI, and KT5823 antagonized guanabenz. In SHR cardiomyocytes, the "arginine paradox" was not observed: 5 mM L-arginine affected neither NO production nor Ca²⁺ currents. Consistently, guanabenz acetate did not alter NO production and inhibited Ca²⁺ currents to a much smaller extent in SHR cardiomyocytes as compared to SD cardiomyocytes. Taken together, the data of the inhibitory analysis suggest that millimolar L-arginine serves as an agonist of α₂-adrenoceptors, which are coupled to PI3K-Akt pathway as well as downstream NO-cGMP pathway to control activity of L-type Ca²⁺ channels, thus providing new insights into the "arginine paradox" in cardiomyocytes.