α ₁ -Adrenoceptor Activation Increases Ecto-5′-Nucleotidase Activity and Adenosine Release in Rat Cardiomyocytes by Activating Protein Kinase C

Abstract

Background Adenosine is an important regulator of many cardiac functions and is synthesized primarily by ecto- and cytosolic 5′-nucleotidase. We have previously reported that α 1 -adrenoceptor blockade attenuates adenosine release from ischemic myocardium, raising the possibility that α 1 -adrenoceptor activation activates 5′-nucleotidase. This study tested whether activation of protein kinase C by α 1 -adrenoceptor activation increases 5′-nucleotidase activity and augments adenosine release. Methods and Results Cardiomyocytes were isolated from adult male Wistar rats and suspended in modified HEPES-Tyrode’s buffer solution. After stabilization, the cardiomyocytes were incubated with and without an exposure to norepinephrine (10 −9 to 10 −5 mol/L) while being treated with propranolol and yohimbine or with and without an exposure to methoxamine (10 −9 to 10 −5 mol/L). Ecto-5′-nucleotidase activity was increased by norepinephrine and methoxamine during 30 minutes in a dose-dependent manner, whereas cytosolic 5′-nucleotidase was not activated. These increases in ecto-5′-nucleotidase activity were inhibited by GF109203X, an inhibitor of protein kinase C, and mimicked by phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C. The increase in ecto-5′-nucleotidase was not prevented by cycloheximide. When ecto-5′-nucleotidase activity increased, adenosine release was augmented in methoxamine- and PMA-treated cardiomyocytes (1299±252% and 1372±149%, respectively) compared with the untreated group (578±26%). The increase in adenosine release was blunted by GF109203X and α,β-methyleneadenosine 5′-diphosphate, an inhibitor of ecto-5′-nucleotidase. Conclusions Thus, we conclude that α 1 -adrenoceptor–mediated increases in ecto-5′-nucleotidase activity are attributed to activation of protein kinase C in rat cardiomyocytes.