| Abstract: | Nitric oxide (NO) and reactive oxygen species (ROS) are known to play various functional and pathophysiological roles as an intracellular messenger in the heart. In this study, we investigated whether the increased production of NO and/or ROS was involved in the cholinergic regulation of rhythmic contraction in spontaneously beating cultured cardiac myocytes from neonatal rats. Exposure of cultures to carbachol, an agonist of muscarinic acetylcholine receptors (mAchR), produced a dose-dependent decrease in the beat rate of cultured cardiac myocytes, and such a effect was significantly attenuated by pre-treatment with an NOS inhibitor, as well as an NO scavenger. In addition, exposure to an NO donor (SNAP) also decreased the beat rate dose-dependently. Carbachol- or SNAP-induced suppression of the contraction rhythm was significantly attenuated by co-treatment with 5-hydroxydecanoate (5-HD). In contrast, treatment with diazoxide decreased the beat rate dose-dependently. Carbachol treatment increased the intensity of 2',7'-dichlorodihydrofluorescein fluorescence, suggesting that the production of ROS was enhanced by the treatment. In addition, the carbachol- or diazoxide-induced suppression of contraction rhythm was attenuated by co-treatment with 2-mercaptopropionyl glycine, a scavenger of ROS. The present study has suggested that the mAchR-NO-mitoK ATP -ROS pathway is a factor responsible for carbachol-induced suppression of contraction rhythm in cultured cardiac myocytes. |
