The importance of the endothelial nitric oxide synthase on the release of 6-nitrodopamine from mouse isolated atria and ventricles and their role on chronotropism.

6-nitrodopamine (6-ND) is released from rat isolated atria, where it acts as a potent positive chronotropic agent. The release of 6-ND from rat isolated atria and ventricles is significantly reduced when pre-incubated with l-NAME, and the release was not affected by tetrodotoxin pre-treatment, indicating that in the heart, the origin of 6-ND is not neurogenic. Since l-NAME inhibits all three isoforms of NO synthase, it was investigated the basal release of 6-ND from isolated atria and ventricles from nNOS, iNOS and eNOS mice of either sex. The release of 6-ND was measured by LC-MS/MS. There were no significant differences in the 6-ND basal release from isolated atria and ventricles from male control mice, as compared to female control mice. The 6-ND release from atria obtained from eNOS mice was significantly reduced when compared to atria obtained from control mice. The 6-ND release in nNOS mice was not significantly different compared to control animals whereas the 6-ND release from atria obtained from iNOS mice was significantly higher when compared to control group. Incubation of the isolated atria with l-NAME caused a significant decrease in the basal atrial rate of control, nNOS, and iNOS mice, but not in eNOS mice. The results clearly indicate that eNOS is the isoform responsible for the synthesis of 6-ND in the mice isolated atria and ventricles and supports the concept that 6-ND is the major mechanism by which endogenous NO modulates heart rate.