TYROSINE-HYDROXYLASE-IMMUNOREACTIVE AND NITRIC-OXIDE-SYNTHASE-IMMUNOREACTIVE NERVE-FIBERS IN MITRAL-VALVE OF YOUNG-ADULT AND AGED FISCHER-344 RATS

Using confocal fluorescence microscopy we studied, in whole mounts of heart mitral valves of young adult and aged Fischer 344 rats, the dist ribution of nerves containing the catecholamine marker tyrosine hydrox ylase (TH) or the synthetic enzyme marker for nitric oxide, nitric oxi de synthase (NOS). TH-IR was localized in two separate nerve plexuses which do not intermingle. The 'major' plexus arose from the annulus re gion, traversed the basal zone of the valve, and ramified in the inter mediate zone to form a dense network of fine fibers. The 'minor' plexu s was restricted to the distal zone and originated from bundles that a scended the chordae tendineae to enter the valve cusp. A concentric zo ne located between the major and minor plexuses was devoid of TH-TR ne rve fibers. Both plexuses demonstrated (i) nerves that contained numer ous varicosities along the length of each fiber, (ii) many terminal ax ons and (iii) different shaped terminal axon endings. With age, the de nsity of TH-IR innervation in the mitral valve was markedly reduced; a nd nerve fibers of the minor plexus were limited to the chordae tendin ae, without extending into the valve cusp itself. NOS-IR fibers in the mitral valve formed a loose network that extended from the annulus to more than halfway down the cusp. The varicose beads of the terminal N OS-IR axons appeared to become progressively smaller and less intensel y fluorescent until they disappeared at the terminal endings, which sh owed no specializations. No NOS-IR fibers were observed in the distal zone of the valve leaflet or in the chordae. In the aged mitral valve, the density of NOS-IR nerves was decreased, as compared with NOS-IR i nnervation in the young adult valve. The existence of TH and NOS as we ll as other signal molecule markers in heart valve nerves and the disp arate patterns of their distribution and localization provide evidence supporting the theory that heart valve nerves form a complex reflexog enic control system in the mitral heart valve. In summary, two distinc t neural architectures are described for TH-IR and NOS-IR valve nerves , respectively. The former are believed to be axons dedicated to sympa thetic motor functions. The NOS-IR valve nerves may have sensory and/o r postganglionic parasympathetic motor functions. An implication of th ese findings is that different, but perhaps related, valve functions m ay be mediated by separate, dedicated circuits.