The origin of nitergic nerve terminals in the guinea-pig heart

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The origin of nerve fibers projecting to the guinea pig heart that contain nitric oxide synthase (NOS) was studied by unilateral cervical vagotomy. Three kinds of NOS-immunoreactive (NOS-ir) nerve fibers are distributed in the control guinea pig heart : the sparse network covering the right atrium, the basket-like endings around intracardiac neuronal cell bodies in the small ganglia located in the left atrium and the interatrial septum, and the axons situated in the septal region. The sparse network in the right atrium did not change after vagotomy of right or left side. In the wholemount preparations of right atrium, we often traced labeled axons from the somata to join the network covering the right atrium. Therefore, most of these networks of labeled fibers must be of intrinsic origin. Because the basket-like endings around neuronal cell bodies in the ganglia in the left atrium and the septum disappeared completely after vagotomy of left side, they seem to be parasympathetic preganglionic fibers come from the left vagus nerve. NOS-ir cell bodies and the positive fibers in the atrioventricular nodal region survived after vagotomy. All of such nerve fibers were unmyelinated axons. Therefore, they seem to be the postganglionic fibers arising from the ganglia located in the left atrium or the septum.