Angiotensin II binding sites in the conduction system of rat hearts

Angiotensin II binding sites were localized and quantified in the conduction system of the rat heart by autoradiography in combination with computerized microdensitometry. Tissue sections (16-microns thick), containing cardiac vagus ganglia, sinus node, and atrioventricular node, were incubated with 125I-Sar1-angiotensin II to generate autoradiograms that were compared with other autoradiograms from 125I-labeled standards. Angiotensin II binding sites were highly localized in both the sinoatrial and atrioventricular nodes and in the cardiac vagus ganglia. In contrast, binding to the angiotensin converting enzyme, determined by incubation of adjacent tissue sections with the specific enzyme inhibitor 125I-351A, was very low in these areas but high in the cardiac endothelium. Our results suggest that angiotensin II may have direct chronotropic effects through stimulation of specific receptors in the conduction system of the heart and in intrinsic parasympathetic ganglia.