An ultrastructural and immunocytochemical study of thoracic aortic endothelium in aged Sprague-Dawley rats

The distribution of various vasoactive agents [nitric oxide synthase (NOS)- type I, endothelin-1 (ET-1), arginine-vasopressin (AVP), serotonin (5-HT), histamine and substance P (SP)] in the thoracic aortic endothelium of aged Sprague-Dawley rats was investigated using electron microscopic immunocytochemical methods. The aged thoracic aortic intima was characterized by a large number of leukocytes that adhered to the endothelium, an accumulation of a flake-like precipitate and clusters of leukocytes and smooth muscle cells (SMC) in the subendothelium. Age-associated alterations were also seen in the medial and adventitial layers of the vascular wall. An extensive vasa-vasorum was present in the adventitia from which leukocytes penetrated into perivascular tissue. Some vasa-vasorum showed mast cells adhered to perivascular pericytes. Immunocytochemistry showed about 70% endothelial cells (EC) with positive immunostaining for the brain isoform NOS-type I, compared to 10% in adult mature rats. About 10% of cells showed a positive immunoreaction for ET-1, which is about the same as for the mature adult thoracic aorta (8-9%). Subendothelial macrophages often showed positive immunostaining for antibodies against ET-1. The percentage of EC immunopositive to AVP, 5-HT, and histamine was 16-18, 15 and 12%, respectively compared to 5-8, 7-8 and 6% in mature adult rats. A few cells showed an immunopositive reaction for SP. In summary, the ageing vessel was characterized by a large number of leukocytes adhering to the endothelium and also by the presence of many macrophages and SMC in the subendothelial layer. The percentage of EC in rat thoracic aorta showing NOS immunostaining increased substantially from 10% in mature rats to 70% in aged rats. The percentage of EC immunopositive for AVP, 5-HT and histamine also increased about twofold compared to mature adult rats, while no changes were seen for ET-1.