Early ultrastructural changes in choroidal arterioles of spontaneously hypertensive rats.

Ultrastructural changes in the vascular smooth muscle cells of spontaneously hypertensive rats (SHR) aged 7-9 months were studied in the long posterior ciliary artery (LPCA) and they were compared with the findings in the same aged control Wistar Kyoto rats (WKY). The systolic blood pressure was higher than 200 mmHg in the SHR at the postnatal fourth month. The muscle cells in the media of the LPCA in the SHR could be divided into three types according to the electron density of their sarcoplasm, i.e., dark, intermediate or light cells. The electron density depended on the content of actin filaments; the number of the filaments per one micron 2 was determined in the specimens where cross sections of the filaments prevailed, and it averaged 48.7 in the dark cells, 34.1 in the light cells. In WKY, all the muscle cells showed the intermediate density and the number of actin filaments averaged 37.9 per micron 2. It was also observed that the muscle cells migrated into the intima and they were called the intimal cells. These cells in SHR appeared as an aggregated mass underneath the endothelium, but in WKY the individual intimal cells were scattered. These findings were thought to be early ultrastructural changes in response to a rapid elevation of the arterial pressure.