Aortic actomyosin content of maturing normal and spontaneously hypertensive rats

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Abstract
The maximum contractile response of aorta from spontaneously hypertensive (SHR) rats is evidently reduced compared to that of normotensive controls. The maximum contractile response of aorta from Wistar-Kyoto (WKY) rats is less than that of aorta from Wistar rats. A possible explanation for these differences is that there are differences in the amount of actomyosin per smooth muscle cell. To test this hypothesis, actomyosin, total protein and DNA contents of thoracic aorta from 3, 6, 12 and 23 wk old SHR and WKY rats were determined and compared to similar values previously published by this laboratory for Wistar rats. The absolute amount of actomyosin increased with increasing age in aorta from both WKY and SHR rats, but increased more rapidly in the SHR so that by 23 wk it was significantly greater than that in the WKY. This increase was not correlated with changes in systolic blood pressure during maturation. Sufficient hyperplasia (based on DNA) was observed in the aorta of SHR relative to WKY rats, so that the amount of actomyosin per smooth muscle cell was not different between these 2 groups at any age studied. Comparison of aortas from SHR, WKY and Wistar rats indicated that Wistar aortas had more actomyosin and a greater number of cells (based on DNA) per gram of media than aortas from either SHR or WKY rats, but at any age, all 3 groups had the same amount of actomyosin per smooth muscle cell. It appears that the lower maximum contractile response of aorta from SHR rats is not due to a reduction in the amount of actomyosin per smooth muscle cell, there are biochemical differences in aorta between subspecies of rats, and the WKY rat may be a better control for studies involving SHR rats because the biochemical characteristics of their aortas are similar.