Differential Effects of Antihypeftensive Drug Therapy on Vascular Smooth Muscle Cell Hypertrophy, Hyperploidy, and Hyperplasia in the Spontaneously Hypertensive Rat

The present report extends our previous studies of smooth muscle cell hypertrophy, hyperploidy, and hyperplasia in the 5-month-old spontaneously hypertensive and Wistar-Kyoto rats to include analyses of 3- and 7-month-old rats and explores the effects of antihypertensive drug treatment on the accelerated growth of vascular smooth muscle in aortas of spontaneously hypertensive vs. Wistar-Kyoto rats. Drug-treated rats were administered a combination of reserpine, hydralazine, and chlorathiazide in their drinking water, either between 3 and 5 months or between 5 and 7 months of age. Drug treatment decreased the blood pressure of spontaneously hypertensive rats to values at or below those of Wistar-Kyoto rats for both age-treatment groups. Smooth muscle growth was evaluated by morphometric analyses of aortic smooth muscle content, flow cytometric and microdensitometric measurements of the frequency of polyploid smooth muscle cells, biochemical estimates of aortic medial smooth muscle cell number, and microdensitometric measurements of individual smooth muscle cell protein content. The following results were obtained. (1) Aortic medial smooth muscle content was not significantly increased in 3- month spontaneously hypertensive compared to Wistar-Kyoto rats, indicating that aortic smooth muscle hypertrophy occurred post-3 months, as well as after blood pressure was elevated. (2) In 5-month-old spontaneously hypertensive and Wistar-Kyoto rats, medial smooth muscle hypertrophy could be accounted for by cellular hypertrophy without hyperplasia; in contrast, medial hypertrophy in 7-month-old spontaneously hypertensive rats involved both cellular hypertrophy and hyperplasia. (3) Antihypertensive treatment prevented the accelerated growth of vascular smooth muscle that occurred in spontaneously hypertensive rats via cellular hypertrophy and hyperploidy, but it did not prevent an increase in smooth muscle cell number in spontaneously hypertensive rats between 5 and 7 months of age. Furthermore, it had no effect on the parallel increases in aortic medial smooth muscle cell number that occurred in both spontaneously hypertensive and Wistar-Kyoto rats between 3 and 5 months of age. (4) Whereas drug treatment prevented accelerated development of smooth muscle cell polyploidism in spontaneously hypertensive rats, in no case (spontaneously hypertensive or Wistar-Kyoto rats) did it reverse changes in ploidy that existed at the time of initiation of drug treatment, although it did cause cellular atrophy in smooth muscle cells of each ploidy class. (5) Reductions in vascular smooth muscle content with drug treatment were due to reductions in smooth muscle cell size, not to reductions in cell number. The differential effects of blood pressure lowering on hypertrophic vs. hyperplastic smooth muscle cell growth clearly suggests that the mediating factors for these two growth processes are different. Data support the hypothesis that hypertrophic smooth muscle cell growth represents a response to increased blood pressure or wall stress, but suggest that factors other than elevated blood pressure per se are responsible for smooth muscle cell hyperplasia.