Quantitative changes in the capillary bed during developing, peak, and stabilized cardiac hypertrophy in the spontaneously hypertensive rat.

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Abstract
The effects of developing, peak and stabilized cardiac hypertrophy on the capillary bed of spontaneously hypertensive rats was studied. Wistar-Kyoto rats served as controls. Histological measurements were based on 1.5-.mu.m cross sections of left ventricular specimens from perfuse-fixed hearts arrested in diastole. The decline in capillary density, in general, paralleled the decrease in capillary surface area which was lowest at peak hypertrophy (7 mo.) in spontaneously hypertensive rats. Between 7-15 mo. cardiocyte diameter remained constant and capillary density increased to Wistar-Kyoto rat values. Radioautographic data showed that in 12-mo.-old spontaneously hypertensive rats, 3H-thymidine labeling of capillary nuclei was > 2-fold higher than in Wistar-Kyoto rats. Mean capillary diameter increased in both spontaneously hypertensive and Wistar-Kyoto rats between 1-2.5 mo. but remained constant thereafter. Whereas multiple regression analysis showed that capillary density is the major determinant of capillary surface area, significant changes were detected earlier in capillary surface area than in capillary density. Although peak cardiac hypertrophy affected a 12.5% capillary density decrement in spontaneously hypertensive rats, capillary surface area was 24% lower than in Wistar-Kyoto rats. This substantial decrease in capillary surface area was associated with only a 1.1-.mu.m decrease in minimal intercapillary distance. Since mean capillary diameter was similar in the 2 strains, the drop in capillary surface area during peak hypertrophy apparently was due to a relative decrease in anastomotic and branching capillaries, as well as the decline in capillary density. These data provide morphological evidence that: whereas capillary density is, in general, a good predictor of capillary surface area, the latter is a more sensitive measure of capillarity than the former; the decrement in capillary supply to a cardiocyte during peak, but moderate, hypertrophy is probably substantially greater than that estimated by capillary density; and capillary growth during stabilized hypertrophy is sufficient to reverse the decrements in capillary surface area, capillary density and the increase in minimal intercapillary distance.