Heart size in inbred strains of rats. Part 1. Genetic determination of the development of cardiovascular enlargement in rats.

The heart and aorta weights in 23 strains of rats and the four-way cross generation among the M520/N, SHRSP/N, SHR/N, and WKY/N strains were investigated in relation to their blood pressure in an attempt to characterize cardiovascular enlargement (increased weight of heart and aorta) from a genetic aspect. The distribution of blood pressure in these strains at 10 weeks of age was clearly divided into hypertensive and normotensive groups. In the hypertensive group, heart weight increased in proportion to blood pressure. In contrast, there was no relationship between blood pressure and heart weight in the normotensive group in spite of large strain differences in heart weights. The result of variance analysis exhibited a significant strain difference in heart weight, and the degree of genetic determination was estimated to be 65%-75%. A similar genetic influence was apparent for normotensive strains excluding hypertensive strains. The distribution of blood pressures in the four-way cross generation showed the segregation of three phenotypes consisting of normotensive, intermediate and hypertensive groups. A large variability was seen in heart weight of each group. However, the increase in average heart weight of these three groups was very small. The degree of genetic determination from the cross analysis was estimated to be 45%-65%. These results indicate that heart weight is a highly heritable trait, and that the effect of genetic factors on cardiac enlargement is larger than that of blood pressure. A similar result was obtained for the aorta weight. However, the effect of genetic factors was less important for aorta weight than for heart weight since the degree of genetic determination was estimated to be 45%-65% from the strain comparison and 35%-60% from the cross analysis.