Alterations in population dynamics of arterial smooth muscle cells during atherogenesis: I. Activation of interphase cells in cholesterol-fed swine prior to gross atherosclerosis demonstrated by “postpulse salvage labeling”

The effect of a high-cholesterol diet on the cell dynamics of swine aortic smooth muscle was investigated by pulse labeling in vivo with 8H-thymidine autoradiography, and the determination of nuclear grain-count distributions. The shift in grain-count distributions with time after labeling was analyzed with the aid of a mathematical model. The conclusions inferred were that: (1) the initially labeled cells virtually all completed their partial cell cycle from S phase through mitosis within 2 days, and gave no evidence of further division up to 30 days; (2) control and cholesterol-fed animals did not differ in this respect; and (3) the known effect of cholesterol diet on labeling index is accounted for by newly recruited G 0 cells or by acceleration of slowly moving G 1 cells. This was demonstrated by postpulse labeling, presumably resulting from reutilization of label salvaged from dead cells in the intestine and elsewhere which continued for at least 7 days. These conclusions were supported by estimation of the G 2 population withthe aid of microspectrophotometry, by a detailed analysis of grain counts and mitoses during the first 2 days with the help of successive 5-hour colchicine collections, and by a comparison of labeling in carotid artery cells in which one artery was excluded from the systemic circulation during the initial pulsing period and subsequently readmitted to the circulation. Since no observations were made beyond 30 days, it was not possible to decide whether the diet ultimately did affect the generation time of the pulse-labeled cells, nor whether the cells newly recruited by the diet resulted from activation of G 0 cells or from acceleration of G 1 cells. However, it was possible with the help of preliminary data on the S period and some further mathematical analysis to estimate a range of values for the generation time and for the fraction of cells in G 0.