Abstract
Following partial hepatectomy in rats, there were two bursts of hepatocyte DNAβsynthetic and mitotic activity which were produced by two subpopulations having different rates of (nearly synchronous) proliferative development. Only about 50% of the cells in both subpopulations could initiate DNA synthesis and enter mitosis when exposed to the hypocalcemic conditions in the parathyroprivic rat for 24 hours before partial hepatectomy. The proliferatively incompetent hepatocytes in these hypocalcemic rats could be induced to initiate their DNA synthetic and mitotic activity by an intraperitoneal injection of the calciumβmobilizing parathyroid hormone (50 USP units/100 g) as late as 12 hours after partial hepatectomy. Single intraperitoneal injections of calcium (0.25 mg/100 g) could also restore the proliferative competence of these hepatocytes, but only when injected at specific periods following partial hepatectomy. The injection of calcium 12 to 15 hours after partial hepatectomy induced hepatocytes in the first subpopulation to finish their development and enter mitosis, but did not affect the second, more slowly developing, subpopulation. Calcium had to be injected 25 hours after partial hepatectomy to stimulate proliferation in this second subpopulation. These data suggest that the hepatocytes which became proliferatively incompetent by prolonged exposure to a hypocalcemic environment are proliferatively activated by partial hepatectomy, but their proliferative development stops at a calciumβdependent stage near the end of the preβreplicative phase of development.