Incorporation of [3H]thymidine by isolated fetal myoblasts and fibroblasts in response to human placental lactogen (HPL): Possible mediation of HPL action by release of immunoreactive SM-C

We investigated the actions of human placental lactogen (HPL) and human growth hormone (HGH) on [3H]thymidine incorporation and the release of immunoassyable somatomedin‐C (SM‐C) by isolated myoblasts, dermal fibroblasts, and costal cartilage explants taken from human fetuses, at 11–21 weeks of gestation. The incorporation of [3H] thymidine by myoblasts and fibroblasts was significantly increased after incubation for 20 hr or 44 hr, and cell number after incubation for 7 days, in the presence of 50–250 ng/ml HPL. Incubation with HPL did not increase [3H]thymidine incorporation into cartilage explants, whereas incubation with HGH failed to enhance the uptake of this isotope by any of the tissues. Following extraction with acid‐ethanol, culture medium conditioned by exposure to myoblasts or fibroblasts for 44 hr, and to cartilage explants for 7 days, contained radioimmunoassayable SMC. Myoblast‐conditioned medium contained significantly more SM‐C [1,609 ± 953 mU/mg cell protein (mean ± SD) n = 10] than did that conditioned by fibroblasts (637 ± 323; n = 5; P < 0.02). In 1 week of culture, cartilage explants released 4.1 ± 1.1 mU/mg wet weight (n = 7). The release of immunoassayable SM‐C from cultured cells was significantly increased in the presence of 250 ng/ml HPL in five of eight experiments with myoblasts and two of four experiments with fibroblasts. Neither fibroblasts or myoblasts showed increased SM‐C release following exposure to HGH. The results suggest that HPL, but not HGH, is growth‐promoting for some human fetal tissues in vitro and that this action is mediated, at least in part, by an increased release of somatomedins.