EFFECTS OF BLEOMYCIN ON THE EPIDERMAL CONTENT OF GROWTH‐REGULATORY SUBSTANCES (CHALONES)

Hairless male mice were given 2 mg Bleomycin i.p. on two successive days. At different time intervals from 1 to 10 days after the last Bleomycin injection, groups of animals were killed and water extracts of homogenized skin were made. These extracts, supposed to contain the epidermal G 1 and G 2 chalones, were injected into female hairless mice, and their growth inhibitory potency determined by two methods. 5 mg of lyophilized crude skin extract were injected i.p. together with Colcemid, and the animals killed 4 hr later. The number of Colcemid‐arrested mitoses was determined, and was considered to be a measure of the G 2 inhibitor present in the skin extracts. 10 mg of the same extracts were injected i.p., and these animals also got 3 H‐TdR i.p. 12 hr later, and were killed after a subsequent 30 min. The epidermal LI was determined, and was considered to be a measure of the epidermal G 1 factor in the skin extracts. The results obtained were compared to the effect of Bleomycin alone and to the effects of skin extracts from non‐Bleomycin‐treated animals. The results show that Bleomycin provoked slight alterations in the growth‐inhibitory potency of the G 1 chalone, whereas significant effects were seen in the G 2 chalone. There was an increased amount of growth‐inhibiting factors on days 2 and 3, and on days 8–10. The results are discussed and it is concluded that the most probable hypothesis is that Bleomycin, in addition to its known inhibitory effect on epidermal cell proliferation, exerts growth inhibition by accumulation of cells with high growth inhibitory potency. An initial, additional direct effect of Bleomycin on the chalone system cannot be excluded.