The Effects of Insect Juvenile Hormone on the Growth of Some Protozoans in vitro. I. Crithidia sp

SYNOPSIS. Experiments were designed to investigate the effects of insect juvenile hormone (JH) on the over‐all growth and macromolecular synthesis of Crithidia sp. in vitro. Cells grown in the presence of 10−5M‐10−3M JH showed a concentration‐dependent inhibition of growth, which appeared to result from both a prolongation of generation time and a delay in the onset of logarithmic growth. Juvenile hormone (10−3M) inhibited the incorporation of [3H]thymidine, [3H]uridine and [3H] leucine into logarithmically growing cells by 50, 70 and 40% respectively. The incorporation of [3H]uridine into acid insoluble material could be stopped within 1 hr of application of the hormone (10−3M). The inhibitory effect was reversible in terms of cell numbers in subcultures of washed cells but an examination of the reversibility of RNA synthesis inhibition suggested that the resumption of RNA synthesis at an optimal level would require a lag period of at least 1–3 hr. It is suggested that JH may act by interfering with RNA synthesis either directly or indirectly by primarily acting at the level of the plasma membrane.