Developmental Physiology of Cestodes: Cyclic Nucleotides and the Identity of Putative Crowding Factors in Hymenolepis diminuta

Worm-conditioned saline (WCS) was prepared by incubating Hymenolepis diminuta from crowded infections for 12 hr in a balanced salt solution. The effect of the WCS on the incorporation of [3H] thymidine into DNA in the anterior regions of fresh H. diminuta was compared to effects produced by the cyclic nucleotides in the WCS. Cyclic AMP and cGMP were found in the WCS, and cGMP but not cAMP (at the concentration in WCS) caused some inhibition of DNA synthesis. For further study of the effects of cyclic nucleotides, worms were incubated with theophylline, caffeine, 3-isobutyl-1-methyl xanthine, 2-deoxy cGMP, and L-ascorbic acid, all of which produced some inhibition of [3H] thymidine incorporation. Treatment of WCS with 3′,5′ cyclic nucleotide phosphodiesterase abolished part of its inhibitory activity, i.e., that part presumed to be due to cGMP. When worms were incubated in the presence of succinate, acetate, D-glucosaminic acid, and cGMP simultaneously and in the concentrations each was found in the WCS, DNA synthesis was inhibited to a degree equal to that found in the WCS. Thus these substances apparently represent the putative crowding factors in the WCS. WCS prepared with worms from different population densities contained the same levels of cAMP but varied in content of cGMP, which decreased as the worm density increased. WCS prepared with patent worms contained high levels of cAMP, but the same amounts of cGMP as WCS prepared with 10-day-old worms. At least some inhibitors of cyclic nucleotide phosphodiesterase inhibited the secretion of cGMP by the worms. Levels of cGMP in the host intestine varied with the presence or absence of worms, number of worms, and area of the intestine.