Effect of thyrotropin-releasing hormone and its metabolites on the secretion of sulfated polysaccharides by foot integument of a pond snail.

The effect of thyrotropin-releasing hormone (TRH), TRH metabolites, and a TRH analog on the secretion of 35S-labeled sulfated polysaccharides from in vitro incubated foot integument of the pond snail Lymnaea stagnalis palustris was assessed. Whereas TRH significantly inhibited the secretion of 35S-labeled polysaccharides, its metabolite deamido TRH significantly stimulated polysaccharide secretion. Histidyl-proline-diketopiperazine did not alter the secretion of 35S-labeled sulfated polysaccharides. Foot integument exhibited considerable TRH-peptidase activity in vitro. The principal proline-containing TRH metabolites were deamido-TRH and proline. The deamido-TRH content of the foot, mantle, and hemolymph was determined by radioimmunoassay. All tissues contained deamido-TRH. These findings suggest that the secretory activity of skin mucus glands of L. stagnalis palustris is the result of the stimulatory action of deamido-TRH and the inhibitory action of TRH. 3Methyl-2histidyl-TRH, TRH, vasotocin, and bombesin did not alter the polysaccharide secretion by in vitro incubated foot integument, indicating that the changes in mucus secretion after TRH and deamido-TRH administration are specific and that the gastropod "TRH receptors" may differ from the mammalian brain and anterior pituitary gland TRH receptors.