INHIBITION OF CONTRACTION RESPONSES OF HYDRA

The contraction reponse ot hydra to intermittent light stimulationmay be inhibited by exposing the animal to reduced glutathione(GSH). Such inhibitory acthity is dependent on: (1) the concentrationof GSH; (2) the pH of the medium; (3) previous exposure to GSH;and (4) the nutritional state of the animal. Hydra adapt tolO^–5 M GSH so that after approximately an hour the frequenciesof lightinduced contractions are restored to control levels.Such adaptation to GSH is due to changes occurring within theanimal rather than to the degradation of the glutathione molecule.S-methyl glutathione blocks contractions in response to light,showing that the sulfhydryl group is not essential for inhibition.Analogs with sterically large groups substituted for the sulfhydrylgroup, such as oxidized glutathione and S-acetyl glutathione,have no inhibitory activity. These compounds, however, reducethe inhibitory effect of GSH, indicating competition for theGSH receptor. Contractions of hydra in response to intermittent mechanicalagitation are also inhibited by GSH. The duration of inhibitionis dependent on the GSH concentration. Both oxidized glutathioneand S-acetyl glutathione reduce the inhibitory effect of GSH. Hydra adapted to the dark for 24 hours show a marked suppressionof contractions in response to mechanical agitation when exposedto light. On exposure to light, such animals elongate to approximatelyone and a half times their dark adapted length, and are relativelyinsensitive to agitation. The mechanisms by which such stimuliinhibit the contraction responses of hydra remain to be determined.