CALCIUM-ANTAGONIST, TMB-8, PREVENTS THE INDUCTION OF ADAPTIVE RESPONSE BY HYDROGEN-PEROXIDE OR X-RAYS IN HUMAN-LYMPHOCYTES

Treatment of human lymphocytes with hydrogen peroxide (10 mu M, 30 min , 37 degrees C in PBS) or with 1 cGy X-rays evoked about a 30% decreas e in the frequency of micronuclei upon subsequent X-irradiation (1.5 G y). In addition to a lower micronuclei frequency, we also found an inc rease in the sedimentation distance of the nucleoids, when measured 90 min (duration of the isolation procedure carried out at 4 degrees C) after the adaptive dose (hydrogen peroxide or X-rays) and preceding th e challenge dose. To test whether Ca2+ is involved in the induction of the adaptive response pathway, we treated cells with the calcium chel ator, EGTA. When EGTA was given at the same time as the adaptive dose, it prevented the development of the adaptive response. In addition, t he calcium antagonist, TMB-8, also prevented the development of the ad aptive response as it prevented the reduction of both micronuclei and increased nucleoid sedimentation. Cellular treatment with TMB-8 increa sed the free [Ca2+] by 40%, when given together with hydrogen peroxide . The faster sedimenting nucleoids from adapted cells were also examin ed by ethidium bromide titration; there was no indication of any chang e in supercoil density or loop size. Psi-tectorigenin, an inhibitor of phosphatidylinositol turnover, did not modify the adaptive response, indicating that inositol (1,4,5)-trisphosphate is not involved in the induction of the adaptive response, but free Ca2+ ions are.