TMB-8 inhibits respiration and cyclic GMP formation in Dictyostelium discoideum

The putative inhibitor of intracellular calcium mobilization, TMB-8 was found to be a powerful inhibitor of respiration in amoebae of Dictyostelium discoideum. Consequently, the previously reported effects of this drug on cyclic GMP formation induced by chemoattractants were reassessed. It was found that TMB-8 abolished both folate and cyclic AMP-mediated accumulation of cyclic GMP in D. discoideum amoebae and that addition of Ca2+ completely restored this response. The Ca2+ chelating agent EGTA did not mimic the effect of TMB-8. The effect on cyclic GMP formation, however, occurred only at a concentration of TMB-8 that was ten times that causing maximal inhibition of respiration, and inhibition of cyclic GMP formation was completely restored by addition of excess Ca2+, whereas inhibition of respiration was only partially restored. The data suggest that TMB-8 has more than one inhibitory action, and because of the differential sensitivity of respiration and cyclic GMP formation to this drug, and the differential antagonism of excess Ca2+, we conclude that the effect of TMB-8 on the cyclic GMP response is probably due to its effect on Ca2+ mobilization, rather than indirectly via its effects on respiration. However, we advise caution in interpretation of data using this inhibitor where the responses measured are prolonged, are energy-requiring or are not freely reversible by excess Ca2+.