The disulfonic stilbene DIDS and the marine poison maitotoxin activate thesame two types of endogenous cation conductance in the cell membrane of Xenopus laevis oocytes

In the present experiments we exposed the intra- or extracellular surface o f excised giant membrane patches of Xenopus laevis oocytes bathed in 140 mm ol/l Na-aspartate solution to the anion transport inhibitor 4,4'-diisothioc yanatostilbene-2,2'-disulfonate (DIDS, 250 mu moL/l). We observed that DIDS activated at least two cation conductances: (1) a non-selective cation (NS C) conductance that was mediated by channels of approximately 27 pS and res embled the stretch-activated cation conductance that has been observed in t he oocyte cell membrane previously, and (2) a Na+-selective conductance, th e single-channel events of which could not be resolved and which resembled the depolarization-induced Na+ conductance that has also been observed in t he oocyte cell membrane previously. Both conductances were blocked by 1 mmo l/l amiloride from the intra- and extracellular surfaces but inhibition of the NSC conductance by extracellular amiloride was less pronounced. Both co nductances activated only slowly with a delay of 15-60 s after application of DIDS and remained active even after DIDS was washed off. This suggests t hat DIDS caused the exocytosis of preformed channels and this interpretatio n was supported by our additional observation that extracellular applicatio n of maitotoxin (MTX) mimicked the effects of DIDS. MTX is a marine toxin t hat has recently been reported to induce exocytosis in Xenopus laevis oocyt es. The fact that DIDS and MTX each carry two sulfonyl groups suggests that they act on the same positively charged binding sites of an exocytosis-ind ucing protein. Our observations demonstrate that using DIDS to inhibit hete rologously expressed anion transporters in the cell membrane of Xenopus lae vis oocytes may compromise proper determination of the transporter currents . This effect can be prevented if the DIDS-activated endogenous cation cond uctances are suppressed by application of amiloride to the cytoplasmic surf ace of the cell membrane.