title: Modulation of the cost of pHi regulation during metabolic depression: a (31)P-NMR study in invertebrate (Sipunculus nudus) isolated muscle.
abstract: Extracellular acidosis has been demonstrated to play a key role in the process of metabolic depression under long-term environmental stress, exemplified in the marine invertebrate Sipunculus nudus. These findings led to the hypothesis that acid-base regulation is associated with a visible cost depending on the rate and mode of H(+)-equivalent ion exchange. To test this hypothesis, the effects of different ion-transport inhibitors on the rate of pH recovery during hypercapnia, on energy turnover and on steady-state acid-base variables were studied in isolated body wall musculature of the marine worm Sipunculus nudus under control conditions (pHe 7.90) and during steady-state extracellular acidosis (pHe 7.50 or 7.20) by in vivo (31)P-NMR and oxygen consumption analyses. During acute hypercapnia (2 % CO(2)), recovery of pHi was delayed at pHe 7.5 compared with pHe 7.9. Inhibition of the Na(+)/H(+)-exchanger by 5-(N,N-dimethyl)-amiloride (DMA) at pHe 7.5 delayed recovery even further. This effect was much smaller at pHe 7.9. Inhibition of anion exchange by the addition of the transport inhibitor 4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) prevented pH recovery at pHe 7.5 and delayed recovery at pHe 7.9, in accordance with an effect on Na(+)-dependent Cl(-)/HCO(3)(-) exchange. The effects of ouabain, DIDS and DMA on metabolic rate were reduced at low pHe, thereby supporting the conclusion that acidosis caused the ATP demand of Na(+)/K(+)-ATPase to fall. This reduction occurred via an inhibiting effect on both Na(+)/H(+)- and Na(+)-dependent Cl(-)/HCO(3)(-) (i.e. Na(+)/H(+)/Cl(-)/HCO(3)(-)) exchange in accordance with a reduction in the ATP demand for acid-base regulation during metabolic depression. Considering the ATP stoichiometries of the two exchangers, metabolic depression may be supported by the predominant use of Na(+)/H(+)/Cl(-)/HCO(3)(-) exchange under conditions of extracellular acidosis.
authors.affiliation: Alfred-Wegener-Institut für Polar- und Meeresforschung, Okophysiologie und Okotoxikologie, Postfach 120161, D-27515 Bremerhaven, Germany. hpoertner@awi-bremerhaven.de
authors.forename: H O
authors.initials: HO
authors.lastname: Pörtner
authors.forename: C
authors.initials: C
authors.lastname: Bock
authors.forename: A
authors.initials: A
authors.lastname: Reipschläger
mesh_terms: D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002245:Carbon Dioxide; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D009682:Magnetic Resonance Spectroscopy; D009132:Muscles / Q000187:drug effects / Q000378:metabolism*; D009348:Nematoda / Q000378:metabolism*; D010101:Oxygen Consumption
chemical_list: D002245:Carbon Dioxide; C039465:5-dimethylamiloride; D000584:Amiloride; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid