Modulation of the cost of pHi regulation during metabolic depression: A P-31-NMR study in invertebrate (Sipunculus nudus) isolated muscle

Extracellular acidosis has been demonstrated to play a key role in the proc ess of metabolic depression under longterm environmental stress, exemplifie d in the marine invertebrate Sipunculus nudus, These findings led to the hy pothesis that acid-base regulation is associated with a visible cost depend ing on the rate and mode of H+-equivalent ion exchange. To test this hypoth esis, the effects of different ion-transport inhibitors on the rate of pH r ecovery during hypercapnia, on energy turnover and on steady-state acid-bas e variables were studied in isolated body wall musculature of the marine wo rm Sipunculus nudus under control conditions (pHe 7.90) and during steady-s tate extracellular acidosis (pHe 7.50 or 7.20) by in vivo P-31-NMR and oxyg en consumption analyses. During acute hypercapnia (2% CO2), recovery of pHi was delayed at pHe 7.5 compared with pHe 7.9. Inhibition of the Na+/H+-exc hanger 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 exch ange by the addition of the transport inhibitor 4,4'-diisothiocyanatostilbe ne-2,2'-disulphonic acid (DIDS) prevented pH recovery at pHe 7.5 and delaye d recovery at pHe 7.9, in accordance with an effect on Na+-dependent Cl-/HC O3- exchange, The effects of ouabain, DIDS and DMA on metabolic rate were r educed at low pHe, thereby supporting the conclusion that acidosis caused t he ATP demand of Na+/K+-ATPase to fall. This reduction occurred via an inhi biting effect on both Na+/H+- and Na+-dependent Cl-/HCO3- (i.e. Na+/H+/Cl-/ HCO3-) exchange in accordance with a reduction in the ATP demand for acid-b ase regulation during metabolic depression, Considering the ATP stoichiomet ries of the two exchangers, metabolic depression may be supported by the pr edominant use of Na+/H+/Cl-/HCO3- exchange under conditions of extracellula r acidosis.