Role of eicosanoids, inositol phosphates and extracellular Ca2+ in cell‐volume regulation of rat liver

Abstract

1 In isolated perfused rat liver, the time‐course of volume‐regulatory K⁺ efflux following exposure to hypoosmolar perfusate resembled the leukotriene‐C₄‐induced K⁺ efflux in normotonic perfusion. Omission of Ca²⁺ from the perfusion fluid had no effect on volume‐regulatory K⁺ efflux, but abolished completely the leukotriene‐C₄‐induced K⁺ efflux. 2 Volume‐regulatory K⁺ fluxes following hypoosmolar exposure (225 mOsmol l⁻¹) and subsequent re‐exposure to normotonic media (305 mOsmol l⁻¹) were not significantly affected by the cyclooxygenase inhibitors indomethacin (5 μmol l⁻¹) or ibuprofen (50 μmol l⁻¹), the leukotriene D₄/C₄‐receptor antagonist 1‐{2‐hydroxy‐3‐propyl‐4‐[4‐(1H‐tetrazol‐5‐yl)butoxy]phenyl}ethanone (LY 171883, 50 μM), the lipoxygenase inhibitor nordihydroguaiaretic acid (20 μM), the phospholipase‐A₂ inhibitor bromophenacyl bromide (50 μM) or the thromboxane‐receptor antagonist 4‐[2‐(benzenesulfonamido)ethyl]‐phenoxyacetic acid (BM 13.177, 20 μM). Also the effects of hypoosmotic cell swelling on lactate, pyruvate and glucose balance across the liver remained largely unaffected in presence of these inhibitors. Neither exposure of perfused rat liver to hypoosmolar (225 mOsmol l⁻¹) nor to hyperosmolar (385 mOsmol l⁻¹) perfusion media affected hepatic prostaglandin‐D₂ release. 3 When livers were ³H‐labeled in vivo by an intraperitoneal injection of myo‐[2‐³H]inositol about 16 h prior to the perfusion experiment, cell swelling due to lowering the perfusate osmolarity from 305 mOsmol l⁻¹ to 225 mOsmol l⁻¹ led to about a threefold stimulation of [³H]inositol release. The maximum of hypotonicity‐induced [³H]inositol release preceded maximal volume‐regulatory K⁺ efflux by about 30 s, but came after the maximum of water shift into the cells. Hypotonicity‐induced [³H]inositol release was largely prevented in presence of Li⁺ (10 mM), but simultaneously inositol monophosphate accumulated inside the liver within 10 min and a small, but significant increase of inositol trisphosphate 1 min after onset of hypoosmolar exposure was detectable. No stimulation of [³H]inositol release was observed during cell shrinkage by switching the perfusate osmolarity from 225 mOsmol l⁻¹ to 305 mOsmol l⁻¹ or from 305 mOsmol l⁻¹ to 385 mOsmol l⁻¹. No stimulation of [³H]inositol release was observed upon swelling of preshrunken livers by lowering the osmolarity from 385 mOsmol l⁻¹ to 305 mOsmol l⁻¹, although the volume‐regulatory K⁺ efflux under these conditions was almost identical to that observed after lowering the osmolarity from 305 mOsmol l⁻¹ to 225 mOsmol l⁻¹. 4 The data suggest that volume‐regulatory K⁺ fluxes in rat liver do not require the formation of eicosanoids or entry of Ca²⁺ from the extracellular space. Hypoosmolar cell swelling, but not cell shrinkage is accompanied by a stimulation of inositol‐phosphate turnover, which could participate in triggering a volume‐regulatory decrease and the modulation of metabolism following cell swelling. However, in view of the experiments presented, the possibility must be envisaged that the swelling‐induced inositol‐phosphate response is rather an epiphenomenon than a trigger of volume‐regulatory K⁺ fluxes and of metabolic alterations.