The erythrocyte Na,K,Cl cotransporter and its circulating inhibitor in Dahl salt-sensitive rats

BACKGROUNDAbnormal Na,K,Cl cotransport is thought to be a pathogenic factor in Dahl salt-sensitive rat models, but the only direct evidence for this is an increased cotransport activity found in erythrocytes from salt-loaded Dahl salt-sensitive rats. OBJECTIVETo re-examine erythrocyte cotransport fluxes and a circulating cotransport inhibitory factor (CIF) in inbred Dahl rats maintained on a low (0.2%) salt diet. Cotransport fluxes were investigated both under basal conditions and after stimulation by cell shrinking. METHODSBlood was drawn from 12 male Dahl salt-sensitive and 12 Dahl salt-resistant rats of the inbred John Rapp strain. Erythrocyte Na,K,Cl cotransport activity was equated to the bumetanide-sensitive fluxes of sodium, rubidium or lithium. Plasma CIF activity was tested in human erythrocytes. RESULTSIn Dahl salt-sensitive rats(1) plasma CIF activity (5.7 ± 0.4 units/ml) was modestly higher than in Dahl salt-resistant rats (2.97 ± 0.12 units/ml, P < 0.0001), but much lower than that previously found in salt-loaded Dahl salt-sensitive rats (16.1 units/ml), and (2) erythrocytes exhibited a similar bumetanide-sensitive sodium efflux (rate constant 0.056 ± 0.008 h) as in Dahl salt-resistant rats (0.047 ± 0.007 h). Following hypertonic shock, the bumetanide-sensitive rubidium influx reacted more to cell shrinkage in Dahl salt-sensitive than in Dahl salt-resistant erythrocytes (cell volume decrease required to stimulate bumetanide-sensitive rubidium influx by 4000 μmol/l cells per h = −4.04 ± 0.36 versus −5.89 ± 0.44 fl, respectively; P < 0.01). CONCLUSIONSWhen fed a low-salt diet, Dahl salt-sensitive rats present slightly increased plasma CIF levels and normal erythrocyte cotransport fluxes under basal conditions, but an increased response to a hypertonic shock. Therefore, if there is any primary cotransport abnormality in Dahl salt-sensitive rats, it appears to be restricted to the renal Na,K,Cl cotransporter BSC1 isoform. Alternatively, any such change may be the consequence of abnormal regulation by osmolarity-dependent mechanisms.