Isoosmotic shrinkage by self-stimulated outward Na-K-Cl cotransport in quail erythrocytes.
In mammalian erythrocytes, outward fluxes by the Na-K-Cl cotransporter NKCC have been clearly characterized, but NKCC fluxes are small and their physiological role, if any, is poorly understood. Avian erythrocytes are nucleated cells, in which a physiologically relevant NKCC acts as a cell volume regulator. Therefore, we further investigated outward cotransport and its relation to cell volume by using quail erythrocytes. Unlike human or rat erythrocytes, quail erythrocytes exhibit outward cotransport fluxes: (1) of high magnitude [maximal rate of bumetanide-sensitive Li+ efflux=12.3+/-1.1 mmol (l cells x h)(-1), mean +/-SEM, n=23] and (2) strongly stimulated by hyperosmotic media (by 100-200% in 500 mosmol/l media). Na+- or Li+-loaded quail erythrocytes exhibited rapid cell shrinkage when incubated in K+-free media. Thus, cell volume remained stationary up to 5-10 min and then started to shrink. Shrinkage was first slow, but progressively accelerated, finally reaching a new stationary state where cell volume had decreased by about 20%. Such rapid cell shrinkage was fully inhibited by bumetanide and was associated with outward cotransport stimulation (self-stimulated or an auto-catalytic process, i.e. a reaction stimulated by its product). External K+ reduced all these phenomena, but significant cell shrinkage was still observed at an external K+ concentration of 2.8 mM. K+ removal failed to stimulate outward cotransport in hypotonic media (250 mosmol/l). Finally, reincubation of shrunken erythrocytes in physiological saline revealed that inward cotransport was stimulated more than outward cotransport. In conclusion, isoosmotic hypokalaemia drives a rapid shrinkage of quail erythrocytes, due to auto-catalytic net outward cotransport stimulation. Whether this is an experimental curiosity or indicates that outward cotransport can have some physiological role deserves further investigation.
Lou
Josè M
JM
Dept. Fisiologìa, Facultad de Medicina, Universidad de Zaragoza, Spain.
Garay
Ricardo P
RP
Gimenez
Ignacio
I
Escanero
Jesus F
JF
Alda
Josè O
JO
D000818:Animals; D048429:Cell Size / Q000187:drug effects / Q000502:physiology; D003370:Coturnix; D004305:Dose-Response Relationship, Drug; D004912:Erythrocytes / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D009994:Osmolar Concentration; D009995:Osmosis / Q000187:drug effects; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D012965:Sodium Chloride / Q000494:pharmacology; D028021:Sodium-Potassium-Chloride Symporters / Q000378:metabolism*; D013395:Sucrose / Q000494:pharmacology
D028021:Sodium-Potassium-Chloride Symporters; D012965:Sodium Chloride; D013395:Sucrose; D011189:Potassium Chloride