Volume‐activated Cl(‐)‐independent and Cl(‐)‐dependent K+ pathways in trout red blood cells.

Abstract

SUMMARY1. Swelling of trout erythrocytes can be induced either by addition of catecholamine to the cell suspension, thus promoting NaCl uptake via fiadrenergic-stimulated Na+-H+ exchange (isotonic swelling) or by suspending red blood cells in a hypotonic medium (hypotonic swelling). In both cases cells tend to regulate their volume by losing K+, but the characteristics of the volume-activated K+ pathways are different: after hormonally induced swelling the K+ loss is strictly Cl-dependent; after hypotonic swelling the K+ loss is essentially Cl-independent.2. In order to determine the nature of these volume regulatory pathways (i.e. whether the net K+ loss was conductive or was by electroneutral K+-H+ exchange or KCl co-transport), studies were performed to analyse ion fluxes and associated electrical phenomena. The cell membrane potential and intracellular ionic activities of volume-regulating and volume-static cells were measured by impalement with conventional microelectrodes and double-barrelled ion-sensitive microelectrodes.3. The information gained from the electrical and ion flux studies leads to the conclusion that both Cl--independent and Cl--dependent K+ loss proceed via electrically silent pathways.4. Experiments were designed to distinguish between electroneutral K+-H+ exchange or KCI co-transport. These were based upon the inhibition of Cl--OHexchange to evaluate the degree of coupling between K+ and Cl-(KCI stoichiometry, pH change). The experimental observations are consistent with the fact that both Cl--independent and Cl--dependent K+ loss are mediated by coupled K+-anion co-transport and not by K+-H+ exchange.5. On the basis of previous data, we suggest that only one type of K+-anion cotransport exists in the cell membrane, for which the selectivity for anions varies according to the change in cellular ionic strength induced by swelling.

Evidence for a K(+)‐H+ exchange in trout red blood cells.

“…ID, Table 1). This electroneutral and chloride-dependent K+ extrusion has been shown to be mediated by a KCI co-transporter which is 'switched on' in response to cell swelling (Borgese, Garcia-Romeu & Motais, 1987;Guizouarn et al 1993). When the same experiment was performed in the presence of acetazolamide (Diamox, Theraplix, Paris), the pattern of the response was drastically modified.…”

“…6) and certain other inhibitors of the anion exchanger, such as niflumic acid, can strongly inhibit K+ movement and the associated H+ movements. It is of interest to note that, in trout red cells, these compounds were previously shown to interact directly with and to inhibit the volume-sensitive K+ pathway , which was subsequently shown to be a K+-anion co-transport (Guizouarn et al 1993).…”

“…The new steady state of cell volume is dynamic in nature since it is maintained by stimulation of a K+ loss which compensates for the Na+ gain induced by catecholamines. The K+ extrusion, which is Cl-dependent (Borgese et al 1987) and electroneutral, is mediated by a KCl cotransporter (Guizouarn et al 1993) which is 'switched on' or 'switched off' according to the cell volume and this tends to restore swollen cells to their original volume. Thus the catecholamine-regulated trout cell volume is delicately balanced by a separate but simultaneous operation of volume increase (VI) and regulatory volume decrease (RVD) processes.…”

“…The choice between these two pathways depends on the changes in cellular ionic strength induced by swelling: when the ionic concentration falls, the AMS 1356 cells adopt the Cl--independent pathway and when it rises, they adopt the Cl--dependent pathway (Motais, Guizouarn & Garcia-Romeu, 1991). The present study originated from experiments designed to characterize the nature of these K+ pathways (Guizouarn, Harvey, Borgese, Gabillat, Garcia-Romeu & Motais, 1993). We report here that, in a particular experimental condition which does not involve a significant volume increase, a K+ loss is observed which could be mediated by a K+-H+ exchanger: after Na+-H+ exchange activation by catecholamine, the Cl--OH-exchanges being inhibited by acetazolamide, a K+ loss occurs; in the absence of chloride movements, this K+ loss electrically balances the Na+ uptake occurring through the Na+-H+ antiport.…”

Use it or lose it? Sablefish,Anoplopoma fimbria, a species representing a fifth teleostean group where the βNHE associated with the red blood cell adrenergic stress response has been secondarily lost

“…Following development of a blood acidosis, adrenaline and noradrenaline are released, activating the RBC bNHE which extrudes protons to protect RBC intracellular pH (pH i ), Hb-O 2 affinity, O 2 uptake at the gill, and therefore O 2 supply to the tissues (Borgese et al, 1987;Cossins and Kilbey, 1991;Nikinmaa et al, 1984;Primmett et al, 1986;Salama and Nikinmaa, 1988) (reviewed by Berenbrink and Bridges, 1994;Fievet and Motais, 1991;Jensen, 2004;Thomas and Perry, 1992). During this cascade of events, not only is pH i protected, sometimes elevated, but also H + extrusion and Na + influx initiate a further chain of events requiring anion exchange to remove HCO 3 -from the RBC in exchange for Cl -, Na + /K + -ATPase activation, as well as influx of osmotically obliged water, resulting in a RBC regulatory volume increase (RVI) and associated increase in haematocrit (Hct) (Borgese et al, 1987;Cossins and Gibson, 1997;Cossins and Kilbey, 1991;Guizouarn et al, 1993;Nikinmaa et al, 1990). Species possessing a RBC bNHE generally possess a pronounced Root effect, and therefore extremely pH-sensitive Hbs (Berenbrink et al, 2005); however, the bNHE has been secondarily lost in four groups of derived teleosts (Berenbrink et al, 2005), a loss that is also associated with a substantial decrease in the magnitude of the Root effect.…”