Most, but not all, cell types release intracellular organic solutes (e.g. taurine) in response to cell swelling to achieve cell volume regulation. Although this efflux is blocked by classical inhibitors of the electroneutral anion exchanger band 3 (AE1), it is thought to involve an anion channel. The role of band 3 in volume‐dependent taurine transport was determined by expressing, in Xenopus oocytes, band 3 from erythrocytes which do (trout) or do not (mouse) release taurine when swollen. AE1 of both species elicited anion exchange activity, but only trout band 3 showed chloride channel activity and taurine transport. Chimeras constructed from trout and mouse band 3 allowed the identification of some protein domains critically associated with channel activity and taurine transport. The data provide evidence that swelling‐induced taurine movements occur via an anion channel which is dependent on, or controlled by, band 3. They suggest the involvement of proteins of the band 3 (AE) family in cell volume regulation.

The ubiquitous plasma membrane Na+/H+ exchanger (termed NHE1) is activated by diverse hormonal signals, with the notable exception of hormones acting through cAMP as second messenger. Therefore, the Na+/H+ exchanger found in the nucleated trout red cell is of particular interest since it is activated by catecholamines, forskolin, and cAMP analogues. We report here that a cloned cDNA encoding the red cell exchanger restores functional Na+/H+ activity when transfected into Na+/H+ antiporter-deficient fibroblasts (i.e., it regulates intracellular pH in a Na-dependent and amiloridesensitive manner). This red cell exchanger represents an ad-

A B S T R A C T It has previously been shown that addition of catecholamines to a suspension of trout erythrocytes induces an enlargement of the cells owing to an uptake of NaC! mediated by a cAMP-dependent, amiloride-sensitive Na+/ H + exchange. In this article, we show that the change in cell volume induced by catecholamines is much greater when the erythrocytes are incubated in N2 than when they are in 02. This difference is explained by an inhibition of the cAMP-dependent Na+/H + exchange by 02. The inhibition is not reversed in cells incubated in O~ but poisoned with cyanide. It cannot be explained by a difference in the content of cAMP in O~ and in N~. In a CO atmosphere, in which the cells are anoxic, swelling and Na permeability are not increased as they are in N~: in CO, the cells behave as they do in O~. Moreover, cells previously exposed to CO and then put in an N~ atmosphere do not show the expected increase in Na+/H + exchange. This strongly indicates that the binding of CO to hemoglobin, which persists during the subsequent exposure to N~, is the primary event responsible for the inhibition. As CO substitutes for O~ in binding to hemoglobin, the effect of O~ in the control of Na+/H + exchange is probably explained by this interaction with heine. (Allen and McManus [1968.

SUMMARY1. An osmolality reduction of the suspending medium leads to osmotic swelling of trout erythrocytes, which is followed by a volume readjustment towards the original level. The regulatory volume decrease (RVD) was not complete after 1 h.2. During RVD the cells lost K+ and Cl-but gained Nat. This entry of Na+, which is about half the K+ loss, explains the incomplete volume recovery (it was complete when Na+ was replaced by impermeant N-methyl-D-glucamine). The cells also lose large quantities of taurine, which accounts for about 53 % of the volume recovery. In addition RVD is accompanied by the activation of a pathway allowing some large organic cations which are normally impermeant, such as choline or tetramethylammonium, to rapidly penetrate the cells.3. The swelling-activated K+ loss is not significantly affected by replacement of Cl-by N03-, indicating that K+ moves through a Cl--independent K+ pathway.Furosemide, DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and niflumic acid inhibit the K+ loss. From experiments performed in high-K+-containing media, it appears that these compounds block the K+ flux, not by inhibiting Cl-movements but by interfering with the K+ pathway. 4. All the volume-activated pathways (K+, Na+, taurine, choline) are fully inhibited by furosemide and by inhibitors of the anion exchanger such as DIDS and niflumic acid. The concentration required for 50 % inhibition (IC50) of both inorganic cations and taurine appears to be similar. It is proposed that DIDS interacts with a unique target which controls all the volume-sensitive transport systems.