Cell volume regulation by trout erythrocytes: characteristics of the transport systems activated by hypotonic swelling.

1. 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 Na+. 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 tetramethyl‐ammonium, to rapidly penetrate the cells. 3. The swelling‐activated K+ loss is not significantly affected by replacement of Cl‐ by NO3‐, 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.