The effect of a hyposmotic shock on amino acid efflux from lactating rat mammary tissue: stimulation of taurine and glycine efflux via a pathway distinct from anion exchange and volume-activated anion channels.

We have examined the effect of a hyposmotic shock, and thus cell swelling, upon the efflux of amino acids, SO4(2-) and I- from lactating mammary tissue. A hyposmotic challenge increased the efflux of taurine and glycine via a 4,4'-diisothiocyanatostilbene-2,2'- disulphonic acid (DIDS)-sensitive pathway. It appears that these amino acids do not exit via an anion-exchange mechanism following cell swelling because sulphate efflux, which uses a DIDS-sensitive exchange mechanism, was unaffected. The hyposmotic-induced efflux of taurine was not dependent upon the Na+ gradient and was not influenced by the nature of the anion in the incubation medium. In addition, taurine efflux was stimulated by incubating mammary tissue in an isosmotic medium that contained urea, suggesting that cell swelling is the stimulating factor rather than a decrease in osmolality per se. The results suggest that mammary tissue uses taurine and glycine as a means of regulating cell volume following swelling. In contrast, the efflux of glutamic acid, alanine and alpha-aminoisobutyric acid was unaffected by a hyposmotic challenge. Similarly, the efflux of I- was unaffected by such a challenge. The results suggest that volume-activated amino acid transport in lactating rat mammary tissue is distinct from volume-regulated anion channels.