Activation and inactivation of volume-sensitive taurine efflux from rat mammary gland.

A knowledge of volume-sensitive solute transport in mammary cells is important in light of evidence that mammary cell metabolism is regulated by the cellular hydration state. In this report we have examined volume-sensitive taurine and K+ (Rb+) transport by lactating rat mammary tissue. A hyposmotic shock increased taurine efflux from rat mammary tissue: taurine release returned to a basal level upon transferring the tissue back to an isosmotic medium. However, the time taken to activate taurine efflux was less than the time taken to inactivate taurine release. A second subsequent osmotic challenge also increased taurine release but to a lesser extent than the first osmotic shock. A similar pattern was observed for bumetanide-insensitive, volume-activated K+ (Rb+) release from mammary tissue explants suggesting that taurine and K+ efflux are acting in concert to regulate mammary cell volume. An abrupt hyposmotic shock increased taurine efflux from mammary explants to a greater extent than a gradual reduction in the osmolality of the incubation medium. Increasing extracellular K+ increased taurine release via a pathway sensitive to niflumic acid, which suggests that activation of volume-sensitive taurine efflux does not require a change in the ionic strength of the incubation medium or a decrease in intracellular osmolality. A hyposmotic shock also stimulated taurine efflux from rat mammary acini. In contrast, a hyposmotic challenge had no effect on taurine uptake measured under sodium-free conditions. Hyposmotically induced taurine efflux was not dependent upon extracellular calcium. The results suggest that taurine and K+ transport may allow mammary cells to volume-regulate and consequently help to control mammary metabolism.