The effect of hyposmotic and isosmotic cell swelling on the intracellular [Ca-2+] in lactating rat mammary ACinar cells

The effect of hyposmotic and isosmotic cell swelling on the free intracellular calcium concentration ([Ca2+]i) in rat mammary acinar cells has been examined using the fura-2 dye technique. A hyposmotic shock (40% reduction) increased the [Ca2+]i in rat mammary acinar cells in a fashion which was transient; the [Ca2+]i returned to a value similar to that found under isomotic conditions within 180 sec. The increase in the [Ca2+]i was dependent upon the extent of the osmotic shock. The hyposmotically-activated increase in the [Ca2+]i could not be attributed to a reduction in extracellular Na+ or a change in the ionic strength of the incubation medium. Thapsigargin (1 M) enhanced the hyposmotically-activated increase in the [Ca2+]i. Isosmotic swelling of rat mammary acinar cells, using urea, had no significant effect on the [Ca2+]i. Similarly, a hyperosmotic shock did not affect the [Ca2+]i in rat mammary acinar cells. It appears that the effect of cell swelling on the [Ca2+]i in rat mammary acinar cells depends on how the cells are swollen (hyposmotic vs. isosmotic). This finding may have important physiological implications given that it is predicted that mammary cell volume will change in vivo under isomotic conditions.