Changes in element-composition of A6 cells following hypotonic stress

Cellular element concentrations and dry weight contents were determined in A6 epithelia using electron microprobe analysis. This was done to assess th e quantitative contributions of Na, K and Cl to the regulatory volume decre ase (RVD) and isovolumetric regulation (IVR) after decreasing the basolater al osmolality from 260 to 140 mosmol/kg in a stepwise or gradual way. Two m inutes after inducing acute hypotonic stress the cells behaved almost like ideal osmometers, as indicated by a pronounced increase in cell height and decreases in the cellular dry weight and concentrations of all measured ele ments by about the same degree. Sixty minutes after inducing acute hypotoni c stress the dry weight and concentrations of the impermeant elements P, Mg and Ca had returned approximately to control values, indicating normalized cell volume. Na, K and Cl concentrations, however, remained greatly reduce d. The cellular amounts of Na, K and Cl diminished during RVD by approximat ely 31%, 24% and 46%, respectively. The dry weights and element concentrati ons measured 60 min after inducing acute hypotonic stress were similar to t hose obtained after a continuous reduction of basolateral osmolality. The c ellular loss of Na and K following hypotonic stress exceeded that of Cl by about 40 mmol/kg wet wt., suggesting the exit of an other anion and/or the titration. of fixed negative charges. The contribution of Na, K and Cl to t otal cellular osmolality increased from about 75% under control conditions to about 85% during RVD and IVR. Since only approximately 70% of the loss o f cellular osmolytes necessary for the observed RVD and IVR is accounted fo r by the cellular exit of Na, K and Cl, other osmolytes, possibly amino aci ds, must leave the cells following hypotonic stress.