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The functional expression of a ouabain-resistance gene has been studied in a transfected cell line, OR-6 cells. These cells possess a ouabain-inhibitable, amiloride-resistant Na+-K+ pump under control conditions. They can be induced by brief culture in the presence of low concentrations of ouabain to express a ouabain-resistant, amiloride-sensitive Na+-K+ pumping activity. They revert to the uninduced phenotype shortly after removal of ouabain from the culture medium. When placed in low-potassium, low-serum, or dibutyryl adenosine 3'-5'-cyclic monophosphate-containing media, these cells produce long cellular extensions. Thus a single inducible gene appears to regulate sensitivity of the Na+-K+ pumping activity to cardiac glycosides and to amiloride, and it may be related to important characteristics of cellular morphology.

Nonperfused proximal S2 segments from rabbit kidney cortex have been shown to keep cell volume constant as medium osmolality is slowly raised but to shrink and not exhibit regulatory volume increase (RVI) if medium osmolality is abruptly elevated (J. Lohr and J. Grantham. J. Clin. Invest. 78: 1165-1172, 1986). In the current study, 0.5 mM butyrate in the medium 1) extended the range from 361 to 450 mosmol/kgH2O over which cells maintained volume constant as osmolality was gradually raised and 2) restored RVI after cell shrinkage when osmolality was rapidly raised from 295 to 400 mosmol/kgH2O. Volume regulation was associated with net increases in intracellular Na+ and Cl- but no change in K+ (measured by electron probe). The increments in Na+ and Cl- were insufficient to account for the total addition of osmolytes required for volume maintenance or restoration. The fraction of the expected increase in intracellular osmoles accounted for by the increase in [(K+)i + (Na+)i + (Cl-)i] was 52 and 21% for gradual and rapid osmotic changes, respectively. We conclude that butyrate enhances the capacity of S2 segments to regulate volume in hyperosmotic medium by promoting addition of Na+ and Cl- and by other undermined factors.

1. By incubating 6‐day post‐coitum (p.c.) blastocysts in medium supplemented with either 20 mM‐sucrose or 10 mM‐KCl it was possible to show that the trophectoderm of the blastocyst prevents the concentration of K in the blastocoele fluid rising to external concentrations. 2. The concentrations of K in the blastocoele fluid are maintained predominantly by leakage of K from the trophoblast cells into the blastocoele and by ouabain‐sensitive transport of K into the trophoblast cells from the blastocoele fluid. 3. Exposure of blastocysts to ouabain on the juxtacoelic, but not abcoelic, surface of the trophectoderm may inhibit blastocoele fluid accumulation.

Glucose uptake was measured by a noninvasive fluorescence technique on a total of 165 morula- and blastocyst-stage murine embryos in two different culture media. Eighty-four embryos were tested in M2 medium, and the remaining 81 embryos were tested in M16. Embryos assayed in M2 took up significantly less glucose over the 4-h assay period than did embryos assayed in M16. The lower uptake of glucose by embryos in M2 corresponded with a decrease in the quality of embryos cultured overnight in M2 as judged by morphological criteria. Embryos that were judged to be degenerate or had gross abnormalities took up significantly less glucose than did normal embryos. Glucose uptake in both populations of embryos covered a wide range of values and was normally distributed. A significant effect between mothers was noted in glucose uptake for embryos assayed in both M16 and M2 media. The possible uses of noninvasive measures of glucose uptake as a test of embryo viability or for optimizing culture conditions are discussed.