1. Previous studies have shown that myeloid leukaemic blast cells contain a heat stable factor which inhibits bidirectional sodium transport in normal erythrocytes. This study was undertaken to establish whether leukaemic promyelocytes in culture secrete this factor.

2. Two cell-lines of leukaemic promyelocytes (HL-60 and JR) were grown and culture media from both reduced significantly the ouabain-insensitive sodium efflux rate constant, whereas conditioned culture medium (incubated like the cells in culture) had no inhibitory effect.

3. Promyelocyte extract reduced significantly (P < 0.01) the total sodium efflux rate constant from 0.393 ± 0.030 (sd) to 0.311 ± 0.060, and ouabain-insensitive efflux rate constant from 0.131 ± 0.008 to 0.079 ± 0.009 (P<0.001).

4. The inhibitory factor was heat stable (80°C for 30 min) and it inhibited sodium efflux through a pathway which was not inhibited by ouabain or frusemide.

5. These studies suggest that leukaemic promyelocytes secrete the previously identified passive sodium transport inhibitory factor.

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