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With an Appendix

1. A method for measuring the lipogenesis from [¹⁴C]glucose by single fat cells is described: (i) after incubation with `carrier-free' [U-¹⁴C]glucose (0·55 µ-mole/ml.), collagenase-isolated fat cells were fixed with osmium tetroxide; (ii) similarly incubated pieces of epididymal fat pads were treated with osmium tetroxide for 90 sec, whereby only the superficial cells are fixed, and the tissue was then disintegrated by shaking with collagenase. The osmium-fixed free cells were washed, sucked into a micropipette, measured under a microscope and assayed individually for ¹⁴C-activity.

2. There was a quantitative recovery of ¹⁴C-lipid activity from osmium-fixed single cells.

3. Both collagenase-isolated cells and in situ fixed surface cells were normally distributed with respect to diameters (for both cell groups from ad lib. fed rats of ca. 110 g; mean diameter, about 55 µm; S.D. about 7 µm).

4. Frequency distribution curves (number of fat cells versus ¹⁴C-lipogenesis per cell) were asymmetric and very broad (coefficient of variation about 50%) for collagenase-isolated cells incubated with insulin (10³ µ-u./ml.). Frequency distribution curves for surface cells obtained from similarly incubated pieces of epididymal fat pads showed a coefficient of variation of the same magnitude, whereas the mean lipogenesis of these cells was only about one third of that of the isolated cells.

5. Collagenase-isolated cells incubated in the presence of insulin (10³ µ-u./ml.) showed a weak but highly significant positive correlation between fat cell diameter and ¹⁴C-lipogenesis (eight rats, r about 0·5 and P < 0·001 for each rat). Analysis of the relationship: lipogenesis = k x diameter to the exponent of showed that the estimates of varied significantly from rat to rat (range: 1·3-2·9). Similar correlations between cell size and lipogenesis were found both for cells incubated with insulin in various submaximal concentrations and for cells incubated without insulin.

6. Small and large cells from the same rat were equally sensitive to insulin.

7. Statistical analysis of frequency distribution curves (number of cells versus ¹⁴C-lipogenesis per unit surface area) representing cells from the same rat incubated with insulin 0, 2·5, 5, 10, and 10³ µ-u./ml., respectively, suggests that insulin exerts a graded influence on the lipogenesis of each fat cell.