A tissue sampling scheme for tandem assessments of whole-organ physiology and ultrastructure was applied to the lower intestine (coprodaeum) of White Plymouth Rock hens on low- and high-NaCl diets. The objective was to correlate net amiloride-sensitive Na transport determined using the Ussing chamber with the plasma membrane surface areas due to microvilli at the epithelial cell apex. Hens kept on the low-NaCl diet for 3-4 weeks displayed a substantial increase in short-circuit current and in total microvillous membrane surface area. The latter rose from a group mean +/- S.E.M. of about 90 +/- 9.7 cm2 to one of 200 +/- 38 cm2 per organ. An increase in epithelial cell membrane contributed to, but did not fully explain, the increase in microvillous area. No differences in mean cell height or mean cell volume were found but the average cell in the low-NaCl birds was better developed in possessing a greater surface area of microvilli. On the high-NaCl diet, the epithelium was 33 +/- 2.7 microns tall and contained about 270 +/- 15 million cells. Each cell had a volume, on average, of 540 +/- 59 microns 3 and a microvillous surface of 32 +/- 2.6 microns 2. After NaCl depletion, there were 420 +/- 75 million cells and the average microvillous surface was 49 +/- 5.3 microns 2 per cell. The morphological adaptations alone do not explain the increased net Na transport found on the low-NaCl diet. Of cardinal importance is greater density of open Na channels in apical cell membranes.

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