Function and regulation of the avian caecal bulb: influence of dietary NaCl and aldosterone on water and electrolyte fluxes in the hen (Gallus domesticus) perfused in vivo

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Abstract
The function of the caecal bulb, and its adaptation to chronic high- or low-Na+ intake, was investigated by in vivo perfusion of anaesthetised birds. Effects of acute aldosterone injection (125 μg·kg−1 body mass) were also measured. Evidence was found for primary active net absorption of Na+, inducing parallel Na-linked absorption of water and Cl− and secretion of K+. Around 20–35% of total Cl− absorption and K+ secretion were independent of Na+ fluxes, and these components appear to be driven by passive processes with apparent conductances of 6.3×10−3 (G Cl) and 1.1×10−3 (G K) S·cm−2. Acetate (40mM) stimulated Na+ fluxes (8.5–9.9 μEq·cm−2·h−1) and Na-linked water fluxes (27–44 μl·cm−2·h−1). Increased coupling ratios (2.9–4.6 μl·μEq−1) and other data indicate that these effects may be due to increased osmotic permeabilities of barriers involved in the Na-linked water transfer pathway. Low-Na+ maintenance enhanced EPD (49–69 mV, serosa positive) and all net fluxes:J Na (6.8–11.6);J K (−3.2–−4.3);J Cl (4.3–5.6 μEq·cm serosal area−2·h−1);J v (28–43 μl·cm−2·h−1) (mucosal-serosal fluxes positive). Acute aldosterone enhancedJ Na (10.8–14.0 μEq·cm−2·h−1) and EPD (54–66 mV) by 3 h after injection, but had no effect on the Na-linked components ofJ K orJ Cl.