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Department of Physiology, St George's Hospital Medical School, London.

1. The whole trachea of the ferret has been isolated in vitro in an organ bath and used to study the transport of bovine serum albumin (BSA) and two dextrans (70,000 and 9000 Da) from external buffer solution to air-filled lumen, assessed by fluorescent-labelled tracers. 2. In control conditions, when mucus secretion was not stimulated by drugs, the concentration of albumin in the lumen was over half that in the buffer, and about six times greater than those of the two dextrans. 3. Methacholine and phenylephrine caused large increases in mucus secretion and albumin output and decreases in albumin concentration. The responses were proportional to drug concentration. We concluded that albumin output is increased but diluted with submucosal gland secretion. 4. Salbutamol caused a small increase in mucus secretion and large increases in output and concentration of albumin. The concentration of albumin became greater than that in the external buffer medium. The responses were proportional to concentration of salbutamol. 5. Histamine increased mucus secretion and albumin output and concentration. 6. None of the four drugs increased the output of dextran-70,000. Methacholine and phenylephrine increased the output of dextran-9000, but to a far less extent than for albumin. 7. Cooling the trachea and buffer to 4 degrees C almost abolished the stimulation of mucus and albumin outputs due to methacholine. 8. Increasing the concentration of albumin external to the trachea did not proportionally increase albumin secretion, the logarithmic relationship suggesting saturation of an active transport system. 9. We conclude that albumin is secreted by active transport into the tracheal lumen, and that the rate of transport can be augmented by salbutamol to build up a higher concentration in the lumen than in the external buffer.

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