1. Nineteen patients with a spectrum of immunologically related disorders were studied before and immediately after plasmapheresis for changes in plasma aldosterone, insulin and arginine vasopressin (AVP). Renin was also measured in 11 of these patients by direct radioimmunoassay. 125% of the initial plasma volume was replaced, which corresponded to a predicted removal of 72% for any plasma constituent.

2. The intial, final (experimental) and final (predicted) concentrations (means ± sem) were 337 ± 50, 185 ± 23 and 100 ± 16 pg/ml respectively for renin, 465 ± 86, 146 ± 38 and 124 ± 22 pmol/l respectively for aldosterone, 218 ± 35, 69 ± 11 and 63 ± 11 pmol/l respectively for insulin, 7.2 ± 1.9, 6.1 ± 0.5 and 1.8 ± 0.2 pmol/l respectively for AVP. The predicted final concentration was calculated from the initial concentration and the fraction of plasma volume exchanged. The experimental final concentration was lower than the initial concentration for renin, aldosterone and insulin (P < 0.001) but not for AVP. The predicted final concentration was lower than the experimental final concentration for AVP and renin (P < 0.001) but not for aldosterone and insulin. Plasma volume, osmolality, glucose, sodium and potassium concentrations did not change significantly.

3. The concentrations of renin, aldosterone, insulin and AVP in the removed plasma were 84 ± 17 pg/ml, 179 ± 36, 98 ± 15 and 4.8 ± 0.7 pmol/l respectively. The amount subtracted expressed as percentage of the total amount present in plasma was markedly greater for AVP than for the three other plasma constituents.

4. These results demonstrate that the response of the releasing cells to the fall in the plasma concentration of their secretory product does not follow the same kinetics for all substances. There is probably for AVP a fast releasing pool stimulated by a negative feedback.

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