The total Na+/K + ATP-ase activity of the thick ascending limb of the loop of Henlé may be stimulated by arginine-vasopressin (AVP). Lysine-vasopressin (LVP), oxytocin (OT), and arginine-vasotocin (AVT) produce less than 5% of the enzyme activity induced by the same concentration of AVP. Physiological concentrations of a mixture of other hormones with known activity on the kidney (T3, T4 aldosterone, angiotensin II, and OT) did not significantly increase total Na+/K + ATP-ase activity. Specific AVP antiserum consistently removed greater than 90% of the stimulatory effect of plasma. The concentration of AVP in plasmas from dehydrated subjects was greater than l0 times that of the same subjects hydrated. Intra-assay coefficient of variation was 35% and 52% from 200 μl and 20 μl of plasma respectively. The interassay coefficient of variation was 53% and 55% from plasma pools with high and low AVP content.

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