238 



METABOLIC HORMONES 



to be able to conserve more water than this, and in fact it is 

 commonly found that the urine is markedly hypertonic. Recent 

 evidence shows that in addition to the action of an antidiuretic 

 hormone (ADH injected as vasopressin in graded doses in Fig. 

 5-25) there is a process of active reabsorption of water. Although 

 the exact means by which it is brought about is still under dis- 

 cussion (e.g. Wirz, 1957 and Sawyer, 1957), it is clear that it is 



2-8 



2-4 



e2-0 

 E 



Q 1-6 

 ^^ 



ro 



§ 1-2 



5 08 



04 







Time, lOmin intervals 



Fig. 5-25. Urine output (ordinates) of an unanaesthetized rat, 

 Rattus, after being given a standard water load by stomach pump 

 (5 % body weight, or ca.W -Sg) and then a dose of antidiuretic hor- 

 mone (vasopressin). Successive doses were given intravenously, in 

 the order b, c, d, a, e, at the times marked by arrows and were 

 graded : « = 100, 6 = 200, c = 400, d = 600, e = 800 |j. units. The 

 antidiuretic effect, shown by the drop in urine output, approaches 

 a maximum after dose d^ and the further increase in hormone 

 given at e has practically no greater effect. The method can be 

 adapted for assaying antidiuretic substances over the lower range 

 of concentrations (from Ginsburg and Heller, 1953). 



independent of hormone control. This has been demonstrated in 

 water-loaded dogs, in which urine flow is maximal and antidiuretin, 

 ADH, is therefore not being secreted (Berliner and Davidson, 

 1957). Acute unilateral reduction of G.F.R., induced by occluding 

 the blood supply to one kidney only, so reduces the volume of the 

 filtrate reaching the ''concentrating mechanism" of that side, that 

 the urine collected separately from the two ureters becomes 



