§ 5.32 WATER BALANCE 219 



Mammalia. Removal of the neurohypophysis, and therefore of 

 the supply of oxytocin from rats (last line of Table 26), mitigates 

 considerably the effects of adrenalectomy, at least as far as plasma 

 sodium is concerned. Plasma sodium is practically equivalent to 

 that of the control, and potassium is lowered, as compared with 

 specimens that had had the adrenals only removed; plasma 

 potassium is, however, not fully restored to normal, and the balance 

 of ions in the muscles is decidedly abnormal. A series of such 

 experiments shows that the neurohypophysis is at least partially 

 responsible for the loss of sodium in adrenalectomizcd animals, and 

 that its action must be to inhibit the active tubular reabsorption of 

 sodium ions (Fig. 5-156). Its action on chloride ions may be 

 similar to that on sodium ; but that on potassium is, as yet, far less 

 clear (Chester Jones, 1957^). 



The interesting if tentative suggestion has been made that the 

 effect of neurohypophysial removal may be due to the loss of the 

 oxytocin secretion, rather than the antidiuretic fraction, ADH. 

 Injections of oxytocin, at least in pharmacological doses, are more 

 effective than ADH in increasing sodium excretion in dogs with 

 a low rate of urine flow (Brooks and M. Pickford, 1957). 



If this last suggestion is further substantiated, the situation 

 would be that sodium as well as chloride ions, at least in rats 

 (Dicker and Heller, 1946), are controlled by the balance between 

 aldosterone, favouring their reabsorption, and oxytocin, favouring 

 their excretion. Potassium ions are excreted during periods of 

 aldosterone activity. Secretion by the adrenal cortex is usually 

 stimulated by ACTH, but this has less effect upon aldosterone than 

 on other cortical hormones ; the only control of neurohypophysial 

 secretion is nervous. 



5.32 WATER BALANCE 



The direction of movement of water through a cell surface is 

 determined by osmotic forces ; but the rate of movement can be 

 affected by hormones, which vary the permeability of the surface. 

 Decrease in permeability, which is associated with diuresis, will 

 be considered first (§ 5.321), as it tends to concentrate the blood 

 salts and to accompany salt reabsorption (§ 5.311); increase m 

 permeability associated with antidiuresis will be taken second 



