Postural changes in atrial natriuretic peptide (ANP) and fluid balance in conscious goats.

To gain insight into how weak physiological stimuli suffice to release ANP, the effects of 15 degrees head-up and head-down tilt, 90 min each in succession, on plasma atrial natriuretic peptide (ANP), plasma cortisol, plasma and urinary Na, K and osmolality, plasma proteins and haematocrit were studied in five conscious goats before and after 2 wk daily training for the tilting procedure. In the trained goats the 15 degrees tilt did not affect the plasma ANP, cortisol or the urine excretion significantly. Total plasma proteins decreased significantly. In the untrained goats, on the other hand, an increasing trend was observed in the plasma ANP and cortisol as well as in the urine Na excretion during the head-up tilt. During the head-down tilt, the levels of ANP and Na excretion remained elevated. The plasma ANP was significantly increased after 40 min, by 28% as compared to the pre-tilting level. The plasma cortisol was first elevated, but then returned to the starting level. The results suggest that in the trained goats the responses to tilting were unmasked. Despite minor effects of the 15 degrees tilt in itself, increased plasma ANP seemed to be associated with increased natriuresis.