STIMULATION OF THE GASTRIC SODIUM MONITOR REDUCES HEPATIC ANGIOTENSIN-CONVERTING ENZYME-ACTIVITY

1. The natriuresis engendered by stimulation of the gastric sodium mon itor is mediated in part by a decrease in the circulating concentratio n of angiotensin II (AngII). This decrease is due to a decrease in syn thesis rather than to an increase in metabolism, We investigated the r ole of changes in plasma and hepatic angiotensin-converting enzyme (AC E) activity in this decrease in AngII synthesis. 2. Male Sprague-Dawle y rats were equilibrated on a low-sodium diet for 7 days, On the day o f experiment, rats were anaesthetized and received either a sodium loa d of 1.5 mmol/kg as 3 mol/L saline or an equivalent volume of an iso-o smotic urea solution by direct gastric puncture. Blood was sampled and livers were harvested at 0 and 30 min after sodium or urea administra tion. Angiotensin-converting enzyme was measured in serum and tissue h omogenates by generation of histidyl-leucine. 3. In the liver, ACE act ivity decreased from control after both sodium (P < 0.005) and urea (P < 0.025) administration. The decrease was greater in the group that r eceived saline compared with rats that received urea (P < 0.05). Serum ACE decreased in response to urea (P < 0.025) but not sodium administ ration, 4. We conclude that stimulation of the gastric sodium monitor results in a decrease in ACE activity in the liver, This decrease in A CE activity may be contributory to the decrease in AngII synthesis.