Untersuchungen über Mechanismen der Osmo- und Volumenregulation: II. Untersuchungen über den Einfluß intravenös, intraporitle

Plasma arginine vasopressin response to oral, gastric, and intravenous water load in patients with cirrhosis

BACKGROUND/AIMS

Hepatic osmoreceptors are sensitive to changes in portal blood osmolality and cause variations in plasma antidiuretic hormone and water diuresis, which prevent major systemic osmotic changes. Insensitivity of hepatic osmoreceptors might contribute to maintaining increased plasma levels of antidiuretic hormone and negative free water clearance after oral hydration in some cirrhotic patients.

METHODS

We measured free water clearance and plasma arginine vasopressin levels in basal conditions and after oral, intravenous, and intragastric water overload in control subjects (group I) and patients with cirrhosis (some with a positive [group II] and some a negative [group III] free water clearance).

RESULTS

In groups I and II, no significant differences in plasma arginine vasopressin levels were found regardless of the route used for hydration. In group III, plasma levels were significantly higher between 0 and 75 minutes after intragastric hydration than after oral or intravenous hydration. No significant changes in plasma osmolality were detected between minute 0 and 20 after the end of oral or intragastric hydration, although plasma arginine vasopressin decreased significantly only 5 minutes after the oral hydration. Group III patients showed evidence of autonomic neuropathy. However, this dysfunction was unrelated to the insensitivity to gastric hydration.

CONCLUSIONS

Hepatic osmoreceptors seem to be disturbed in some cirrhotic patients, although this dysfunction may be compensated for by normally functioning oropharyngeal and hypothalamic osmoreceptors.

Splanchnic control of vasopressin secretion in conscious rats

Saline solutions (NaCl, 2 ml, pH 7.4, 10-598 mosmol/kgH2O) were infused over 4 min in conscious rats, via tail artery catheter or intragastric tube. Intragastric infusions of hyper- and hypotonic solutions caused, within 14.4 +/- 2.2 min, a maximal increase and decrease, respectively, of plasma vasopressin (AVP) relative to time controls (r = 0.97; P less than 0.00001) without affecting systemic plasma osmolality (r = -0.09; P less than 0.92). Mean changes of plasma AVP between 11 and 21 min were also correlated with the osmolality of gastric infusion (r = 0.72; P less than 0.000001), whereas systemic osmolality was unchanged (r = 0.14; P less than 0.42). Systemic infusions caused within 9.0 +/- 2.0 min a maximal change in both plasma AVP (r = 0.82; P less than 0.00001) and systemic osmolality (r = 0.97; P less than 0.00001). However, mean changes of plasma AVP between 11 and 21 min weakly correlated with the osmolality of systemic infusions (r = 0.27; P less than 0.20), although correlations between mean changes of systemic osmolality and the osmolality of systemic infusions were significant (r = 0.72; P less than 0.00001). Lack of correlations with mean arterial pressure and heart rate suggest that hemodynamic changes did not mediate the AVP responses. Pretreatment with atropine methyl bromate (2 mg/kg) abolished the AVP response to gastric but not systemic infusions of hypertonic saline. These results indicate that a splanchnic cholinergic receptor mechanism modulates AVP secretion during a moderate gastric intake of salt or water.

[Comparison of the effects of gastroenteric and parenteral saline loading on kidney function after inhibition of prostaglandin synthesis (author's transl)]

We inhibited the prostaglandin synthesis with indomethacin in rats during a 7-h (periods: 1--4h; 4--5.5h; 5.5--7h) gastroenteric and parenteral infusion of isotonic saline. Indomethacin was given in a dosis of 10 mg/kg of b. wt. by stomach tube after 5.5-h infusion of saline. We studied how far prostaglandins are involved in the different function of the kidney by measuring the water and sodium excretion and the cortico-medullary osmotic and albumin concentrations of the kidney.

RESULTS

1. a) Studies Without Indomethacin. As compared to infusion of isotonic saline into the stomach water excretion is decreased by 30% (P less than 0.02) and 35% (P = n.s.) in the second and third 90 min period during parenteral infusion of saline. As compared to gastroenteric infusion we found an increase of the total osmolarity by 13% (P less than 0.02) in the cortex of the kidney. b) Studies with Indomethacin. As compared to saline infusion into the stomach indomethacin-treated rats decrease their urinary flow rates in 90 min by 26% (P = n.s.) in the presence of equivalent saline infusion into the aorta. Total osmolarity of the parenteral loaded rats increases by 17% (P less than 0.01) in the cortex of the kidney. 2. Indomethacin leads to an increased concentrating ability of the kidney in both gastroenteric and parenteral loaded rats. 3. In the presence of indomethacin plasma albumin of the kidney increases in the cortex and in the outer medulla of the kidney. In the inner medulla, and in the papillary tip, however, albumin concentration decreases significantly. As compared to gastroenteric saline infusion, the osmolarity of the kidney tissue increases and the urinary flow rate decreases during equivalent parenteral infusion of isotonic saline. The prostaglandins are not responsible for the pronounced diuresis during infusions of isotonic saline into the stomach.

Role of hepatic portal osmoreception in the control of ADH release

Dogs were prepared for experiments by chronic implantation of catheters into the hepatic portal vein. Intraportal infusions of hypertonic saline, 5 ml/min, increased plasma antidiuretic hormones (rat bioassay) in absence of significant changes in systemic plasma osmolality. In the 5th min of 1.8, 2.7, or 3.6% saline infusion, plasma antidiuretic hormone (ADH) increased significantly from 3.2 +/- 0.7 to 14.3 +/- 3.6 (SE) microU/ml, from 2.3 +/- 0.4 to 42.0 +/- 7.6 microU/ml, and from 1.9 +/- 0.6 to 64.4 +/- 16.8 microU/ml, respectively. In control experiments plasma ADH did not change with isotonic saline infused intraportally or with 3.6% saline given into a systemic vein. The transsection of hepatic vagal afferent fibers abolished plasma ADH increase dependent on hypertonic intraportal infusion. The data support the existence of osmoreceptors within the portal vascular bed and indicate ADH involvement in the control of fluid balance by intrahepatic blood osmalality. The information on osmolality changes is neurally transmitted to the hypothalamus via hepatic vagal afferents. A demonstration of an increase in portal but not systemic blood osmolality following food intake suggests that intrahepatic osmoreception may be important in physiological regulation of water metabolism.