Participation of the liver receptors in the regulation of ion composition osmolality and extracellular fluid volume

Effect of an increase in brain serotonin on the osmoregulatory response to a hypo- or hyperosmotic load in Wistar and vasopressin-deficient Brattleboro rats

Serotonin and its receptor agonists stimulate the release of arginine vasopressin (AVP) into peripheral blood under intraventricular injection. To test the hypothesis that brain serotonin can modulate the development of natural osmoregulatory responses, the effect of an increase in endogenous brain serotonin on the response to an intragastric hypo- or hyperosmotic loading was studied in Wistar and AVP-deficient Brattleboro rats. 5-Hydroxytryptophan (5-HTP), the rate-limiting serotonin biosynthesis precursor known to increase the brain level of serotonin, was injected intraperitoneally (5 mg/100 g body weight). The renal functional parameters (glomerular filtration rate [GFR], free water reabsorption, and urine flow rate) were monitored during the 4 h after intragastric infusion of water or a 2% NaCl solution (5% of body weight). Plasma AVP was measured by radioimmunoassay. In Wistar rats, intraperitoneal injection of 5-HTP at the same time as water loading prevented the development of the renal diuretic response: there was no increase in urine flow rate and GFR, and free water reabsorption remained at the high level. In AVP-deficient Brattleboro rats, unlike Wistar rats, 5-HTP treatment was without effect on the renal function parameters. In Wistar rats, injection of 5-HTP at the peak of water diuresis produced an abrogation of the diuretic response to water loading due to the increase in free water reabsorption. Plasma AVP increased from 1.2 +/- 0.4 to 4.2 +/- 1.6 pg/ml (n = 8 in each group, p < 0.01). Hyperosmotic treatment of Wistar rats with a 2% NaCl solution stimulated AVP secretion compared to baseline (from 3.2 +/- 0.1, n = 7 to 5.6 +/- 0.9, n = 7, p < 0.01), and the saluretic response developed on the background of high free water reabsorption. When injected concomitantly with NaCl solution, 5-HTP revealed no additive effect on plasma AVP and on free water reabsorption. We conclude that the 5-HTP-caused increase in brain serotonin contributed significantly to the dynamics of changes in the osmoregulatory response to the hypo-osmotic challenge due to stimulation of AVP secretion. 5-HTP had no additive effect on the osmoregulatory response to hyperosmotic loading. Peripherally injected 5-HTP had no effect on the renal function, being absent in AVP-deficient Brattleboro rats.

Celiac-superior mesenteric ganglionectomy, but not vagotomy, suppresses the sympathoadrenal response to insulin-induced hypoglycemia

Afferent innervation of the portal vein has been shown to be critical in hypoglycemic detection, but the neural pathway by which these afferents ascend remains unknown. To ascertain the role of vagal afferents versus spinal afferents in hypoglycemic detection, the catecholamine response to hypoglycemia was assessed in male Wistar rats undergoing hepatic vagotomy (HV), total subdiaphragmatic vagotomy (TSV), or celiac-superior mesenteric ganglionectomy (CSMG). After recovering from the surgery, the animals were exposed to a hyperinsulinemic-hypoglycemic clamp, with glucose infused peripherally via the jugular vein. In all animals, systemic hypoglycemia (2.64 +/- 0.03 mmol/l) was induced via jugular vein insulin infusion (25 mU x kg(-1) x min(-1)). No significant differences were observed among the groups with respect to arterial glucose or insulin concentration. When hypoglycemia was induced in sham-operated control animals, epinephrine was observed to rise from a basal value of 0.84 +/- 0.10 to 25.18 +/- 1.24 nmol/l. Neither HV nor TSV had any significant impact on the epinephrine response to hypoglycemia. In contrast, CSMG animals demonstrated a significant suppression in the epinephrine response to whole-body hypoglycemia (11.25 +/- 1.21 vs. 22.32 +/- 0.86 nmol/l in CSMG vs. controls; P < 0.05). The norepinephrine response for controls, 2.00 +/- 0.22 at basal and rising to 8.95 +/- 0.20 nmol/l in hypoglycemia, was not significantly different from that of the HV and TSV animals. As with epinephrine, the norepinephrine response to hypoglycemia was significantly suppressed in CSMG compared with control animals (4.72 +/- 0.48 vs. 7.15 +/- 0.76 nmol/l; P < 0.05). These findings are consistent with the idea that hypoglycemic detection at the portal vein is mediated by spinal, and not vagal, glucose-sensitive afferents.

Neural control of hepatic osmolytes and parenchymal cell hydration

Liver cell function is strongly influenced by cell hydration and osmolyte content via osmosensing and osmosignaling pathways. Electrical stimulation of hepatic nerves increases the hepatocellular hydration state via an alpha-adrenergic mechanism, promotes taurine release from parenchymal cells and myo-inositol release from hepatic stellate cells. Although changes in liver cell hydration and osmolyte content are known regulators of liver cell function, the functional relevance and integration of nerve-stimulation-dependent alterations of liver cell volume and osmolyte content in the overall hepatic response to towards signals from the nervous system remains to be established.

Hepatoportal bumetanide-sensitive K(+)-sensor mechanism controls urinary K(+) excretion

To determine whether a K(+)-sensor mechanism exists in the hepatoportal region, periarterial hepatic afferent nerve activity responses to intraportal injection of KCl were examined in anesthetized rats. Hepatic afferent nerve activity increased in response to intraportal injection in a K(+) concentration-dependent manner, and the increase was attenuated by inhibition of the Na(+)-K(+)-2Cl(-) cotransporter by bumetanide in a dose-dependent manner. These results suggest that a bumetanide-sensitive K(+)-sensor mechanism exists in the hepatoportal region. Stimulation of this mechanism by intraportal KCl infusion elicited an immediate and powerful kaliuresis with no significant change in the plasma K(+) concentration; this was significantly greater than the kaliuresis induced by intravenous KCl infusion and was attenuated by severing the periarterial hepatic nervous plexus. These results indicate that a hepatoportal bumetanide-sensitive K(+)-sensor mechanism senses the portal venous K(+) concentration and that stimulation of this sensor mechanism causes kaliuresis, which is mainly mediated by the periarterial hepatic nervous plexus.

Innervation of the liver: morphology and function

Although it has been known for many years that the liver receives a nerve supply, it is only with the advent of immunohistochemistry that this innervation has been analysed in depth. It is now appreciated not only that many different nerve types are present, but also that there are significant differences between species, especially in the degree of parenchymal innervation. This has stimulated more detailed investigation of the innervation of the human liver in both health and disease. At the same time, functional studies have been underlining the important roles that these nerves play in processes as diverse as osmoreception and liver regeneration. This article briefly reviews current understanding of the morphology and functions of the hepatic nerve supply.

Calcitonin gene-related peptide innervation of the rat hepatobiliary system

The digestive system is densely innervated by calcitonin gene-related peptide (CGRP)-immunoreactive neurons. The present study investigated a) the distribution and origin of CGRP-immunoreactive fibers in the rat hepatobiliary tract, and b) their relation with substance P/tachykinin (SP/TK) immunoreactivity using immunohistochemical and radioimmunoassay techniques. CGRP-containing fibers form dense networks in the fibromuscular layer of the biliary tree and surrounding the portal vein. Thin, varicose fibers are present at the base of the mucosa of the ducts. In the liver, labeled fibers are restricted to the portal areas and the stromal compartment. Neonatal treatment with capsaicin, a neurotoxin for primary afferent neurons, or celiac/superior mesenteric ganglionectomy depletes CGRP-containing fibers in the biliary tract, and reduces those associated with the portal vein. In contrast, subdiaphragmatic vagotomy does not appreciably modify the density of these fibers. Radioimmunoassay studies show a reduction of CGRP-immunoreactive contents in the biliary tract and portal vein by 84% and 65%, respectively, following capsaicin treatment, and by 80% and 66%, respectively, following ganglionectomy. By contrast, CGRP concentrations in vagotomized animals are comparable to those of controls. Most CGRP-positive fibers appear to contain SP/TK immunoreactivity, as indicated by double-label studies. These results demonstrate that the rat hepatobiliary tract is prominently innervated by CGRP- and CGRP/SP/TK-immunoreactive fibers, which are likely to originate from spinal afferent neurons. The abundance of these fibers and their association with a variety of targets are in line with the involvement of these peptidergic visceral afferents in regulating hepatobiliary activities, including hemodynamic functions of the hepatic vasculature.