The liver is well placed to monitor the uptake of nutrients from the digestive tract and, given its role in smoothing out this erratic supply to maintain more stable levels in the general circulation, it must be able to detect concentrations or rates of uptake of metabolites such as glucose and amino acids. There are important nervous links between the liver and the central nervous system and it is perhaps surprising that little serious attention was paid to the role of the liver as a source of signals for controlling food intake until the early 1970s (reviewed by Anil and Forbes, 1987).
The potent effect of portal glucose infusion is reduced by liver denervation, and electrical activity is induced in some neurones in the lateral hypothalamus by portal infusion of glucose. The activity was modified by either vagotomy or splanchnotomy. Nerve endings have not been found in liver parenchyma but there are many nerve endings in the wall of the portal vein, and superfusion of the exposed inner wall of the hepatic portal vein with glucose solution decreases firing of some vagal fibres.
There is considerable evidence that the supply of oxidizable substrates to the liver is monitored in simple-stomached animals with transmission of information to the central nervous system via the nerves of the hepatic plexus, and that this plays a part in the control of feeding (Forbes, 1988a).
The liver is an important site of fat metabolism in mammals, where in the rat the intake-depressing effects of fatty acids are blocked by substances that block fatty acid oxidation. Vagal section prevents these effects, further strengthening the belief that fatty acids can play a significant role in satiation via the liver. The hypophagic effect of metabolites is linked to a particular step in their metabolism, principally in the liver (Scharrer et al., 1996). Whereas glycerol and malate depress intake, their oxidation products dihydroxyacetone and oxaloacetate do not, indicating that the oxidation process itself is important. Lactate and its product of metabolism, pyruvate, depress intake to an equal extent and, as lactate is metabolized to pyruvate and as pyruvate itself is oxidized to acetyl-CoA, the importance of the oxidation step is supported.
When a high-fat diet was offered, lactate and pyruvate no longer showed the intake-depressing effect, but fat is known to depress pyruvate oxidation; it is likely that such a depression occurred in this experiment because blood levels of these metabolites remained elevated for longer after injection in rats fed the high-fat diet compared with those on a high-carbohydrate diet. All of this evidence points to the oxidation step in liver metabolism being the key to effects of the liver on food intake (Forbes, 1988a).
Several studies have failed to confirm an important role for the liver in sensing nutrients and transmitting the information via the nervous system, but M.G. Tordoff and M.I. Friedman (personal communication) have noted that those experiments in which there has been no effect on feeding of increasing nutrient flow to the liver have exposed animals to different, contrasting, treatments at short intervals, thus confusing them by not allowing adequate time for them to learn the effects of different infusions (see Chapter 6). Despite the evidence presented here, there is still considerable controversy as to whether the liver is important in the control of intake under physiological conditions.
Poultry
Several experiments have investigated the glucostatic theory of intake control in birds without finding any significant depression due to glucose injections or infusions. In the first recorded infusions of glucose solutions into the coccygeo- mesenteric vein (leading into the liver) of chickens, Shurlock and Forbes (1981a) infused a solution of 60 g glucose/l at a rate of 1.2 ml/min into cockerels over a period of 3 h. Even though this high rate of infusion had no effect on food intake when given into the jugular vein (32 versus 28 g/3 h), it almost completely suppressed feeding when given into the liver (34 versus 2 g/3 h). Further, when a range of solutions from 1 to 60 g/l was given, there was a highly significant negative correlation between the concentration of glucose in the solution and the weight of food eaten during the infusion (see
Fig. 4.1). The slope of the relationship is steep at lower concentrations, which is the physiological range, suggesting that liver sensitivity to glucose has the potential to exert a powerful control over feeding in the normal chicken.
The effects of overnight fasting on the response of cockerels to portal infusion of glucose were investigated by Shurlock and Forbes (1981a). Intake was depressed only by the highest dose of glucose when access to food was allowed during the infusion. When food was withheld until the end of the infusion there was a large, dose-dependent inhibition of voluntary intake. This result, together with observations made during the post-infusion period in previous experiments, showed that the effect of glucose lasted beyond the infusion period and was more likely to be a result of changes in liver glycogen or glucose content rather than simply an effect of the concentration of glucose in the blood perfusing the liver.
An amino acid mixture infused into the portal vein also depressed intake, while jugular introduction at the same rate had a much smaller effect (Shurlock and Forbes, 1984); effects of glucose and amino acids were additive and possibly affected the same system in the liver. Lysine infused at 50–150 mg/3 h caused a reduction in intake during the 3-h treatment, whereas ammonia at an equivalent rate did not and leucine had a delayed effect (Rusby and Forbes, 1987a). Vagotomy at the level of the proventriculus blocked these effects of portal infusion of glucose and lysine (Rusby et al., 1987; Fig. 4.2), although doubt is cast on this conclusion by the lower intake of the vagotomized birds during the control infusion, compared with the intact chickens, suggested as being due to the changed feeding pattern of the former (see below).
0 10 20 30 40 50 60 70 80 90
0 1 2 3 4 5 6 7
Concentration of glucose in infusate (g/l)
Food intake (g)
Fig. 4.1. Effect of infusing at 1.2 ml/min isotonic saline (including glucose) into the liver of cockerels on food intake over 1 h (䡲) and 3 h (●) (from Shurlock and Forbes, 1981a).
Adrenaline injected into the hepatic portal vein at low doses of up to 0.1 mg/kg depresses intake, an effect which is blocked by section of the hepatic branch of the vagus nerve (Howes and Forbes, 1987a). As adrenaline causes glycogenolysis, these results suggest that it is glucose concentration in or around hepatocytes that is important rather than the direction of flow of glucose through the hepatocyte membrane. In order to study the mode of action of adrenaline, an ␣-adrenergic blocker, phenylephrine, was given into the liver at a wide range of doses, but without effect; a -blocker, salbutamol, depressed intake but the range of doses used (500–2000 g) was probably too high to achieve its effect just by a physiological blocking of endogenous adrenaline. The intake-depressing effect of injecting glucagon into the liver of chickens is also blocked by vagal section (Howes and Forbes, 1987b), but whether the dose used was physiological is open to question.
In birds, most fat synthesis occurs in the liver, and plasma levels of triglycerides are much higher than in mammals. Cockerels of an egg-laying strain were sensitive to fat infused into the hepatic portal vein, but not into the jugular vein, while in broilers there was no effect, irrespective of site of infusion (Lacy et al., 1986). Has selection of meat-type chickens bred out some of the liver’s sensitivity to metabolites?
If the liver is important in the control of food intake, it would be expected that liver denervation would disrupt feeding behaviour. Local denervation has not been undertaken in the chicken, but section of both vagus nerves as they cross the proventriculus (the equivalent of sub-diaphragmatic vagotomy in mammals) has been carried out, this affecting many visceral organs, not just the liver. It is perhaps surprising, therefore, that Savory and Hodgkiss (1984) found no differences in feeding behaviour between vagotomized and
30 40 50 60 70
Control Lysine Glucose Lysine + glucose
Food intake (g/3 h)
Fig. 4.2. Food intake during 3-h infusions of glucose and/or lysine solutions into the liver of intact (solid bars) or vagotomized (open bars) chickens (from Rusby et al., 1987).
sham-vagotomized chickens, although Rusby et al. (1987) observed signi- ficantly fewer meals, each of a larger size, but there was no effect on total daily intake. If, as suggested in Chapter 10, many feedback factors are integrated by the CNS, the loss of some of these would not necessarily result in a change in daily intake, but would be likely to result in larger meals.
Pigs
Infusion of a solution of 150 g glucose/l, or of 250 ml of a solution of 100 g amino acids/l, into the jugular vein or hepatic portal vein during feeding had no effect on the voluntary intake of pigs trained to eat in one session per day under operant conditions (Stephens, 1980). These results are in contrast to those with other mammals and birds, but there is no a priori reason why the pig should be different to other mammals, as its liver metabolism is similar to that of the rat. It seems likely that the feeding regime used in experiments with pigs, with only one or two periods of access to food per day, makes them reliant on different stimuli to terminate feeding compared with animals with continuous access to food. Perhaps the large amount they must eat in the short period of access emphasizes the physical aspects of gut-fill as a limit to meal size.
Ruminants
Ruminant animals absorb most of their energy from the digestive tract in the form of volatile fatty acids (VFAs), of which propionate is gluconeogenic and has been shown to depress intake in sheep (see below). The only direct study involving infusions into the hepatic portal vein of cattle was that of Elliot et al. (1985), who noted reduced intake during 20 out of 30 infusions of 20–50 mmol/min of sodium propionate into the portal vein of growing cattle;
they ascribed the variability of their results to their method of feeding, which was restricted to 2.5% of body weight/day, given in 24 hourly meals.
It had been found in sheep that propionate depressed intake to a greater extent when infused into the ruminal vein than into other vessels, and the existence of propionate receptors at that site was proposed. Anil and Forbes (1980b) showed that a 3-h infusion of 1.2 mmol/min of sodium propionate into the hepatic portal vein almost completely prevented feeding in sheep, while the same rate of infusion into the jugular vein had little effect. It seems highly likely, therefore, that the ruminant liver is sensitive to its rate of utilization of propionate.
The route(s) taken by information from the liver to the CNS have been studied by sectioning the hepatic plexus of nerves, by sectioning or anaesthetizing the splanchnic nerves and by sectioning the hepatic branch of the vagus nerve (Anil and Forbes, 1980b, 1988). Almost complete section of the hepatic plexus, bilateral splanchnotomy (Fig. 4.3a) or temporary blockade of nervous trans- mission in the splanchnic nerves (Fig. 4.3b) all prevented the intake-depressing effect of 3-h infusions of sodium propionate into the hepatic portal vein at
0 50 100 150 200 250 300 350
Intact Splanchnotomy Hepatic vagotomy
Hepatic denervation
**
Food intake (g/3 h)
(a)
Fig.4.3. (a) The effect on food intake of saline (solid bars) and sodium propionate (NaPr) solution (open bars) infused into the hepatic portal vein of sheep subject to bilateral splanchnotomy, section of vagal innervation of the liver or total liver denervation; (b) effect on food intake of sodium propionate infused into the hepatic portal vein of sheep with and without splanchnic blockade with anaesthetic; **, highly significant effect of propionate infusion (from Anil and Forbes, 1988).
Food intake (g/3 h)
(b)
0 50 100 150 200 250 300 350
Saline NaPr NaPr + block
1.2 mmol/min. The paradoxical situation whereby either one of the two likely routes for transmission of afferent information from liver to CNS results in blockage of the effects of propionate given into the liver has not been resolved, but may be due to some efferent involvement with the control of blood flow or enzyme activity that is prevented by nerve section.
These clear-cut effects of propionate infusion into the portal vein on feeding by sheep have not always been seen by other workers. De Jong (1981) infused a mixture of the sodium salts of VFAs into the hepatic portal vein of goats at a rate similar to that used for sheep by Anil and Forbes (1980b), but found no change in food intake despite a doubling of plasma levels of VFAs. The protocols used by the two groups were very similar, and it seems unlikely that there is a fundamental difference between such closely related species – perhaps the tips of the cannulae in de Jong’s animals were beyond the receptors in the wall of the veins entering the liver, while those of Anil and Forbes ended in the mesenteric vein into which they were introduced, i.e. short of the portal entry to the liver (see below). The effect observed in the sheep is not likely to be simply osmotic, because sodium acetate infused into the portal vein at 4 mmol/min (i.e. more than three times the osmotic load of the sodium propionate infusion) had no effect on feeding (Anil and Forbes, 1980b); acetate is not utilized by the liver in significant quantities.
Denervation of the liver does not affect daily food intake in several species, including sheep, and this has been cited as evidence that the liver does not play a role in the control of food intake. However, the meals are larger and less frequent than those of normal animals (Anil and Forbes, 1980b), as would be expected if some, but not all, of the negative feedback information from the visceral organs had been cut off.
A nervous pathway from the liver to the nucleus of the solitary tract in the medulla oblongata via the vagus nerve has been identified (M.H. Anil, P.
Chaterjee and J.M. Forbes, unpublished observations), and it would appear that the sheep is similar to the rat in that both vagal and splanchnic afferent pathways are involved in the transmission of information from liver to brain (Anil and Forbes, 1988).
Attempts in our laboratory to study vagal discharges from chemoreceptors in the liver of the sheep have not revealed any consistent responses to propionate injected into the hepatic portal vein (M.H. Anil and J.M. Forbes, unpublished observations). Nor have we been able to find receptors in liver tissue of sheep or chickens by electron microscopy (P. Chaterjee, M.H. Anil and J.M. Forbes, unpublished results). This is in line with the failure to demonstrate receptor nerve endings in the parenchyma of the liver of several other species, including the rat (see above). Structures with the appearance of chemoreceptors have, however, been located in the hepatic portal vein of the rat, so further studies are required in farm animals. The finding that propionate depressed food intake by sheep when given for 20 min at 2 mmol/min into a mesenteric vein – but not the portal vein (where the tip of the infusion catheter would lie within the liver) – supports the idea that the receptors are in the portal vein rather than in liver parenchyma (Leuveninket al., 1997).
Ingestion by sheep of the plant Lantana camaraleads to ruminal stasis and anorexia, which McSweeney and Pass (1983) showed to be due to liver damage rather than to direct effects of the toxin on receptors in the reticulo- rumen. Liver denervation of intoxicated animals reduced the severity of the reduction in ruminal muscular activity, and it was concluded that the effects of lantana poisoning on the rumen were indirect and that the ruminal stasis was reflex in nature, the primary lesion being in the liver.