Farm Animal Metabolism and Nutrition

rate of impulses generated in afferent fibres
of the autonomic nerves terminating in the
liver. In ruminants, glucose normally is
synthesized from propionate and branched-
chain amino acids, rather than being
oxidized, and it is not surprising that
infusions of glucose have little or no effect
on feeding behaviour.
The importance of the liver innerva-
tion in communication of its metabolic
status to the CNS has been shown by
sectioning the branches of the vagal and
sympathetic nerves entering the liver. The
clear intake-depressing effect of glucose
infused into the avian equivalent of the
portal vein (the coccygeo-mesenteric vein)
is also partly prevented by bilateral section
of the vagus nerves as they pass over the
surface of the proventriculus (stomach)
(Shurlock and Forbes, 1981b).
Ruminant animals rely on liver uptake
of propionate and its use for glucose
synthesis. Figure 15.5 shows that infusion
of sodium propionate into the hepatic
portal vein of sheep depresses food intake
relative to a control infusion of sodium
chloride, while infusion of propionate into

the general circulation via the jugular vein

had no effect (Anil and Forbes, 1980).

Denervation of the liver prevents the effect

of portal vein infusion of propionate, as

does temporary blockade of impulses

travelling towards the brain in the

splanchnic nerves (Anil and Forbes, 1988),

clearly demonstrating that the liver

transmits its metabolic information to the

CNS via the nervous system rather than in

the blood.

One problem in the acceptance of the

importance of the liver in the control of

food intake has been the lack of a signifi-

cant effect of liver denervation on daily

food intake. It would be expected that, if

the role of the liver was central, intake

would increase in the absence of the

negative feedback information to the CNS.

However, there are clear changes in feeding

behaviour after liver denervation in several

species (rabbit, sheep, chicken) in which

the meals become much larger but less

frequent. This is what would be expected if

there were other mechanisms, in addition

to liver sensitivity, involved in the control

of intake; the absence of signals from the

Physiological and Metabolic Aspects 325

Fig. 15.5.Intake of pelleted food by sheep infused with (A) saline (Cont) or sodium propionate into the
mesenteric (Prop) or jugular (Jugul) veins (Anil and Forbes, 1980); (B) saline (Cont) or sodium propionate
into the mesenteric vein without (Prop) or with (Block) anaesthetic blockade of the splanchnic nerves (Anil
and Forbes, 1988).

300

250

200

150

100

50

0

Food intake (g 3 h

^1

)

Cont Port Jugul Cont Prop Block

Treatment

*

**

(A) (B)