The arteriovenous differences in the caecum of the rat have been compared for
volatile fatty acids (VFA) and for
electrolytes. Our results suggest the possibility of an exchange between VFA and
chloride at the level of the caecal wall, rather than a net exchange between VFA and
bicarbonate; however, the role of
bicarbonate or Cl- at the cellular level is still unknown.
Acetate uptake by the liver was enhanced when
acetate in the afferent plasma was increased in fed as in starved rats, showing that
acetyl CoA synthetase was still active during
starvation. A release of endogenous
acetate was only observed in situations of very active ketogenesis (
starvation at the end of pregnancy). In physiological conditions,
propionate and
butyrate reaching the liver were almost quantitatively removed. However,
butyrate was taken up by the liver at a higher rate than
propionate after intracecal loads.
Propionate was very efficiently utilized as a glucogenic substrate and without noticeable disturbance of
lactate metabolism. After administration of
acetate loads in starved rats, hepatic ketogenesis increased slightly. There was a marked difference between ketogenesis from
butyrate in fed and starved rats. The low ketogenesis from
butyrate in the fed rats stressed the important role of metabolic pathways of
acetyl-CoA utilization in the control of ketogenesis. In contrast to
alanine or
lactate,
propionate was poorly antiketogenic in the rat.