Effects of Glucagon and Butyrate

1. Isolated lamb liver cells were prepared from 24h-starved animals by venous perfusion of the excised caudate lobe with buffer containing collagenase. On the basis of Trypan-Blue exclusion, rate of 02 uptake, adenine nucleotide content and retention of constitutive enzymes, these cells werejudged to be intact. 2. Isolated caudate-lobe liver cells showed rates of gluconeogenesis from O0mM-propionate and 10mM-lactate that compared favourably with rates determined in isolated median-lobe cells and with rates determined with the isolated perfused lamb liver. 3. The gluconeogenic potential of substrates tested depended on the lamb's age. Cells prepared from suckling lambs (up to 20 days of age and essentially non-ruminant) showed highest rates from galactose, serine and alanine; those prepared from post-weaned lambs (older than 30 days of age and ruminant) showed highest rates from propionate, lactate and fructose. 4. Gluconeogenic rates from endogenous precursors, 10mM-propionate and I0mM-galactose, were linear for 1 h and were both stimulated by 1 pM-glucagon. Provided the endogenous rate ofgluconeogenesis remained unchanged after substrate addition, glucagon caused a net stimulation of gluconeogenesis from each of these substrates. 5. Gluconeogenic capacity and glucagon sensitivity were examined in cells maintained in substrate-free oxygenated buffer at 370, 220 and 0°C. Even under the best of the three conditions of storage that were tested (i.e. at 22°C in gelatin-containing buffer) deterioration of the lamb cells proceeded rapidly, and loss of glucagon responsiveness preceded the loss of ability to convert precursor into glucose. 6. n-Butyric acid, 2-methylpropanoic acid and 3-methylbutanoic acid at concentrations comparable with those found in lamb portal-vein blood each stimulated gluconeogenesis from 10mM-galactose or 10mM-propionate; gluconeogenesis from galactose was stimulated to the greater extent. 7. The regulatory effects of glucagon and sodium butyrate on lamb liver-cell gluconeogenesis and glycogenolysis were compared. Glucagon (1 uM) and 2mM-butyrate accelerated the rate of glucose formation in liver cells of 24h-starved animals from lactate+pyruvate or fructose. Insulin (20nM) decreased both gluconeogenesis and the efficacy of 1 uM-glucagon. For lactate + pyruvate as substrate, the stimulatory effect of butyrate was additive to that of 1 uM-glucagon and for both lactate + pyruvate and fructose the stimulatory effect of butyrate was not influenced by 20nM-insulin. In contrast with glucagon, which stimulated the rate of glycogenolysis in cells prepared from fed lambs, butyrate (0.1-20mM) had no effect. 8. It is concluded that glucagon and butyrate stimulate lamb liver-ell gluconeogenesis by different mechanisms.