The Contrasting Responses of Splanchnic and Renal Glucose Output to Gluconeogenic Substrates and to Hypoglucagonemia in 60-h-fasted Humans

Abstract

SUMMARY Arterial concentrations and splanchnic as well as renal exchange of glucose, lactate, pyruvate, glycerol, and alanine were studied in the basal state and during intravenous infusion of alanine or glycerol in healthy subjects who had fasted for 60 h. After 45 min of infusion of alanine or glycerol, somatostatin administration (SRIF, 9 μg/min) was added to induce hypoglucagonemia. Alanine infusion, at a rate of 1 mmol/min, was accompanied by an 80% rise in splanchnic glucose output (SGO) and a small (16%) rise in arterial glucose. Splanchnic alanine uptake, which rose fivefold to sixfold above basal, could account for the entire rise in SGO. Intravenous administration of glycerol (1 mmol/min) resulted in an elevation of blood glucose (+ 9%) but no significant change in SGO, even though glycerol uptake by the splanchnic tissues rose eightfold above basal levels. Measurements of renal exchange of glucose indicated a small but statistically significant release of glucose from the kidney (0.04 ± 0.01 mmol/min) in the basal state, which accounted for 10% of total (renal plus splanchnic) glucose production. Administration of glycerol at a rate of 2 mmol/min resulted in a rise in renal glucose output to 0.16 ± 0.03 mmol/min, which could be accounted for in its entirety by renal glycerol uptake. In contrast, infusion of alanine (2 mmol/min), did not alter the renal glucose exchange. Infusion of SRIF was accompanied by a significant fall in the arterial glucagon level in all studies. Concomitante, large decreases in arterial glucose and SGO were observed during both alanine and glycerol administration. However, splanchnic uptake of gluconeogenic precursors was not influenced by SRIF administration. The elevated renal glucose output seen during glycerol administration was not altered by SRIF infusion. It is concluded that, in 60-h-fasted man, (a) renal glucose output contributes significantly to total glucose production, (b) administration of glycerol stimulates renal glucose production, (c) alanine infusion increases splanchnic but may not affect renal glucose output, and (d) hypoglucagonemia inhibits splanchnic but not renal glucose output. These data indicate that hepatic and renal gluconeogeneses respond differently with respect to stimulation by specific substrates and to their dependence on basal glucagon levels.