SENSITIVITY OF EXERCISE-INDUCED INCREASE IN HEPATIC GLUCOSE-PRODUCTION TO GLUCOSE SUPPLY-AND-DEMAND

It was hypothesized that the exercise-induced changes in glucoregulatory hormones and glucose production (R(a)) occur as a result of a small deficit in glucose availability. To test this, 18-h fasted dogs performed 150 min of treadmill exercise with either the liver as the sole source of glucose (controls, n = 5) or with glucose infused from 0 to 50 min (period 1) and from 100 to 150 min (period 3) at rates designed to track the glucose utilization (R(d)) response (ExoGlc, n = 5). The liver alone supplied glucose from 50 to 100 min (period 2). Isotopic and arteriovenous methods were used to assess R(a), R(d), and gluconeogenesis (GNG). Variable [H-3]glucose infusion and frequent sampling were used to facilitate R(a) measurements. Arterial glucose declined by -3.1 +/- 1.4, -4.3 +/- 2.9, and -6.4 +/- 3.7 mg/dl in periods 1-3 in controls (changes are mean values during each of the 50-min periods; P < 0.05). In ExoGlc, arterial glucose did not deviate from basal in periods 1 (+ 0.1 +/- 1.8 mg/dl) and 3 (+ 1.5 +/- 4.5 mg/dl) but fell from basal (P < 0.05) by the same amount as controls in period 2 (-5.7 +/- 2.1 mg/dl). Matching the R(d) response with exogenous glucose led to increases in arterial and portal vein plasma insulin levels (P < 0.05) but did not affect glucagon, norepinephrine, epinephrine, and cortisol levels. R(a) was elevated by 3.1 +/- 0.5, 4.0 +/- 1.1, and 4.7 +/- 1.1 mg.kg(-1).min(-1) in periods 1-3 in controls (P < 0.05). In ExoGlc, R(a) rose by 0.0 +/- 0.4, 4.1 +/- 1.4 (P < 0.05), and 0.4 +/- 0.7 mg.kg(-1).min(-1), respectively, in periods 1-3. The rise in R(a) was reduced in periods 1 and 3 of ExoGlc compared with controls (P < 0.02). GNG rose to similar to 250% basal in controls and did not respond with any significant difference in ExoGlc. In summary, the exercise-induced increases in counterregulatory hormones and GNG are present even when a deficit in glucose supply is eliminated by an exogenous glucose infusion. In contrast, the fall in insulin and the rise in hepatic glycogenolysis are greatly attenuated. The regulatory components affected by exogenous glucose predominate at the liver as deviations in plasma glucose of similar to 4% correspond to similar to 60% changes in R(a). In conclusion, the high sensitivity of the pancreatic beta-cell and liver to small deviations in glucose availability created by an exogenous glucose infusion supports the hypothesis that the exercise-induced rise in R(a) is controlled, to a large extent, by a feedback signal proportional to the ratio of glucose supply and muscle glucose demand.