Galactose clearance and carbohydrate metabolism across the gastrointestinal tract in the newborn lamb

We prepared 16 newborn lambs with chronically indwelling catheters in the portal vein, mesenteric vein, femoral vein, and femoral artery to study galactose clearance, portal venous blood flow, and carbohydrate metabolism across the gastrointestinal (GI) tract. Galactose clearance was measured by infusing galactose into the femoral vein to achieve a steady-state galactose concentration in the femoral artery. We observed a curvilinear relationship between galactose clearance and the steady-state galactose concentration. The relationship could be modeled as an apparent Michaelis-Menten system: Clearance = V_max (K_m + [Gal]_a^ss), where V_max = 17.0 ± 2.5 mg/min/kg body weight and K_m = 11.0 ± 0.4 mg/dL. Substrate/oxygen quotients across the viscera drained by the portal vein were measured in the fasted state and during systemic galactose infusion. A net uptake of glucose and galactose by the GI tract was found with quotients of 0.19 ± 0.07 and 0.05 ± 0.02, respectively. There was a relatively large net efflux of lactate across the portal circulation, with a quotient of -0.13 ± 0.03. The indicator-dilution technique was used to estimate portal venous blood flow (PVBF) in the neonatal period with a resting, fasted state value of 92.8 ± 4.4 mL/min/kg body weight. This study in the healthy, unstressed, growing newborn lamb demonstrates (1) galactose clearance is a nonlinear function of galactose concentration; (2) the maximal capacity of newborn lambs to metabolize galactose is ∼12 mg/kg/min; (3) GI tract is an extrahepatic site of galactose consumption; (4) there was a significant plasma galactose concentration in the fasting state; and (5) there was a net glucose consumption and lactate production by the GI tract of approximately equal magnitude. These observations of resting galactose concentration and galactose clearance are consistent with resting endogenous galactose production.