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Abstract
These experiments were conducted to evaluate the role of D(-)lactate in digestive disturbances and elucidate the metabolic potential for D(-)lactate elimination by the bovine.
In experiment one eight Hereford heifers (302 kg) were fed alfalfa hay (R) or a 90% concentrate diet (C) for 28 days. Intakes of metabolizable energy were equal for both diets. Glucose turnover rates were 143.3 and 154.3 mmol/h for R and C respectively. Turnover rates for D(-)lactate were .67 mmol/h for R and .57 mmol/h for C. Rates of oxidation and gluconeogenesis from D(-)lactate were .61 and .22 mmol/h for R and .55 and .16 mmol/h for C. Recovery of ('14)C in urine and expired CO(,2) accounted for 3.73 and 44.3% for R and 2.13 and 43.5% for C.
The rate of oxidation and gluconeogenesis from D(-) and L(+)lactate were determined in slices of kidney cortex, liver, heart and sternomandibularis muscle from cattle. Rates of oxidation were similar for both isomers (P < .05) at .1 and 1 mM for kidney and for gluconeogenesis at .1 mM for kidney and liver. Oxidation of D(-)lactate declined to 43, 39, 24 and 25 percent of the rate of L(+)lactate as the lactate concentration increased to 50 mM for liver, kidney, heart and muscle respectively.
A third experiment utilized 5 crossbred steers (347 kg) surgically fitted with rumen fistulae, hepatic portal, abdominal aortic and mesenteric venous catheters to quantify acid absorption from the gut. Treatments were glucose (12 g/kg) dosed intraruminally (G) or a 70% concentrate diet ad libitum (A). Samples were taken at time 0 then every 2 hrs for 48 hrs. Blood pH and HCO(,3)('-) were only slightly depressed for G despite peak rumen levels of 47 and 77 mM for D(-) and L(+)lactate respectively. Mean rates of net portal absorption were 10.5 and 96.6 mmol/h for A and 71.4 and 164.4 mmol/h for G for D(-) and L(+)lactate respectively. Volatile fatty acid absorption was 4.4, 4.1, 5.7, 5.4 and 13.3 times greater for A than G for acetate, propionate, butyrate, isobutyrate and isovalerate respectively.
