Transport of Niacin and Niacinamide in Perfused Rat Intestine

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Abstract
A one pass perfusion system was used to study the absorption of niacin (NA) and niacinamide (NAm) by the small intestine of the rat. To avoid the complication of uptake and metabolism by erythrocytes, an erythrocyte-free Krebs-Ringer phosphate-bicarbonate buffer containing bovine serum albumin, glucose, glutamine, norepinephrine, and dexamethazone was used. Intestinal preparations appeared normal for more than 1 hour using this perfusate. Comparisons were made between several types of perfusions: one pass or recirculating perfusions where compounds were injected into the lumen, recirculating perfusions where compounds were injected into the perfusate, recirculating perfusions where NAm was placed in the perfusate and the lumen irrigated throughout the experiment and live animal studies where the compounds were injected into the lumen. [14C]NAm was absorbed more rapidly into the perfusate than [14C]NA and at a rate proportional to dosage from 0.16 to 1,600 µmole. [14C]NA was absorbed less rapidly into the perfusate, but more of the isotope was retained by the intestine. For NA, the uptake into the intestine and perfusate was not proportional to dosage at 160 µmole and above. NA was taken up more readily by the intestine when administered in the perfusate than was NAm but little passed into the lumen. When NAm was administered there was a larger accumulation of isotope in the lumen and when the lumen was irrigated considerably more isotope passed into the contents at the expense of both the intestine and perfusate.