Intestinal amino acid and monosaccharide transport in suckling pigs fed milk replacers with different sources of carbohydrate.

Omnivorous mammals are able to adaptively modulate rates of intestinal nutrient transport to match changes in diet. Because adaptive responses during suckling, when dietary composition is relatively constant, have not been adequately determined, we measured in vitro sugar and amino acid uptake [nmol/(mg tissue.min)] in suckling pigs fed milk replacers with either lactose (LAC) or a 60:40 mixture of maltodextrin and sucrose (MDS). The MDS-fed pigs initially grew slower, but had intestinal dimensions similar to those of LAC-fed siblings when normalized to body weight. Carrier-mediated uptake for three monosaccharides (glucose, galactose, fructose) did not differ between LAC- and MDS-fed pigs at 5, 10, 15 and 20 d of age. Interdiet differences in rates of leucine and proline uptake, despite identical types and concentration of protein in both milk replacers, are indicative of non-specific responses to diet during suckling. Uptake capacities (grams of monosaccharide absorbed per 24 h) never exceeded estimates of monosaccharide intake by more than fourfold and were less than aldohexose intake during early suckling. Our results indicate 1) age-related changes in rates of nutrient uptake are genetically programmed and little influenced by diet; 2) any responses to diet are nonspecific and likely involve a shift in the timing of the genetic program; and 3) at birth and throughout suckling, pigs are capable of absorbing limited quantities of alternative nutrients.

Postnatal development of monosaccharide transport in pig intestine.

In vitro brush-border transport of three monosaccharides by pig intestine was studied as a function of postnatal age from immediately after birth before suckling to after weaning. Rates of transport normalized to tissue weight or surface area for glucose (Glc), galactose (Gal), and fructose (Fru) were highest at birth, with a steep decline after the onset of suckling, probably caused by any combination of three or more factors: reduced transporter site density, shifts in relative abundances of different monosaccharide transporters, and/or changes in activities of individual transporters. Whereas highest rates of Glc and Fru transport shifted from proximal to midintestine after weaning, Gal transport remained highest in proximal intestine. Postnatal increases in Km values for Gal, but not Glc, indicate there are multiple aldohexose transporters that undergo separate developmental trajectories. The presence of Fru transport in neonatal pigs may reflect a more advanced state of development than neonatal rats and rabbits, or may be an adaptation for early weaning. Changes in Fru-to-Glc and Gal-to-Glc transport ratios before weaning suggest transporter development is partly genetically hard-wired, apparently to prepare pigs for weaning. Curiously, Fru-to-Glc transport ratios were lower than those of rat and rabbit, but closely paralleled those of the carnivorous cat.