Ontogenetic development of nutrient transporters in bullfrog intestine

The ratio of intestinal glucose (Glc) to amino acid (AA) transporter activity rises with age in animals of species in which the ratio of dietary carbohydrate to protein also rises. Does this shift function to match transporter activity to current dietary substrate levels or to the high AA requirements of young growing animals? We compared intestinal brush-border uptake of Glc and the AA proline (Pro) in adult and tadpole bullfrogs, since with age this species changes from an herbivore to a carnivore and hence its dietary carbohydrate-to-protein ratio decreases rather than increases. Like typical adult herbivores, tadpoles have a long, highly coiled, narrow-bore, thin-walled intestine with a long, heavy colon, whereas adult bullfrogs have a short, wide-bore, thick-walled intestine typical of carnivores. The ratio of Glc to Pro uptake capacity is essentially the same in adults as in larval tadpoles and is typical of carnivores but unprecedently low for herbivores. This suggests that both functions proposed for developmental shifts of Glc and AA transport in other species are significant and that their effects cancel each other in bullfrogs. Transporter kinetic constants vary predictably with developmental stage. Intestinal daily uptake capacity is comparable to actual daily intake for Glc in adults and for Pro in both tadpoles and adults but is much lower than intake for Glc in tadpoles, probably because most carbohydrate ingested by tadpoles is in the form of high molecular weight polymers that are not utilized.