[Frontiers in Bioscience 2, e108-115, November 1, 1997]

	[Frontiers in Bioscience 2, e108-115, November 1, 1997] Reprints PubMed CAVEAT LECTOR
	EFFECT OF AGING AND CALORIC RESTRICTION ON INTESTINAL SUGAR AND AMINO ACID TRANSPORT Ronaldo P. Ferraris Dept of Pharmacology and Physiology, UMDNJ-New Jersey Medical School, 185 South Orange Ave., Newark, NJ 07103 Received 7/17/97 Accepted 10/2/97 4. EFFECT OF AGING ON AMINO ACID TRANSPORT There are considerably fewer studies on the effect of aging on intestinal amino acid transport. There were modest decreases in rates of tyrosine, phenylalanine, tryptophan and histidine uptakes per mg in everted rings (33) or brushborder membrane vesicles (27, 34) of aging rat jejunum. Chen et al (31) found similar decreases in transepithelial tyrosine transport per g, but only in 36-, and not in 24- or 30-month old mice. Ferraris et al (32) found no statistically significant age-related changes in transapical membrane transport of five amino acids between 7- and 24-month old mice, although they found transport of each amino acid to exhibit a decreasing trend with age. Probably there are several reasons why studies on age-dependence of transport activity differ in their conclusions. First and probably most important, is the method of normalization of uptake rates. An excellent case in point is the study of Goodlad and Wright (35) who found absorption per intestine or per unit length of intestine to increase with age in rats. The main mechanism underlying this increase is a marked increase in intestinal length and weight per cm, even though absorption per mg decreased. Review of literature showed that many studies reporting decreased intestinal transport with age use methods that normalize uptake to intestinal weight or to amount of protein (32). On the other hand, some studies which reported increased intestinal transport with age instead normalized uptake to length. Second, the choice of age is critical. Age-related decreases in uptake remain rather modest until well past the median lifespan. In many rodents that have a median lifespan of about 24 mo, significant changes in uptake become easily discernible only at 27 months or later (29, 31, 32). Third, initial transmembrane uptake rates should be measured at substrate concentrations yielding V_max and should be corrected for diffusive uptake. This is due to the fact that uptake measured at low concentrations is subject to significant unstirred layer effects, while uptake at overly high concentrations may contain a large diffusive component (see Karasov and Diamond (36) for a detailed discussion).

[Frontiers in Bioscience 2, e108-115, November 1, 1997]
Reprints
PubMed
CAVEAT LECTOR

EFFECT OF AGING AND CALORIC RESTRICTION ON INTESTINAL SUGAR AND AMINO ACID TRANSPORT

Ronaldo P. Ferraris

Dept of Pharmacology and Physiology, UMDNJ-New Jersey Medical School, 185 South Orange Ave., Newark, NJ 07103

Received 7/17/97 Accepted 10/2/97

4. EFFECT OF AGING ON AMINO ACID TRANSPORT

There are considerably fewer studies on the effect of aging on intestinal amino acid transport. There were modest decreases in rates of tyrosine, phenylalanine, tryptophan and histidine uptakes per mg in everted rings (33) or brushborder membrane vesicles (27, 34) of aging rat jejunum. Chen et al (31) found similar decreases in transepithelial tyrosine transport per g, but only in 36-, and not in 24- or 30-month old mice. Ferraris et al (32) found no statistically significant age-related changes in transapical membrane transport of five amino acids between 7- and 24-month old mice, although they found transport of each amino acid to exhibit a decreasing trend with age.

Probably there are several reasons why studies on age-dependence of transport activity differ in their conclusions. First and probably most important, is the method of normalization of uptake rates. An excellent case in point is the study of Goodlad and Wright (35) who found absorption per intestine or per unit length of intestine to increase with age in rats. The main mechanism underlying this increase is a marked increase in intestinal length and weight per cm, even though absorption per mg decreased. Review of literature showed that many studies reporting decreased intestinal transport with age use methods that normalize uptake to intestinal weight or to amount of protein (32). On the other hand, some studies which reported increased intestinal transport with age instead normalized uptake to length. Second, the choice of age is critical. Age-related decreases in uptake remain rather modest until well past the median lifespan. In many rodents that have a median lifespan of about 24 mo, significant changes in uptake become easily discernible only at 27 months or later (29, 31, 32). Third, initial transmembrane uptake rates should be measured at substrate concentrations yielding V_max and should be corrected for diffusive uptake. This is due to the fact that uptake measured at low concentrations is subject to significant unstirred layer effects, while uptake at overly high concentrations may contain a large diffusive component (see Karasov and Diamond (36) for a detailed discussion).