[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 5. PROXIMATE MECHANISMS OF AGE-RELATED DECREASES IN TRANSPORT RATES There are a number of mechanisms through which age can alter nutrient transport rates. 5.1 Nonspecific mechanisms. There are a large number of studies on age-related changes in intestinal surface area, cell proliferation rate and membrane permeability, each of which can nonspecifically alter intestinal transport. A detailed discussion of these important parameters is beyond the scope of this review. Almost all studies find intestinal mass to increase with age all the way through senescence. Early studies as exemplified by Moog (37) suggest an increase in mouse intestinal weight with age. While some of the increase is due to an increase in amounts of connective tissue, there is also an increase in the number of villi, thereby increasing mucosal mass and absorptive surface area in rats (38). In rats, most of the increase in villus epithelium is in the distal gut regions (22, 39, 40), and the increase is sufficient enough to compensate for age-dependent loss in jejunal function (41). Ferraris et al (32) showed that the age-dependent increase in tissue weight per cm in the ileum is tightly correlated with age-dependent increases in villus heights in this region. This suggests that changes in intestinal weights due to age can be accounted for by changes in amount of intestinal mucosa. In humans, age has no significant effect on intestinal surface area (42). An increase in surface area would enhance the transport per cm or per intestine for all nutrients, and is an easy parameter to measure. Because cells in the small intestine turnover every 3 - 5 days, a change in intestinal mass is the likely outcome of a change in the rate of intestinal cell proliferation. There is a vast literature on intestinal cell proliferation rate in aging, perhaps because of its cancer implications. This subject is beyond the scope of this review but a review by Atillasoy and Holt (43) suggest that there is a state of hyperproliferation of epithelial cells of the small intestine, especially in aging rodents. In rats, Holt and Yeh (44) found a significant increase in intestinal cell production rate, especially in distal regions. Goodlad and Wright (35) found modest or insignificant increases in the proliferation of intestinal cells while Ecknauer et al (38) found cell production rate to be independent of age. In mice, Ferraris and Vinnakota (45) found rates of enterocyte turnover and migration rate along the crypt/villus axis to be independent of age, except in the distal region where there was a modest and statistically borderline increase in migration rate. Levels of polyamines, factors thought to be required for growth of the gastrointestinal tract, are much higher in aged than in young rats (46). Age-related changes in proliferation rates may not only affect mass and surface area, but also the ratio of absorptive to nonabsorptive cells (14). If this ratio increases, there is a corresponding increase in nutrient absorption rate per mg tissue or cell protein. If this ratio decreases, the absorption rate per mg also decreases. A change in ratio of absorptive to nonabsorptive cells is difficult to demonstrate experimentally. We have attempted to detect changes in this ratio by in situ hybridization of SGLT1 and GLUT5 mRNA, but failed to obtain any significant difference. There is probably only a single study able to demonstrate a change in ratio of differentiated (probably absorptive) to undifferentiated cells in aged rats (47). Holt et al assayed for enzyme activity in cryostat sections collected perpendicular to the crypt villus axis of the small intestine. They concluded that reduced enzyme specific activities in mucosal homogenates from aging animals is due to an increase in the proportion of relatively undifferentiated villus cells. This approach cannot be used in transport studies which requires a much higher number of cells for transport assays. Ligand binding to nutrient transporters, a method used by Ferraris and Diamond (48) to demonstrate diet-induced changes in site density of SGLT1 transporters along the crypt/villus axis, cannot detect small changes in ratios. A potentially important factor affecting transport is an age-related change in passive permeability of intestinal cells which would nonspecifically change transport rate. The permeability of the rat small intestine to high molecular weight probes is independent of age. However, the permeability to low molecular weight probes increases modestly with age (49). In humans (50, 51) and mice (32), however, there is no age-dependent change in passive permeability. There are other nonspecific mechanisms which have either a direct (e.g. change in membrane potential) or indirect (e.g. intestinal motility) effect on transport, but these factors have either not been studied or were found to be independent of age. The absence of an overshoot in studies utilizing brushborder membrane vesicles from aged animals suggest age-related changes in the electrochemical gradient for sodium (25, 26, 27). In human small intestine, motility is independent of age (52). 5.2 Specific mechanisms. An often mentioned mechanism underlying age-related decreases in intestinal glucose transport is a decrease in the number of Na⁺-dependent glucose carriers (30, 31). Ferraris et al (32) provided direct evidence in support of this hypothesis by finding that the number of specific, glucose-protectable phlorizin sites (presumably the number of glucose carriers since phlorizin competitively binds to the glucose carrier) decreases with age. The decrease in site number is tightly correlated with the decrease in intestinal glucose transport. In mice, the age-related decrease in transporter number is modest until senescence (> 27 months old), which reflects the difficulty in getting good evidence for age-related decreases in transport. Paradoxically, Western blots show no significant difference in amounts of SGLT1 in brushborder membranes of aged and young mice (32), presumably because changes in amounts of protein are too small to detect by scanning densitometry. It is also possible that similar amounts of immunoreactive SGLT1 are actually found in both ages, but that SGLT1 from aged mice are less efficient in binding phlorizin and in transporting glucose. Steady state levels of SGLT1, GLUT5 and GLUT2 mRNAs as shown by Northern blot analysis or by RT-PCR (Casirola et al in press), are similar. This suggests that age-related decreases in transporter activity are not correlated with changes in steady state levels of mRNA coding for those transporters.

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

5. PROXIMATE MECHANISMS OF AGE-RELATED DECREASES IN TRANSPORT RATES

There are a number of mechanisms through which age can alter nutrient transport rates.

5.1 Nonspecific mechanisms.

There are a large number of studies on age-related changes in intestinal surface area, cell proliferation rate and membrane permeability, each of which can nonspecifically alter intestinal transport. A detailed discussion of these important parameters is beyond the scope of this review.

Almost all studies find intestinal mass to increase with age all the way through senescence. Early studies as exemplified by Moog (37) suggest an increase in mouse intestinal weight with age. While some of the increase is due to an increase in amounts of connective tissue, there is also an increase in the number of villi, thereby increasing mucosal mass and absorptive surface area in rats (38). In rats, most of the increase in villus epithelium is in the distal gut regions (22, 39, 40), and the increase is sufficient enough to compensate for age-dependent loss in jejunal function (41). Ferraris et al (32) showed that the age-dependent increase in tissue weight per cm in the ileum is tightly correlated with age-dependent increases in villus heights in this region. This suggests that changes in intestinal weights due to age can be accounted for by changes in amount of intestinal mucosa. In humans, age has no significant effect on intestinal surface area (42). An increase in surface area would enhance the transport per cm or per intestine for all nutrients, and is an easy parameter to measure.

Because cells in the small intestine turnover every 3 - 5 days, a change in intestinal mass is the likely outcome of a change in the rate of intestinal cell proliferation. There is a vast literature on intestinal cell proliferation rate in aging, perhaps because of its cancer implications. This subject is beyond the scope of this review but a review by Atillasoy and Holt (43) suggest that there is a state of hyperproliferation of epithelial cells of the small intestine, especially in aging rodents. In rats, Holt and Yeh (44) found a significant increase in intestinal cell production rate, especially in distal regions. Goodlad and Wright (35) found modest or insignificant increases in the proliferation of intestinal cells while Ecknauer et al (38) found cell production rate to be independent of age. In mice, Ferraris and Vinnakota (45) found rates of enterocyte turnover and migration rate along the crypt/villus axis to be independent of age, except in the distal region where there was a modest and statistically borderline increase in migration rate. Levels of polyamines, factors thought to be required for growth of the gastrointestinal tract, are much higher in aged than in young rats (46).

Age-related changes in proliferation rates may not only affect mass and surface area, but also the ratio of absorptive to nonabsorptive cells (14). If this ratio increases, there is a corresponding increase in nutrient absorption rate per mg tissue or cell protein. If this ratio decreases, the absorption rate per mg also decreases.

A change in ratio of absorptive to nonabsorptive cells is difficult to demonstrate experimentally. We have attempted to detect changes in this ratio by in situ hybridization of SGLT1 and GLUT5 mRNA, but failed to obtain any significant difference. There is probably only a single study able to demonstrate a change in ratio of differentiated (probably absorptive) to undifferentiated cells in aged rats (47). Holt et al assayed for enzyme activity in cryostat sections collected perpendicular to the crypt villus axis of the small intestine. They concluded that reduced enzyme specific activities in mucosal homogenates from aging animals is due to an increase in the proportion of relatively undifferentiated villus cells. This approach cannot be used in transport studies which requires a much higher number of cells for transport assays. Ligand binding to nutrient transporters, a method used by Ferraris and Diamond (48) to demonstrate diet-induced changes in site density of SGLT1 transporters along the crypt/villus axis, cannot detect small changes in ratios.

A potentially important factor affecting transport is an age-related change in passive permeability of intestinal cells which would nonspecifically change transport rate. The permeability of the rat small intestine to high molecular weight probes is independent of age. However, the permeability to low molecular weight probes increases modestly with age (49). In humans (50, 51) and mice (32), however, there is no age-dependent change in passive permeability.

There are other nonspecific mechanisms which have either a direct (e.g. change in membrane potential) or indirect (e.g. intestinal motility) effect on transport, but these factors have either not been studied or were found to be independent of age. The absence of an overshoot in studies utilizing brushborder membrane vesicles from aged animals suggest age-related changes in the electrochemical gradient for sodium (25, 26, 27). In human small intestine, motility is independent of age (52).

5.2 Specific mechanisms.

An often mentioned mechanism underlying age-related decreases in intestinal glucose transport is a decrease in the number of Na⁺-dependent glucose carriers (30, 31). Ferraris et al (32) provided direct evidence in support of this hypothesis by finding that the number of specific, glucose-protectable phlorizin sites (presumably the number of glucose carriers since phlorizin competitively binds to the glucose carrier) decreases with age. The decrease in site number is tightly correlated with the decrease in intestinal glucose transport. In mice, the age-related decrease in transporter number is modest until senescence (> 27 months old), which reflects the difficulty in getting good evidence for age-related decreases in transport. Paradoxically, Western blots show no significant difference in amounts of SGLT1 in brushborder membranes of aged and young mice (32), presumably because changes in amounts of protein are too small to detect by scanning densitometry. It is also possible that similar amounts of immunoreactive SGLT1 are actually found in both ages, but that SGLT1 from aged mice are less efficient in binding phlorizin and in transporting glucose. Steady state levels of SGLT1, GLUT5 and GLUT2 mRNAs as shown by Northern blot analysis or by RT-PCR (Casirola et al in press), are similar. This suggests that age-related decreases in transporter activity are not correlated with changes in steady state levels of mRNA coding for those transporters.