J Prakasa Rao
Department of Physiology, Kasturba Medical College, Manipal, India

https://doi.org/10.36866/pn.67.24

In my opinion the simplest tool ever devised to measure the active transport in the intestine is the everted gut sac! Segments of intestine can be turned inside out and made into sacs by tying both ends, after filling with a physiological solution containing the substance whose transport one wishes to study. The sacs are incubated in the same solution under physiological conditions for a fixed period. If an active transport of the substrate under study occurs, its concentration inside the sac builds up while the outside solution loses the substrate. Using this method my graduate student Mary and I studied the phosphate transport in the mouse intestine. Easy availability, less food consumption and cheaper cost were the considerations in choosing the mice. Mary, returning to research after many years of teaching, had a way of handling men and mice alike. Both need to be caged, she often used to say! We noted that the duodenal segment showed the maximum transport which dropped to the minimum in the adjoining jejunal segment only to rise again in the ileum to reach the submaximal level (Mary & Rao, 2004)

Hence it was a bit of consternation to see two papers (Radanovic et al. 2005 and Marks et al. 2006) that showed duodenum to be the least effective segment in transporting phosphate in mice. While it is easy to take shelter under headings like differing experimental conditions and newer technologies, I thought it may not be a bad idea to delve deeper into these investigations.

The pattern of increasing transport of phosphate from jejunum to ileum is common to all the three studies quoted above. Duodenal transport in our studies seems to be quite high when compared to others who used brush border membrane vesicles or in situ ligated loop to monitor the transport of phosphate. Both detected mRNA responsible for the synthesis of NaPillb in the mouse duodenum, even though the transporter itself is almost unnoticeable. And both mention that modes other than the sodium dependent NaPillb mediated transport, may exist. Such possibility is fortified by a recent report (Williams & De Luca, 2007) showing that phosphate transport in vivo does not require sodium! It is therefore most likely that the everted sacs we employed might have used all the transport modalities, sodium dependent and independent, to push phosphate uphill into the serosal compartment.

In our study we found that duodenum while transporting phosphate maximally also shows a gender difference, being higher in males. This difference has not been reported in either of the two differing papers since they have used only male mice for experimentation. Gender difference has been reported in rats with reference to intestinal calcium transport (Uhland-Smith & De Luca, 1993). Moreover, oestrogen (Colin et al. 1999) and testosterone (Hope et al. 1992) seem to stimulate it, specifically in the duodenum. Therefore, it is not unreasonable to assume that sex hormones may be responsible for the observed differences in phosphate transport too. Then how do we account for the poor duodenal transport of phosphate reported in the two studies, since the age and weight of mice employed in all the three studies is about the same. An old publication (Mirskaia & Crew 1930) gave me the hint. Time of onset of puberty and sexual maturity show a lot of variability in mice! A recent publication (Miller et al. 2002) also endorses such variability. Since I have become a ‘social scientist’ (if you wish further enlightenment on this term, refer to Physiology News 54, 33-35) may I appeal to Marks et al. to reinvestigate the effect of sex hormones on the duodenal transport. In mice of course!

References

Colin EM, Van Den Bend GJ, Van Aken M, Christales S, De Jouge HR, De Luca HF, Prahl JM, Birkenhagen JC, Buurman CJ & Van Leeuven JP (1999). Evidence for involvement of 17 beta-estradiol in intestinal calcium absorption independent of 1, 25 dihydroxy vitamin D3 level in rat. J.Bone Min Res 14, 57-64.

Hope WG, Ibana MJ & Thomas ML (1992). Testosterone alters duodenal calcium transport and longitudinal bone growth in parallel in the male rat. Proc Soc Exp Biol Med 200, 536-541

Marks J, Srai SK, Biber J, Murer H, Unwin RJ & Debnam ES (2006). Intestinal phosphate absorption and the effect of vitamin D: a comparison of rats with mice. Exp Physiol 91,531-537.

Mary PL & Rao JP (2004). Gender difference in phosphate transport by the mouse intestine. Curr Sci 86, 511-512.

Miller RA, Harper JM, Dysko RC, Durkee SJ & Austad SN (2002). Longer life spans and delayed maturation in wild derived mice. Exp Biol Med 227, 500-508.

Mirskaia L & Crew FAE (1930). On the genetic nature of the time of attainment of puberty in the female mouse. Quart J Exp Physiol 20, 299-304.

Radanovic T Wagner CA, Murer H & Biber J (2005). Regulation of intestinal phosphate transport 1. Segmental expression and adaptation to low Pi diet of the type llb Na Pi cotransporter in the mouse intestine. Am. J Physiol 288, G496-G500.

Uhland-Smith A & De Luca HF (1993). 1, 25 Dihydroxy chole calciferol analogs cannot replace vitamin D in normocalcemic male rats. J Nutri 123, 1777-1785.

Williams KB & De Luca HF (2007) Characterization of intestinal phosphate absorption using a novel in vivo method. Am J Phys Endo Met (in press).