Review: Phosphorus in Fish Nutrition
Review: Phosphorus in Fish Nutrition
Review: Phosphorus in Fish Nutrition
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isoforms are IIb (see Werner & K<strong>in</strong>ne 2001), but the renal and <strong>in</strong>test<strong>in</strong>al isoforms are fairly different <strong>in</strong> sequence,<br />
except flounder that has an identical NaPi <strong>in</strong> both tissues. Pi uptake stimulator (PiUS) or <strong>in</strong>ositol hexakisphosphate<br />
k<strong>in</strong>ase (Schell et al. 1999) may be <strong>in</strong>volved <strong>in</strong> the absorption of dietary P. Norbis et al. (1997) first isolated PiUS<br />
mRNA from rabbit duodenum. Katai et al. (1999) subsequently isolated the correspond<strong>in</strong>g sequence from rat<br />
small <strong>in</strong>test<strong>in</strong>e. These PiUS cRNA markedly <strong>in</strong>creased Na-dependent Pi-uptake when <strong>in</strong>jected <strong>in</strong>to Xenopus laevis<br />
oocytes. Katai et al. (1999) further described that dietary P restriction (7 days) up-regulated PiUS genes (~2 times,<br />
per mRNA abundance) and Pi uptake from BBMV (~2 times) <strong>in</strong> the rat <strong>in</strong>test<strong>in</strong>e. In fish, PiUS <strong>in</strong> both the <strong>in</strong>test<strong>in</strong>e<br />
and kidney appears to be only slightly responsive to chronic dietary P restriction.<br />
Dietary regulation of Pi transport and NaPi-II mRNA/prote<strong>in</strong> expression <strong>in</strong> the <strong>in</strong>test<strong>in</strong>e have been studied<br />
<strong>in</strong> mammals (Hattenhauer et al., 1999; Katai et al., 1999; Huber et al., 2000; Huber et al., 2002). In mammals,<br />
the ma<strong>in</strong> site of dietary P absorption is the proximal small <strong>in</strong>test<strong>in</strong>e (Danisi & Murer 1991), and unlike kidney,<br />
apical expression of NaPi-IIb prot e<strong>in</strong> <strong>in</strong> the small <strong>in</strong>test<strong>in</strong>e, is mostly <strong>in</strong> response to longer term (days) situations<br />
(Murer et al. 2001). In ra<strong>in</strong>bow trout, Avila et al. (2000) reported that active Pi transport is higher <strong>in</strong> the proximal<br />
than distal <strong>in</strong>test<strong>in</strong>e. They also have functionally characterized P-transport <strong>in</strong> trout us<strong>in</strong>g everted <strong>in</strong>tact <strong>in</strong>test<strong>in</strong>al<br />
sleeves. It was shown to have a saturable, career-mediat ed active component and a diffusive non-saturabl e<br />
component. The study also showed that the dietary Pi level did not affect Pi-uptake rat e <strong>in</strong> trout <strong>in</strong>test<strong>in</strong>e at day-7,<br />
but significantly down-regulat ed Pi-uptake at day 28, <strong>in</strong>dicat<strong>in</strong>g that the response of NaPi-II <strong>in</strong> trout <strong>in</strong>test<strong>in</strong>e to<br />
dietary P concentration is not acute, but chronic. They did not measure NaPi-II prote<strong>in</strong> or mRNA abundance of the<br />
tissues. Avila et al. (1999) <strong>in</strong>cubated everted trout <strong>in</strong>test<strong>in</strong>e for one hour <strong>in</strong> a R<strong>in</strong>ger solution conta<strong>in</strong><strong>in</strong>g either<br />
vitam<strong>in</strong> D3, 25(OH)D or 1,25 (OH) 2D; however, none of these pre-<strong>in</strong>cubation did <strong>in</strong>crease 32 P uptake from the<br />
<strong>in</strong>test<strong>in</strong>e.<br />
Pyloric caeca (PC) are f<strong>in</strong>ger-like diverticula stemm<strong>in</strong>g from the duodenal region of the small <strong>in</strong>test<strong>in</strong>e of<br />
many fish species. The physiological functions of PC have not been well researched; however, they are thought to<br />
be the auxiliary to the proximal small <strong>in</strong>test<strong>in</strong>e because of their histological simalities. Budd<strong>in</strong>gton & Diamond<br />
(1987) studied PC of ra<strong>in</strong>bow trout: It has about 56 PC, which collectively contribute about 70% of the total gut<br />
surface area. They also studied the contribution of PC to the total uptake capacity (of the entire gut) for glucose,<br />
the dipeptide carnos<strong>in</strong>e, and n<strong>in</strong>e am<strong>in</strong>o acids, which ranged between 68 and 81%. Thus, the functional<br />
contribution of PC corresponds well to its contribution to the total gut surface area. A recent study has shown that<br />
Pi absorption <strong>in</strong> trout PC is largely a passive paracellular process facilitated by high lum<strong>in</strong>al Pi concentrations.<br />
Approximately 89% of total Pi absorption takes place <strong>in</strong> PC, and about 92% of the Pi absorption <strong>in</strong> PC is diffusive at<br />
physiological lum<strong>in</strong>al [Pi] of 20 mM (i.e., lum<strong>in</strong>al [Pi] when a normal P diet is fed). This expla<strong>in</strong>s the fractional Pi<br />
absorption, which is cont<strong>in</strong>uous over the wide range of dietary P levels, even at concentrations much higher than the<br />
dietary requirement. The active Pi absorption is modulated by several factors, <strong>in</strong>clud<strong>in</strong>g temperature, lum<strong>in</strong>al Pi<br />
concentration, fish P status, and <strong>in</strong> particular lum<strong>in</strong>al pH. An isoform of NaPi isolated from trout PC is similar to<br />
the <strong>in</strong>test<strong>in</strong>al NaPi <strong>in</strong> base sequence, but functionally different one another.<br />
NaPi transporters are sodium-dependent symporters. Zebrafish and flounder NaPi-IIb are<br />
Na+-dependent. Trout <strong>in</strong>test<strong>in</strong>al NaPi-IIb is Na+ dependent (Avila et al., 2000). Trout PC NaPi is<br />
Na+-dependent at alkal<strong>in</strong>e pH, but not at neutral pH. Pi uptake <strong>in</strong> PC is much higher at alkal<strong>in</strong>e pH, and this<br />
<strong>in</strong>creas e is strictly Na+-dependent, which is similar to that of the mammalian renal NaPi isoforms (Hilfiker et al.,<br />
1998; Murer et al., 2001). Danisi & Murer (1991) reported Na+-<strong>in</strong>dependent, active (carrier-mediated) Pi<br />
transport <strong>in</strong> rat and chick <strong>in</strong>test<strong>in</strong>al basolateral membrane and <strong>in</strong> dog renal basolateral membrane; however, the<br />
molecular identity of the transporter is unknown. Mammalian <strong>in</strong>test<strong>in</strong>al NaPi isoforms have higher transport rates<br />
at neutral to acidic pH (Berner et al., 1976; Borowitz & Ghishan, 1989; Danisi et al., 1984; Lee et al., 1986;<br />
Tenenhouse, 1999; Xu et al., 2002). Thus, <strong>in</strong>test<strong>in</strong>al NaPi are functionally different between mammals and fish.<br />
However, <strong>in</strong> chick, pig and sheep <strong>in</strong>test<strong>in</strong>es, Pi transport rate is higher at alkal<strong>in</strong>e pH than acidic pH (Danisi &<br />
Murer, 1991), which is similar to <strong>in</strong>test<strong>in</strong>al and renal NaPi isoforms of zebrafish, flounder and trout (Forster et al.,<br />
1997; Graham et al., 2003; Kohl et al., 1996; Nalbant et al., 1999). S<strong>in</strong>ce at pH higher than 7.2 divalent ions<br />
2– –<br />
(HPO4 ) will be the dom<strong>in</strong>ant species than monovalent ions (H2PO4 ), the preferred ions for the fish NaPi isoforms<br />
and mammalian renal NaPi isoforms may be the divalent ion, whereas for mammalian <strong>in</strong>test<strong>in</strong>al isoforms, the<br />
preferred species may be the monovalent form.<br />
Dietary Acidification and P availability<br />
Heitzmann (1873) stated that lactic acid <strong>in</strong>creased <strong>in</strong> the course of rickets and brought about dissolution of the salts<br />
of the bones. Anderson (1878) hypothesized about the etiology of scurvy as follows; "The conditions<br />
characteristic of this disease (i.e., scurvy) are always accompanied with a deficiency <strong>in</strong> the food of the materials for<br />
© 2000, 2005. Shozo H. Sugiura. All rights reserved.<br />
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