This lack of relationship was probably the consequence of the high variability of P i concentration. Interestingly, jejunal NaPi IIb occurred in two bands, which were inversely modulated by the diets. This indicated the expression of two isoforms of type IIb transporter in the upper small intestine of broilers.
The chicken protein sequence of NaPi IIb is highly homologous to that of zebrafish supporting the idea of the existence of two NaPi IIb isoforms also in broilers. Zebrafish NaPi IIb1 was a low affinity, high capacity, P i transporter, which was active in the presence of high concentrations of P i.
Substrate-based modulation of transporter expression also is a well-known physiological phenomenon for regulation of other intestinal nutrient transport processes such as glucose absorption.
For glucose absorption, sodium-linked glucose transporter 1 SGLT1 as a high affinity, low capacity transporter is replaced by low affinity, high capacity glucose transporter GLUT2 after ingestion of high carbohydrate meals Kellett, In addition, paracellular P i transport also could contribute to the overall P i absorption.
However, differences in transporter protein expression were generally low in this study - although significant - which was most likely due to the moderate differences in tP contents of the diets.
The protein expression of this high affinity, low capacity NaPi IIb was also reduced by supplemented phytase similar to the expression pattern in the jejunum. Furthermore, other transporters may play a role for the ileal P i transport. In both segments of the broiler small intestine, a second sodium-dependent P i transporter type, PiT1, was first described in this investigation. This transporter belongs to the type III sodium-dependent P i transporters and is a well-known housekeeping P i transporter responsible for the basal P i uptake in all body cells Kavanaugh and Kabat, In the ileum, PiT1 expression significantly decreased with phytase supplementation.
Thus, kinetic characteristics of this type III transporter could explain its down-regulation by high concentrations of available P i as provoked by phytase supplementation in the ileum of the broilers similar to the modulation pattern of the high affinity, low capacity NaPi IIb Band2 transporter. However, similar to the decrease in NaPi IIb Band2 transporter expression, the decrease in PiT1 expression in the ileum could not explain the increased amount of net absorbed P with phytase supplementation in this segment.
The exact regulatory processes and the quantitative role of this P i transporter for intestinal P i absorption in chicken still have to be determined. Beside the paracellular route of P i transport, solubility of P i may also play a role for modulation of absorption.
In the present study, diets containing MCP also contained higher concentrations of limestone compared to the diets without supplemental MCP. Hence, increased expression found in birds fed supplemental MCP indicated an enhanced availability of luminal ionized Ca as a result of higher dietary Ca concentration and differing Ca sources which may differ in solubility compared to the diets without MCP supplement. Whether InsP 6 derivatives themselves as produced by phytases and other phosphatases may influence P i transporter expression by so far unknown mechanisms is debatable.
http://co.organiccrap.com/171682.php Especially the InsP 3 derivatives, due to their structural similarity to the second messenger Ins 1,4,5 P 3 , and the end product after complete InsP 6 hydrolysis, myo -inositol, may evoke signaling pathways which modulate intestinal P i transporter expression. Besides regulation by available P i , this indicated a local modulation of intestinal transporter expression by lower InsP derivatives and by myo -inositol.
Since APN did not channel substrates into the transporter, we next hypothesized that the increase in apparent substrate affinity could be caused by an increase of the local substrate con-centration as compared with the bulk concentration. Do ESCs produce a large array of proteins? Rate of perfusion APN function was demonstrated using the peptidomimetic colorimetric substrates L -leucinenitroanilide or L -alaninenitroanilide Supplementary Figure S1B. Diets and water were provided for ad libitum consumption.
Activity of mucosal phytase may provide P i for absorption at the surface of the epithelium in addition to the dietary MCP and P i released by supplemented phytases. Hence, the local P i amount for regulation of P i transporter expression could be also determined by mucosal phytase activity.
However, regulation of this endogenous enzyme has not been well examined. In this group, luminal P i concentrations and amounts of net absorbed P were the highest compared to all other groups indicating that phytase activity and, potentially, protein expression was downregulated in the presence of more P i. Furthermore, lower InsP derivatives and myo -inositol may also be involved in the local regulation of endogenous mucosal phytase expression. However, the quantitative role of this process cannot yet be assessed. To conclude, this study provides evidence that supplemental phytase not only improved intestinal P digestibility but also led to adaptation of P i transporters in broilers.
Most likely, the amount luminal of P i was an important factor to modulate expression of transporter proteins.
However, other factors such as Ca:P ratio and lower inositol phosphates produced by phytase activity might be involved in regulation of transporter expression. P i transport in chicken intestine and its regulatory pathways has to be studied in more detail to optimize dietary intervention regarding P utilization for improved metabolic performance and lower environmental pollution.
The authors thank Kathrin Hansen for her substantial support in performing the Western blot analyses and establishing the phytase assay. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article Navigation.
Volume Article Contents. Editor's Choice. Modulation of small intestinal phosphate transporter by dietary supplements of mineral phosphorus and phytase in broilers Korinna Huber. Oxford Academic. Google Scholar. Ellen Zeller.
Markus Rodehutscord. Revision received:. Cite Citation.
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