Department of Physiology, University of Veterinary Medicine Hannover, Bischofsholer Damm 15/102, 30173, Hannover, Germany.
Pflugers Arch. 2019 Jan;471(1):185-191. doi: 10.1007/s00424-018-2181-5. Epub 2018 Jul 16.
Ruminants have a unique utilization of phosphate (P) based on the so-called endogenous P recycling to guarantee adequate P supply for ruminal microbial growth and for buffering short-chain fatty acids. Large amounts of P enter the gastrointestinal tract by salivary secretion. The high saliva P concentrations are generated by active secretion of P from blood into primary saliva via basolateral sodium (Na)-dependent P transporter type II. The following subsequent intestinal absorption of P is mainly carried out in the jejunum by the apical located secondary active Na-dependent P transporters NaPi IIb (SLC34A2) and PiT1 (SLC20A1). A reduction in dietary P intake stimulates the intestinal P absorption by increasing the expression of NaPi IIb despite unchanged plasma 1,25-dihydroxyvitamin D concentrations, which modulate P homeostasis in monogastric species. Reabsorption of glomerular filtrated plasma P is mainly mediated by the P transporters NaPi IIa (SLC34A1) and NaPi IIc (SLC34A3) in proximal tubule apical cells. The expression of NaPi IIa and the corresponding renal Na-dependent P capacity were modulated by high dietary phosphorus (P) intake in a parathyroid-dependent manner. In response to reduced dietary P intake, the expression of NaPi IIa was not adapted indicating that renal P reabsorption in ruminants runs at a high level allowing no further increase when P intake is diminished. In bones and in the mammary glands, Na-dependent P transporters are able to contribute to maintaining P homeostasis. Overall, the regulation of P transporter activity and expression by hormonal modulators confirms substantial differences between ruminant and non-ruminant species.
反刍动物具有独特的磷酸盐（P）利用方式，基于所谓的内源性 P 循环，以保证瘤胃微生物生长和缓冲短链脂肪酸所需的足够 P 供应。大量的 P 通过唾液分泌进入胃肠道。高唾液 P 浓度是通过 P 通过基底外侧钠（Na）依赖性 P 转运体 II 从血液主动分泌到初级唾液中产生的。随后的 P 肠吸收主要在空肠中通过位于顶端的次级主动 Na 依赖性 P 转运体 NaPi IIb（SLC34A2）和 PiT1（SLC20A1）进行。尽管血浆 1,25-二羟维生素 D 浓度不变，但降低饲粮 P 摄入会通过增加 NaPi IIb 的表达来刺激肠 P 吸收，从而调节单胃动物的 P 稳态。肾小球滤过的血浆 P 的重吸收主要由近端小管顶端细胞中的 P 转运体 NaPi IIa（SLC34A1）和 NaPi IIc（SLC34A3）介导。NaPi IIa 的表达和相应的肾 Na 依赖性 P 能力受高饲粮 P 摄入的甲状旁腺依赖性调节。响应于低饲粮 P 摄入，NaPi IIa 的表达没有适应，表明当 P 摄入减少时，反刍动物的肾 P 重吸收水平较高，无法进一步增加。在骨骼和乳腺中，Na 依赖性 P 转运体能够有助于维持 P 稳态。总的来说，激素调节剂对 P 转运体活性和表达的调节证实了反刍动物和非反刍动物之间存在显著差异。
