Localization of aquaporin-3 proteins in the bovine rumen

Abstract

Urea nitrogen salvaging is a crucial mechanism that ruminants have evolved to conserve nitrogen. Facilitative urea transporter-B proteins are known to be involved in urea transport across the rumen epithelium and thus efficiently facilitate the urea nitrogen salvaging process. Recently, functional studies have suggested that aquaglyceroporin transporters might also play a significant role in ruminal urea transport and aquaporin-3 (AQP3) protein has previously been detected in rumen tissue. In this current study, we investigated the specific localization of AQP3 transporters in the bovine rumen. First, end-point reverse-transcription PCR experiments confirmed strong AQP3 expression in both bovine rumen and kidney. Immunoblotting analysis using 2 separate anti-AQP3 antibodies detected AQP3 protein signals at 25, 32, and 42-45 kDa. Further immunolocalization studies showed AQP3 protein located in all the layers of rumen epithelium, especially in the stratum basale, and in the basolateral membranes of kidney collecting duct cells. These data confirm that AQP3 transporters are highly abundant within the bovine rumen and appear to be located throughout the ruminal epithelial layers. The physiological significance of the multiple AQP3 proteins detected and their location is not yet clear, hence further investigation is required to determine their exact contribution to ruminal urea transport.

The TRPV3 channel of the bovine rumen: localization and functional characterization of a protein relevant for ruminal ammonia transport

“…+ can be assumed. The finding that complex proteins are required to mediate ruminal transport not only of urea [50,65] but also of ammonium across the ruminal wall should end the concept of ruminal nitrogen recycling as the result of a "leaky" epithelium. Instead, nitrogen recycling appears an efficient mechanism to remove protons from the rumen with the energy coming from the liver (Fig.…”

“…Clearly, a clarification of the mechanism responsible for the high efflux of ammonia from the rumen is overdue. The expression profile in the ovine rumen does not appear to support the involvement of either RhCG or RhCB [65]. Systematic studies of ruminal transport of ammonia have established that electroneutral transport of NH 3 is predominant at an alkaline pH of 7.4.…”

“…Unfortunately, large quantities of ammonia are absorbed from the rumen before they can be utilized. However, the ruminal epithelium expresses transport proteins through which urea can reenter the rumen and serve as a source of nitrogen for microbial protein synthesis [2,43,50,65]. Since this protein can be fully digested in the following parts of the gastrointestinal tract, this recycling of nitrogen allows ruminants to subsist on low-grade, poorly digestible fodder while yielding milk and meat.…”

Sex-Related Differences in UT-B Urea Transporter Abundance in Fallow Deer Rumen

“…In addition, a revealing functional study by Walpole and colleagues suggested a potential role in ruminal urea transport for aquaglyceroporins, such as AQP3 [10]. After AQP3 protein was first identified in the rumen [16], it has recently been demonstrated that ruminal AQP3 RNA levels are regulated by diet [17] and that two distinct AQP3 proteins may localize to various layers of cow rumen papillae [18].…”

Effects of butyrate− on ruminal Ca2+ transport: evidence for the involvement of apically expressed TRPV3 and TRPV4 channels

“…Furthermore, both K + channels [ 19 ] and K + transporters such as NKCC [ 50 ] can transport NH 4 + , which is not surprising since the biophysical properties of both ions are similar. Functional data from in vitro studies of the ruminal epithelium show that at an unphysiologically high pH of 7.4, absorption of ammonia involves diffusion of NH 3 [ 3 ]—possibly via AQP3 [ 120 ] since Rh glycoproteins do not appear to be expressed [ 119 ]. However, at the more physiological pH of ~ 6.4 found in vivo, uptake of NH 4 + clearly predominates [ 3 , 11 , 13 ].…”