The transport of nitrogen across the rumen epithelium is characterized by absorption of ammonia from the rumen and by an influx of urea into the rumen. The transport rates of both compounds are large and exhibit wide variation. The transport of ammonia occurs in two forms: in the lipophilic form as NH₃, the magnitude of which is linearly related to the pH in the ruminal fluid at pH values above 7, while at a physiological pH of 6.5 or lower, ammonia is predominantly absorbed as NH₄⁺via putative potassium channels in the apical membrane. The uptake of NH₄⁺depends on the potential difference of the apical membrane, Pd_a, and shows competition with K uptake. The pathway for basolateral exit of NH₄⁺is unknown. Hence, the relative transport rates of NH₃or NH₄⁺are determined by the ruminal pH according to the Henderson–Hasselbalch equation. Transport of ammonia interacts with the transport of Na and Mg mainly via changes of the intracellular pH. Urea recycling into the rumen has been known for many years and the transport across the rumen epithelium is mediated via urea transporters in the luminal and basolateral membrane of the epithelium. Transport of urea occurs by simple diffusion, but is highly variable. A significant increase of urea influx is caused by the fermentation products CO₂and short-chain fatty acids. Conversely, there is some evidence of inhibition of urea influx by ruminal ammonia. The underlying mechanisms of this modulation of urea transport are unknown, but of considerable nutritional importance, and future research should be directed to this aspect of ruminal transport.

Urea transport across the gastrointestinal tract involves transporters of the urea transporter-B group, the regulation of which is poorly understood. The classical stimulatory effect of CO₂and the effect of short-chain fatty acids (SCFA) on the ruminal recycling of urea were investigated by using Ussing chamber and microelectrode techniques with isolated ruminal epithelium of sheep. The flux of urea was found to be phloretin sensitive and passive. At a luminal pH of 6.4, but not at 7.4, the addition of SCFA (40 mmol/l) or CO₂/HCO3− (10% and 25 mmol/l) led to a fourfold increase in urea flux. The stepwise reduction of luminal pH in the presence of SCFA from 7.4 to 5.4 led to a bell-shaped modification of urea transport, with a maximum at pH 6.2. Lowering the pH in the absence of SCFA or CO₂had no effect. Inhibition of Na⁺/H⁺exchange increased urea flux at pH 7.4, with a decrease being seen at pH 6.4. In experiments with double-barreled, pH-sensitive microelectrodes, we confirmed the presence of an apical pH microclimate and demonstrated the acidifying effects of SCFA on the underlying epithelium. We confirm that the permeability of the ruminal epithelium to urea involves a phloretin-sensitive pathway. We present clear evidence for the regulation of urea transport by strategies that alter intracellular pH, with permeability being highest after a moderate decrease. The well-known postprandial stimulation of urea transport to the rumen in vivo may involve acute pH-dependent effects of intraruminal SCFA and CO₂on the function of existing urea transporters.

Seasonal Variation in Blood Constituents of One-humped Camel (Camelus dromedarius)

Veterinarians and animal producers use non-essential compounds via either feed additives or implantation for a long history as growth promoters for farm animals. These compounds, such as anabolic hormones, bovine somatotropin, Beta-agonists, have multiple positive effects and improve animal performance in many meat-exporting countries due to their anabolic activity. However, some of them may cause carcinogenic effects to the consumer. Anabolic hormones are now permitted to be used as growth promoters on a global scale, but have not been permitted in the EU since 1988. Unlawfully, drugs are sometimes added to farm animal feeds to improve muscle growth, feed utilization efficiency, milk production, and nitrogen retention in order to obtain an economic benefit in a shorter period.. In this article, we review the current knowledge on the safety and hazard of an anabolic compound as endocrine disruptor, not only on food, animals and public health but also on the environment, to arrive at recommendations for risk management and avoidance. The U.S. Food and Drug Administration approves seven hormone drugs (testosterone propionate, trenbolone acetate, estradiol, zeranol, progesterone, melengestrol acetate, and bovine somatotropin) for use in food animals for many purposes. Synthetic hormones were higher stability and longer persistence in the environment.

Ruminal fermentation products such as short-chain fatty acids (SCFA) and CO₂acutely stimulate urea transport across the ruminal epithelium in vivo, whereas ammonia has inhibitory effects. Uptake and signaling pathways remain obscure. The ruminal expression of SLC14a1 (UT-B) was studied using polymerase chain reaction (PCR). The functional short-term effects of ammonia on cytosolic pH (pH_i) and ruminal urea transport across native epithelia were investigated using pH-sensitive microelectrodes and via flux measurements in Ussing chambers. Two variants (UT-B1 and UT-B2) could be fully sequenced from ovine ruminal cDNA. Functionally, transport was passive and modulated by luminal pH in the presence of SCFA and CO₂, rising in response to luminal acidification to a peak value at pH 5.8 and dropping with further acidification, resulting in a bell-shaped curve. Presence of ammonia reduced the amplitude, but not the shape of the relationship between urea flux and pH, so that urea flux remained maximal at pH 5.8. Effects of ammonia were concentration dependent, with saturation at 5 mmol/l. Clamping the transepithelial potential altered the inhibitory potential of ammonia on urea flux. Ammonia depolarized the apical membrane and acidified pH_i, suggesting that, at physiological pH (< 7), uptake of NH₄⁺into the cytosol may be a key signaling event regulating ruminal urea transport. We conclude that transport of urea across the ruminal epithelium involves proteins subject to rapid modulation by manipulations that alter pH_iand the cytosolic concentration of NH₄⁺. Implications for epithelial and ruminal homeostasis are discussed.