Regulation of dietary phosphate (Pi) in patients with renal failure is important to prevent secondary hyperparathyroidism and to reduce cardiovascular morbidity and mortality. Expression of sodium phosphate co-transporters (NaPis) at the brush border membrane (BBM) of the small intestine and kidney (NaPi2b, 2a and 2c respectively) is critical to Pi homeostasis. A newly characterized group of regulators, known as phosphatonins, respond to elevated serum Pi levels and down-regulate NaPi expression in the kidney to increase Pi excretion. A phosphatonin-like factor was reported by Berndt et al (2007)[1] to cause rapid Pi excretion (<30 min) following an acute duodenal Pi load, a response that was independent of known regulators of Pi such as PTH, vitamin D, and FGF23. However, this group did not examine potential altered expression of NaPi2a and 2c, the predominant NaPi isoforms in the kidney, and the present work serves to address this. Male Sprague Dawley rats (200-250g) were anaesthetized by injection of pentobarbitone sodium into the intraperitoneal cavity (60mg/kg). The duodenum was cannulated ~ 2cm from the pylorus and 1mL of a solution containing 10mM (simulated post-prandial) or 1.3M Pi (as used by Berndt et al) and matched saline controls was instilled and allowed to flow through the digestive tract in an open loop. After 30 min, the animal was killed via cardiac exsanguination, both kidneys removed, and the cortex isolated and snap frozen for subsequent BBM vesicle preparation[2]. Alkaline phosphatase and protein assays were performed to determine the purity of the BBMVs[3, 4]. For western blotting, 20 µg BBMV was combined with 2x Laemmli buffer at a 1:1 ratio, denatured at 90oC for 2 min, run on a 10% SDS-PAGE gel, transferred to a PVDF membrane, blocked with skimmed milk, probed with an antibody against NaPi2a overnight[5] and an HRP-conjugated anti-rabbit secondary antibody. Blots were visualized using a “homemade” enzyme chemiluminescent reagent and results expressed relative to b-actin (n=6 for all conditions). Statistical analysis used a Student’s unpaired t-test with P ≤0.05 considered significant. No significant difference was seen in the protein expression of NaPi2a at the kidney brush border membrane between animals instilled with 10mM or 1.3M Pi and their respective saline controls (NaPi2a: 10mM Pi vs 154mM saline P=0.56; 1.3M Pi vs. 1.15M saline (osmolarity equal to 1.3 Pi solution) P=0.92). Thus, we conclude that the intestinal phosphaturic factor observed by Berndt et al, if present, did not affect the renal expression of NaPi2a – it is possible that the factor might influence the expression of NaPi2c, Pit1 and Pit2 , though these transporters handle a much smaller percentage of Pi. A duodenal Pi load may also affect the undefined and uncharacterized phosphate transport mechanisms at the renal basolateral membrane.