POTENTIAL ROLE OF AMMONIAGENESIS IN THE HYPOCALCIURIC EFFECT OF PHOSPHORUS IN RATS

Objective: Hypocalciuria associated with a high phosphorus intake is k nown to be both a parathyroid hormone and non-parathyroid hormone depe ndent event, The present study was designed to define the role that am moniagenesis may play in the non-parathyroid hormone dependent pathway . Design: Male rats, initially weighing 160 g, were fed a purified die t containing, in g/kg diet, a single level of protein (200) and variab le inorganic phosphorus (1.8, 4.5, 9.0) for 20 days. Results: Food int ake and body weight were similar for the three groups. Significant inv erse correlations were found for both urinary calcium and phosphorus a nd for urinary ammonia nitrogen and calcium excretion (r = -0.62, p < 0.01). Urinary ammonia nitrogen excretion was highly correlated with b oth phosphorus intake (r = 0.89, p < 0.001) and urinary phosphorus (r = 0.88, p < 0.001). Urinary urea nitrogen tended to vary inversely wit h phosphorus intake, High dietary phosphorus decreased the activity of glutamine synthetase and increased the activity of glutaminase I in k idney. Conclusion: Tying-up some of the hydrogen ions destined for exc retion by phosphorus-stimulated ammoniagenesis could reduce the interf ering effect of hydrogen ion on kidney calcium reabsorption and provid e a mechanism to explain why phosphorus can have a direct positive imp act upon tubular calcium reabsorption.