Colorimetry of p-aminohippurate in the presence of sulfamethoxazole.

Colorimetric quantification of p-aminohippurate (PAH), which is helpful in assessment of renal function, is subject to interference from sulfa compounds such as sulfamethoxazole. We were unable to measure serum PAH in two patients receiving a trimethoprim-sulfamethoxazole combination because of interference by the latter with the colorimetric estimation. We found that isoamyl acetate removes sulfamethoxazole from the samples without influencing PAH. The extraction is simple, inexpensive, and reproducible.

Renal ammonia production from the nitrogens of glutamine in intact acidotic dogs before and after bicarbonate infusions.

Renal ammonia is produced from the amide nitrogen of glutamine, approximately 33-50%. The remainder derives from the amino nitrogen of glutamine and other non amide sources, probably the amino nitrogens of other amino acids. We investigated the acute effects of acid-base perturbations on ammonia production from amide and non amide nitrogen sources to determine how they interrelate. Infusions of glutamine were given to some intact dogs to vary the renal load. Following an acute alkali challenge to dogs in metabolic acidosis, ammoniagenesis from the amide nitrogens decreased significantly when the presentation of glutamine to the kidney was normal or relatively low, but changed less or even increased when the glutamine load was relatively high. In contrast, ammonia from the non amide sources consistently decreased during acute alkalotic challenge at any glutamine load-high or low. Since decreased glutamine deamination leading to glutamate accumulation is generally associated with decreased deamidation in dogs with normal plasma glutamine concentrations, we explain the discrepancy of deamidation at high glutamine loads to an unmasking of a separate effect on the glutaminase (phosphate-dependent) pathway by the acute acid-base changes. Accordingly, our results indicate more than one influence from acute acid-base changes in vivo on renal ammonia formation, one stimulatory and other inhibitory. Nevertheless, the influence of glutamate removal predominates over the other effect on the phosphate-dependent glutaminase pathway at physiological concentrations of glutamine in the intact dog.

Renal ammoniagenesis following glutamine loading in intact dogs during acute metabolic acid-base perturbations.

Adaptation of renal ammoniagenesis during acute metabolic acidosis in intact dogs may be nonexistent or, at least, markedly less than in chronic acidosis. This contrasts to adaptation in acute respiratory acidosis, where levels similar to those attained in chronic acidosis occur within hours. Accordingly, the inability to discern marked changes in acute metabolic acidosis compared with acute respiratory acidosis has been attributed to decreased glomerular filtration rate and renal blood flow seen frequently in the former. In our studies, we found early changes in ammoniagenesis and glutamine metabolism during acute metabolic acidosis, but not of the magnitude seen in chronic acidosis, even considering the changes in renal blood flow (RBF) and glomerular filtration rate (GFR). Exogenous glutamine loading allowed us to discover that the qualitative changes in glutamine metabolism during acute metabolic acidosis differed from control but fell short of those seen in chronic metabolic a acidosis. We also examined glutamine metabolism when renal ammoniagenic adaptation was acutely inhibited in chronically acidotic dogs. Infusing NaHCO3 into chronically acidotic dogs decreased renal ammonia production significantly (247 mumol min-1 100 ml-1 GFR vs 148 mumol min-1 100 ml-1 GFR: P less than 0.001) and glutamine extraction (111.8 mumol min-1 100 ml-1 GFR vs 90.9 mumol min-1 100 ml-1 GFR: P less than 0.02). The qualitative changes in renal glutamine metabolism in both studies suggest that alterations in deamination of glutamate formed from glutamine are responsible, at least in part, for adaptation to acute acid-base perturbations. Compared with respiratory acidosis, adaptation to metabolic acidosis is progressive and prolonged.

Effects of sera from uninephrectomized rats on renal slices: PAH and TEA uptake and QO2.

We followed the effects of sera from unilaterally nephrectomized (uninephectomized) rats compared to sham-operated rats on 3H-p-aminohippurate (3H-PAH) and 14C-tetraethylammonium (14C-TEA) uptake and oxygen consumption (QO2) in incubating rat kidney slices. These studies were based on the assumption that a circulating renotropic substance might also influence various transport mechanisms. Sera were obtained at various times postoperation; the height of renotropic activity occurs 17-24 h after kidney extirpation. Sera removed 17-24 h postuninephrectomy significantly decreased both 3H-PAH and 14C-TEA uptake in incubating kidney slices. Similar to the inability to show significant renotropic activity after 36 h, sera obtained 48, 96, and 168 h postuninephrectomy had no significant influence on 3H-PAH and 14C-TEA uptake. Addition of sera (10% v/v) generally depressed QO2. However, sera obtained from uninephrectomized rats compared to sera from sham-operated rats had relatively more depressive effects on QO2 in renal tissue after 30 min of incubation (p less than 0.01). No significant differences in QO2 were seen when the uninephrectomized and sham-operated sera were added to the tissue immediately or after 60 min of incubation in the sera. These serum studies on 3H-PAH and 14C-TEA uptake simulate in many respects ones performed previously with serum from spontaneously hypertensive and salt-loaded rats and suggest the presence of a common circulating factor.

Substrate oxidation in kidney slices from acidotic and alkalotic rats: role in ammoniagenic adaptation.

The enhanced renal ammoniagenesis that occurs during acidosis may depend on oxidative processes. To determine this, oxygen consumption (Qo2) of renal slices from acidotic rats (medium pH 7.0) and alkalotic rats (medium pH 7.8) in the presence of 4 different substrates (glutamine, lactate, acetate and 2-oxoglutarate) was investigated. Incremental addition of any oxidizable substrate tested to the incubation medium increased Qo2 until a maximal rate was approached (approximately 17-58% above baseline). Comparing acidotic to alkalotic slices, Qo2 was significantly less at low concentrations of lactate and acetate before maximal Qo2 occurred. In contrast, Qo2 was relatively higher at low concentrations of 2-oxoglutarate and glutamine with acidotic compared to alkalotic slices. Accordingly, acid-base perturbations produce different patterns of Qo2 depending on the substrate and its concentration. These observations coupled with others are consistent with the hypothesis that a relative block in the oxidation of lactate and acetate during acidosis provides more NAD+ for glutamine oxidation and participates in acidotic regulation of renal glutamine metabolism.

Effects of diets high in refined carbohydrates on renal ammonium excretion in rats.

Ammonium excretion was investigated in spontaneously hypertensive rats (SHR) and normotensive control rats (WKY) ingesting different diets. SHR and WKY on low protein-high sucrose diets surprisingly showed the same ammonium excretion as rats ingesting a higher protein-lower sucrose diet. This was unexpected, because ammonium excretion correlates positively with protein intake. The relatively high ammonium excretion despite low protein intake (approximately 40% of control) was not associated with acidosis, hypokalemia, hypocalcemia, and/or hypomagnesemia. In a follow-up study, where diets were high in refined carbohydrates (sucrose, glucose, and starch) but more equal in protein content compared with a diet high in carbohydrates of a more complex form (grains), ammonium excretion increased significantly. When we examined the factors known to influence ammonium excretion, the only significant positive correlations found were between norepinephrine, epinephrine, dopamine, and ammonium excretion. These correlations still remained significant when only the data from the rats on the diets high in refined carbohydrates, i.e., rats on the same dietary intake of minerals and proteins, were compared. In vitro, we corroborated that catecholamines significantly increased ammoniagenesis from kidney slices. Our data show that diets high in refined carbohydrates augment both ammonium and catecholamine excretion and suggest that these two events may be interrelated.