Effects of acute acid-base changes on in vivo total ammonia synthesis in the rat

Effects of chronic hypercapnia on ammonium transport in the mouse kidney

Hypercapnia and subsequent respiratory acidosis are serious complications in many patients with respiratory disorders. The acute response to hypercapnia is buffering of H+ by hemoglobin and cellular proteins but this effect is limited. The chronic response is renal compensation that increases HCO3- reabsorption, and stimulates urinary excretion of titratable acids (TA) and NH4+ . However, the main effective pathway is the excretion of NH4+ in the collecting duct. Our hypothesis is that, the renal NH3 /NH4+ transporters, Rhbg and Rhcg, in the collecting duct mediate this response. The effect of hypercapnia on these transporters is unknown. We conducted in vivo experiments on mice subjected to chronic hypercapnia. One group breathed 8% CO2 and the other breathed normal air as control (0.04% CO2 ). After 3 days, the mice were euthanized and kidneys, blood, and urine samples were collected. We used immunohistochemistry and Western blot analysis to determine the effects of high CO2 on localization and expression of the Rh proteins, carbonic anhydrase IV, and pendrin. In hypercapnic animals, there was a significant increase in urinary NH4+ excretion but no change in TA. Western blot analysis showed a significant increase in cortical expression of Rhbg (43%) but not of Rhcg. Expression of CA-IV was increased but pendrin was reduced. These data suggest that hypercapnia leads to compensatory upregulation of Rhbg that contributes to excretion of NH3 /NH4+ in the kidney. These studies are the first to show a link among hypercapnia, NH4+ excretion, and Rh expression.

Arterial ammonemia changes of renal origin induced in the rat by acid and alkaline diets

The aim of this study was to investigate the influence of acid and alkali food supplementation on systemic ammonemia to explain the hyperammonemia previously observed in rats fed a high protein diet. In normal rats, arterial ammonia concentration significantly increases after 4 days of HCl-supplemented diet. Following a NaHCO3-enriched food, there is only a slight but not significant decrease in arterial ammonia level. These changes occur before any variation in arterial acid-base status and are of renal origin. Indeed, there is a positive linear correlation (r = 0.946; P less than 0.001) between arterial ammonia level and the ammonia concentration difference between the renal vein and artery (which varies proportionally to the urinary ammonium excretion). Hindquarter uptake and intestinal release of ammonia do not significantly participate in the arterial ammonia changes observed. Following HCl-enriched diets, increased renal glutamine uptake, enhanced hindquarter glutamine release, and perhaps decreased intestinal glutamine uptake occur simultaneously. In conclusion, acid and alkali food supplementation intervenes on the renal ammonia release into the circulation with concomitant arterial ammonemia changes.

Glutamine transport in the rabbit proximal straight tubule: effect of acute acid pH

Proximal straight tubules (PST) has been shown to be an important nephron segment of renal ammonia production. To clarify the nature of glutamine (substrate of ammonia production) transport in PST, both luminal resorption and peritubular uptake of glutamine were measured in isolated rabbit PST. Luminal glutamine resorption (Jgln) was measured at various perfusate glutamine concentration (0.05 to 20 mM) at 38 degrees C and 12 degrees C. Jgln measured at 12 degrees C were proportional to mean luminal glutamine concentrations. This flux was thought to be a passive glutamine flux. The flux, which was obtained by subtracting passive glutamine flux from Jgln obtained at 38 degrees C, was thought to be active luminal resorption. This flux exhibited saturation kinetics (Vmax 20.9 pmol min-1 X mm-1, km 5.2 mM). When bath pH (HCO3) was lowered from 7.4 to 6.8, Jgln showed no change or a small decrease (12%) at perfusate glutamine concentrations of 0.05 or 5 mM, respectively. When perfusate pH (HCO3) was lowered from 7.4 to 6.8 Jgln showed a small decrease (10%) at 5 mM perfusate glutamine concentration. Peritubular glutamine uptake was determined in isolated nonperfused PST incubated for 5 to 50 min in [14C]-glutamine containing solution. When incubated in pH 7.4 HCO3 buffer solution, cell to medium 14C concentration ratio was higher than unity (3.83 +/- 0.34, P less than 0.001) at 5 min, and reached a maximum (11.37 +/- 1.13) at 30 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandins inhibit renal ammoniagenesis in the rat

We describe the inhibitory effect of prostaglandins (PGs) on in vivo rat renal ammonia synthesis. The influence of systemic pH upon urinary PG excretion and ammoniagenesis was also investigated. Finally, PG production by incubated rat renal cortical slices was suppressed to investigate the PG-ammonia interplay in the absence of changes in renal blood flow, glomerular filtration rate, ambient electrolyte concentrations or extrarenal hormonal factors. In vivo ammonia synthesis doubled and PG excretion fell by 44% in normal rats, after intravenous administration of 1 mg/kg of meclofenamate. Higher doses of meclofenamate further augmented ammonia production and further reduced PG excretion. PG depletion was also associated with an increase in fractional excretion of ammonia (FENH3) that was independent of changes in urine flow rate or pH. Acute metabolic acidosis (AMA) increased total ammonia synthesis but also stimulated PG production. Administration of meclofenamate to rats with mild AMA markedly reduced urinary PG excretion, further augmented ammonia synthesis, and significantly increased the FENH3. Inhibition of stimulated PG synthesis during severe AMA did not increase ammoniagenesis or FENH3. Acute metabolic alkalosis did not alter production of PGs or ammonia, but reduced the FENH3 by 42%. Meclofenamate nearly normalized the FENH3 but stimulated synthesis to a lesser degree than was seen in nonalkalotic rats that received meclofenamate. Inhibition of PG synthesis in incubated rat renal cortical slices also stimulated ammoniagenesis. Conversely, stimulation of PG synthesis decreased ammonia production and acidification of the incubation medium increased prostaglandin F2 alpha production. Thus, in vitro findings support the in vivo results. We conclude that PGs inhibit ammonia synthesis in normal rats and in those undergoing mild AMA. Severe acidosis overrides this inhibitory effect of PGs, whereas metabolic alkalosis suppresses the stimulatory effect of PG synthesis inhibition.