Beatrice Hall

Net uptake of plasma homocysteine by the rat kidney in vivo

Hyperhomocysteinemia is a common finding in dialysis-dependent end-stage renal disease (ESRD) patients, but its etiology and refractoriness to standard homocysteine-lowering B-vitamin therapy are poorly understood. In the absence of actual in vivo data, it has been hypothesized that loss of normal renal parenchymal uptake and metabolism of homocysteine is an important determinant of hyperhomocysteinemia in ESRD, given that urinary homocysteine excretion by healthy kidneys is trivial. We assessed net renal uptake and metabolism of homocysteine using an established rat model for measuring arteriovenous amino acid differences across the rat kidney, along with simultaneous determination of renal plasma flow, urine flow, and urinary homocysteine concentration. Substantial homocysteine uptake and metabolism by normal rat kidneys was demonstrated, and we also confirmed that urinary homocysteine excretion is minimal. These data suggest that loss of the sizable homocysteine metabolizing capacity of the intact kidneys may be an important determinant of the refractory, potentially atherothrombotic hyperhomocysteinemia frequently observed in ESRD.

Hormonal regulation of glycine metabolism

The effects of several hormones on flux through glycine cleavage system (GCS) in rat liver was studied using isolated perfused liver, isolated hepatocytes and in mitochondria isolated from rats previously injected with glucagon. Glucagon (100 nM), epinephrine (1 μM), norepinephrine (1 μM), phenylephrine (10 μM) and vasopressin (100 nM) stimulated flux through GCS in perfused liver by two- to threefold. In isolated hepatocytes, the stimulation of flux by glucagon (0.01 to 1000 nM) was followed closely by increases in intracellular cyclic AMP concentration. Isolated mitochondria were exquisitely sensitive to submicromolar concentrations of free calcium. Mitochondria isolated from rats injected with glucagon 30 min before sacrifice showed 3–4 fold higher rates of decarboxylation of glycine than did mitochondria isolated from rats injected with saline when calcium was absent from incubation medium. Flux through GCS in mitochondria was also highly sensitive to the osmolarity of the medium; hypoosmotic conditions led to stimulation of the flux. The stimulation of the flux by calcium was dependent on medium osmolarity suggesting that the stimulation of flux by calcium may be mediated by changes in mitochondrial matrix volume.

Effect of 3-aminopicolinic acid on renal ammoniagenesis in the rat.

Abstract The effects of 3-aminopicolinate, a known activator of phosphoenolpyruvate carboxykinase (PEPCK), on renal ammoniagenesis were studied in normal rats and in rats allowed to recover for 2 days from ammonium chloride induced metabolic acidosis. The administration of 3-aminopicolinate (5 mg/100 g body wt) did not affect glomerular filtration rate, renal blood flow, arterial blood pH, pO 2 or pCO 2 in either normal or “recovered” rats. A significant increase in renal glutamine extraction and total ammonia production was observed in recovered rats, but not in normal rats, after 3-aminopicolinate treatment. Although this compound activated partially purified renal PEPCK, the profile of metabolites from freeze-clamped kidneys was not consistent with an activation of the enzyme. Thus, the enhancement of renal ammoniagenesis in vivo by 3-aminopicolinate was probably due to an effect other than activation of PEPCK.