Rat renal proximal tubular gluconeogenesis: possible involvement of nonmitochondrial carbonic anhydrase isozymes.
The carbonic anhydrase (CA) inhibitor ethoxzolamide decreases the rate of glucose synthesis from 10 mM pyruvate by tubules incubating in 25 mM HCO3- but not in 50 mM HCO3-: this is evidence that rat renal cortical mitochondrial CA (CA V) provides HCO3- for pyruvate carboxylation in renal tubular gluconeogenesis at physiological total CO2 (CO2 + HCO3-). In renal proximal tubules prepared from 48-h-starved rats and incubating in 10 mM pyruvate in 25 mM HCO3- buffered saline (Krebs-Henseleit buffer) the CA inhibitors acetazolamide (AZ) and benzolamide (BZ) decreased the rate of glucose synthesis. Maximal inhibition was reached with 125 microM AZ or with 450 microM BZ. The rate of glucose synthesis increased with increasing pyruvate concentration from 3.33 to 20 mM; including 600 microM BZ or 188 microM AZ results in glucose synthesis becoming independent of increasing pyruvate concentration. Doubling the physiological concentration of bicarbonate restored the dependence of glucose synthesis on pyruvate concentration and partly, but not completely, alleviated the inhibitory effect of AZ and BZ, leading to the conclusion that AZ and BZ influence gluconeogenesis by affecting enzymes in addition to CA V. Tubules were incubated with substrates which do not require pyruvate carboxylation for synthesis of oxaloacetate. When tubules were incubated in 10 mM malate the rate of glucose synthesis was unaffected by less than 100 microM AZ or 400 microM BZ and was decreased maximally by 40 and 20%, respectively, by 125 microM AZ, 450 microM BZ, and higher concentrations of these drugs. Increasing the malate concentration from 3.33 to 20 mM increased the rate of glucose synthesis; 600 microM BZ inhibited the rate of glucose synthesis only when the malate concentration was greater than 10 mM but 188 microM AZ decreased the rate of glucose synthesis at each concentration of malate. Results were similar when tubules were incubated in glutamine with CA inhibitors. The rate of glucose synthesis differed with the substrate metabolized and the substrate concentration except when 600 microM BZ was included.(ABSTRACT TRUNCATED AT 400 WORDS)
Dodgson
S J
SJ
Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia 19104-6085.
Cherian
K
K
D000818:Animals; D002257:Carbonic Anhydrase Inhibitors; D005016:Ethoxzolamide / Q000494:pharmacology*; D005215:Fasting; D005943:Gluconeogenesis / Q000187:drug effects; D005947:Glucose / Q000096:biosynthesis; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000037:antagonists & inhibitors*; D007687:Kidney Tubules, Proximal / Q000201:enzymology / Q000378:metabolism*; D007700:Kinetics; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*; D008954:Models, Biological; D011773:Pyruvates / Q000378:metabolism; D051381:Rats; D012016:Reference Values
D002257:Carbonic Anhydrase Inhibitors; D007527:Isoenzymes; D011773:Pyruvates; D005947:Glucose; D005016:Ethoxzolamide