Decreased renal dopamine production in diabetic rats results from decreased L-dopa uptake into tubular cells

CARRANZA, ANDREA; KARABATAS, LILIANA; BARONTINI, MARTA; ARMANDO, INÉS

Buenos Aires, Argentina, may 1999

Congreso; XVth International Congress of Nephrology; 1999

International Society of Nephrology

Carranza A, Karabatas L, Barontini M, Armando i: Decreased renal dopamine production in diabetic rats results from decreased L-dopa uptake into tubular cells. XVth International Congress of Nephrology - Buenos Aires, Argentina, may 1999 (Abstract Nº 43, pp. 11, 1999) Clinical studies in both insulin and non-insulin dependent diabetes mellitus patients have suggested a deceased renal dopamine (DA) production that has been related to their impaired ability to excrete a salt loading. This work was aimed to study renal DA production in rats treated with steptozotocin (STZ), a model of insulin dependent diabetes mellitus. STZ (65 mg/kg, ip) or vehicle were administrated to 3 onth-old male Wistar rats. 24 h urines were collected on day 22 after treatment for determination of DOPA, DA, dihydroxyphenylacetic acid (DOPAC) by HPLC-ED, Na+ by flame photometry and urinary glucose by enzymatic assay. STZ treated animals had increased urinary glucose (p<0.01), diuresis (p<0.01), natriuresis (p<0.01) and excretion of DOPA (p<0.01). Excretion of DA and DOPAC, however, were similar to those in vehicle treated animals suggesting a decreased DA production, relative to the availability of its precursor DOPA. This led us to determine the effects of increased glucose on DOPA uptake by isolated tubular proximal cells. In control experiments (glucose 3.4 mM) both the uptake of DOPA (100 nM-100 uM) and the production of DA were dependent of DOPA concentration in the incubation medium. Scatchard analysis of the saturation curves obtained with DOPA showed two uptake sites in these cells. One with high affinity (Km 320±20 nM) and low capacity Vmax 1.22±0.2 pmol/mg prot/min) and other with low affinity (Km 1530±300 nM) and high capacity (Vmax 2.45±0.5 pmol/mg prot/min). The increase of glucose concentration in the medium to 18.4 mM inhibited DOPA uptake by 40 % (p<0.03). Scatchard analysis of the DOPA saturation curves obtained in this condition showed a decrease in the Vmax (to 0.4±0.1 pmol/mg prot/min) of the high affinity site without changes in the Km. These results show that renal DA concentration in the luminal fluid may be responsible for this impairment. Clinical studies in both insulin and non-insulin dependent diabetes mellitus patients have suggested a deceased renal dopamine (DA) production that has been related to their impaired ability to excrete a salt loading. This work was aimed to study renal DA production in rats treated with steptozotocin (STZ), a model of insulin dependent diabetes mellitus. STZ (65 mg/kg, ip) or vehicle were administrated to 3 onth-old male Wistar rats. 24 h urines were collected on day 22 after treatment for determination of DOPA, DA, dihydroxyphenylacetic acid (DOPAC) by HPLC-ED, Na+ by flame photometry and urinary glucose by enzymatic assay. STZ treated animals had increased urinary glucose (p<0.01), diuresis (p<0.01), natriuresis (p<0.01) and excretion of DOPA (p<0.01). Excretion of DA and DOPAC, however, were similar to those in vehicle treated animals suggesting a decreased DA production, relative to the availability of its precursor DOPA. This led us to determine the effects of increased glucose on DOPA uptake by isolated tubular proximal cells. In control experiments (glucose 3.4 mM) both the uptake of DOPA (100 nM-100 uM) and the production of DA were dependent of DOPA concentration in the incubation medium. Scatchard analysis of the saturation curves obtained with DOPA showed two uptake sites in these cells. One with high affinity (Km 320±20 nM) and low capacity Vmax 1.22±0.2 pmol/mg prot/min) and other with low affinity (Km 1530±300 nM) and high capacity (Vmax 2.45±0.5 pmol/mg prot/min). The increase of glucose concentration in the medium to 18.4 mM inhibited DOPA uptake by 40 % (p<0.03). Scatchard analysis of the DOPA saturation curves obtained in this condition showed a decrease in the Vmax (to 0.4±0.1 pmol/mg prot/min) of the high affinity site without changes in the Km. These results show that renal DA concentration in the luminal fluid may be responsible for this impairment.