CELLULAR MECHANISMS OF RENAL TUBULAR TRANSPORT OF L-DOPA AND ITS DERIVATIVES IN RAT - MICROPERFUSION STUDIES

  • 1 January 1976
    • journal article
    • research article
    • Vol. 199  (1) , 17-24
Abstract
Proximal tubular transport of L-dopa and its derivatives were studied in the rat kidney by microperfusion and capillary perfusion techniques in situ. With the use of microperfusion techniques alone, L-dopa, L-3-methoxy-4-hydroxyphenylalanine, L-tyrosine and L-phenylalanine rapidly disappeared from the perfusate. The rates of reabsorption were calculated to be 2.05 .times. 10-12, 2.13 .times. 10-12, 6.35 .times. 10-12 and 7.14 .times. 10-12 mol/cm per s, respectively. L-.alpha.-methyldopa and dopamine were only slightly reabsorbed. The permeability coefficients were calculated to be 0.35 .times. 10-4 and 0.21 .times. 10-4 cm/s, respectively. The rate of reabsorption of L-dopa was greatly reduced in the presence of L-phenylalanine in the perfusate, but was not affected by D-dopa. With the stop-flow microperfusion technique with simultaneous capillary perfusion, the zero net flux transtubular concentration difference (.DELTA.C) of labeled dopa was measured. When the initial concentration of 2 mM labeled L-dopa was perfused, the luminal concentration fell to a plateau level of 1.5 mM after a contact time of 20 s, i.e., .DELTA.C was 0.5 mM. The net flux was much less than the efflux, suggesting the presence of a secretory or passive back flux of L-dopa. The .DELTA.C of L-dopa was not influenced by the decarboxylase inhibitor, MK-486 [carbidopa], and monoamine oxidase inhibitor, pheniprazine, but was significantly reduced by NaCN. Thus the reabsorption of L-dopa in the proximal convoluted tubule is an active process with great structural specificity.

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