Organ-Specific Overexpression of Renal LAT2 and Enhanced Tubular l -DOPA Uptake Precede the Onset of Hypertension

Abstract

Abstract-Spontaneously hypertensive rats (SHR) might have increased renal production of dopamine. L-3,4-Dihydroxyphenylalanine (L-DOPA) uptake in renal epithelial cells is promoted through the type 2 L-type amino acid transporter (LAT2), and this might rate-limit the synthesis of renal dopamine. The present study evaluated L-DOPA uptake in isolated renal proximal tubules of SHR and normotensive controls (Wistar-Kyoto rats [WKY]). Expression of LAT1 and LAT2 in the renal cortex and intestinal mucosa was also evaluated. Tubular uptake of L-DOPA in WKY and SHR was a saturable process, being greater in the latter than the former at both 4 and 12 weeks of age. cDNA fragments (LAT1, 688 bp; LAT2, 729 bp) labeled with 32 P were used as probes for Northern blot analysis. Expression of LAT2 in SHR kidneys was higher than in WKY kidneys. This increase was more marked at 4 than at 12 weeks of age. Intestinal LAT2 expression, however, was identical in SHR and WKY at both 4 and 12 week of age. By Northern blot analysis, the LAT1 transcript was not identified in either the kidney or intestine. Kidney total RNA was then reverse-transcribed and amplified by polymerase chain reaction with specific primers for LAT1. The presence of a fragment of the expected size for LAT1 led to the conclusion that LAT1 mRNA is a rare message in kidney. We conclude that overexpression of LAT2 in the SHR kidney might contribute to the enhanced L-DOPA uptake, which is organ specific and precedes the onset of hypertension.

High-salt intake and the renal expression of amino acid transporters in spontaneously hypertensive rats

“…Studies on the inward transport of L-DOPA by tubular epithelial cells conducted in rat renal cortical slices (43) and cultured renal cell lines (16,55) demonstrated that uptake of L-DOPA is an active process, mediated through amino acid transporters. Recently, we reported that overexpression of type 2 Na ϩ -independent L-type amino acid transporter (LAT2) in the SHR kidney may contribute to the enhanced L-DOPA uptake (41). In line with these findings, immortalized renal proximal tubular epithelial cells from the SHR also overexpressed LAT2 (42).…”

“…Recently, we demonstrated that overexpression of LAT2 in SHR kidney is organ specific, precedes the onset of hypertension, and is accompanied by enhanced ability to take up L-DOPA (41). This led to the suggestion that overexpression of renal LAT2 may constitute the basis for the enhanced renal production of dopamine in the SHR.…”

“…The nonsaturable component of L-DOPA uptake was determined in experiments conducted at 4°C. The saturable component of L-DOPA uptake was derived from the total amount of L-DOPA taken up into the renal tubules at 37°C and subtracted from the values obtained for the nonsaturable component (at 4°C), as previously described (41,55). The incubation was stopped by rapid cooling and an aliquot (300 l) of the incubation medium containing the renal tubules was used for the assay of L-DOPA by HPLC with electrochemical detection (41,55).…”

Functional genomics of the dopaminergic system in hypertension

“…3). The inhibitory effect of dopamine on Na ϩ -K ϩ -ATPase is influenced by intracellular sodium and calcium; dopamine inhibits Na (86,164,165). L-DOPA is the sole source of renal tubular dopamine production.…”

“…Salt loading can also increase in AADC activity in proximal tubules (86). In spontaneously hypertensive rats (SHR), sodium-dependent and LAT-1 L-DOPA uptake may also participate and may explain, in part, the increased dopamine synthesis in SHRs (164,170). Saline loading may also be associated with decreased renal degradation of dopamine by catechol-o-methyl transferase (COMT) (152) to 3-methoxytyramine (3MT) and eventually to homovanillic acid (HVA).…”