5 6 Stimulation of D 1 receptors leads to inhibition of the Na +/H + exchanger as a result of increased adenosine 3′,5′cyclic monophosphate formation and subsequently via protein kinase A–mediated phosphorylation. 4 Inhibition of Na +,K +-ATPase by dopamine occurs via a phospholipase C–coupled protein kinase C–linked pathway. 1 2 At the level of the proximal tubule, the overall increase in sodium excretion produced by dopamine and D 1 receptor agonists results from inhibition of Na +,K +-adenosine triphosphatase (Na +,K +-ATPase) 3 and the Na +/H + exchanger. This may be due in part to the inability of dopamine to inhibit Na +,K +-ATPase activity in old rats.ĭopamine exerts natriuretic and diuretic effects by activating D 1-like receptors located at various regions in the nephron. It is concluded that renal dopaminergic tonus in old rats is higher than in adult rats but fails to respond to HS intake as observed in adult rats. Dopamine inhibited Na +,K +-ATPase activity in proximal tubules obtained from adult rats, but it failed to exert such an inhibitory effect in old rats. l5 min −1) in adult rats (from 67☑ to 93☑) but not in old rats (from 86☒ to 87☒).Aromatic l-amino acid decarboxylase activity was higher in old rats than in adult rats HS intake increased l-amino acid decarboxylase activity (nmol HS intake increased both dopamine levels and dopamine/L-DOPA ratios in the renal cortex of adult rats but not in old rats. Dopamine levels and dopamine/L-DOPA ratios in the renal cortex of old rats were greater than in adult rats. A diuretic response to HS intake was observed in adult rats (from 20.9☒.3 to 37.6☒.8 mL d −1) was less than in adult rats (from 5.2☐.4 to 13.5☒.5 nmol.The natriuretic response to an HS intake in old rats (from 4.7☐.4 to 10.7☒.0 nmol Norepinephrine and L-DOPA plasma levels did not change during HS intake and were similar in both groups of rats. HS intake (1% NaCl) over a period of 24 hours resulted in a 2-fold increase in the urinary excretion of dopamine, DOPAC, and norepinephrine in adult animals but not in old animals. Daily urinary excretion of l-3,4-dihydroxyphenylalanine ( l-DOPA), dopamine, and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid was similar in adult and old rats by contrast, daily urinary excretion of norepinephrine in old rats was almost twice that in adult animals.
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