Salt intake and intestinal dopaminergic activity in adult and old Fischer 344 rats

We have earlier shown that the renal dopaminergic system failed to respond to high salt (HS) intake in old (24-month-old) Fisher 344 rats (Hypertensio n 1999;34:666-672). In the present study, intestinal Na+,K+-ATPase activity and intestinal dopaminergic tonus were evaluated in adult and old Fischer 344 rats during normal salt (NS) and HS intake. Basal intestinal Na+,K+-ATP ase activity (nmol Pi/mg protein/min) in adult rats (142 +/-6) was higher t han in old Fischer 344 rats (105 +/-7). HS intake reduced intestinal Na+,K-ATPase activity by 20% (P <0.05) in adult, but not in old rats. Dopamine ( 1 muM) failed to inhibit intestinal Na+,K+-ATPase activity in both adult an d old Fischer 344 rats (NS and HS diets). In adult animals, co-incubation o f pertussis toxin with dopamine (1 muM) produced a significant inhibitory e ffect in the intestinal Na+,K+-ATPase activity. L-DOPA and dopamine tissue levels in the intestinal mucosa of adult rats were higher (45 +/-9 and 38 /-4 pmol/g) than those in old rats (27 +/-9 and 14 +/-1 pmol/g). HS diet di d not change L-DOPA and DA levels in both adult and old rats. DA/L-DOPA tis sue ratios, an indirect measure of dopamine synthesis, were higher in old ( 1.1 +/-0.2) than in adult rats (0.6 +/-0.1). Aromatic L-amino acid decarbox ylase (AADC) activity in the intestinal mucosa of old rats was higher than in adult rats. HS diet increased the AADC activity in adult rats, but not i n old rats. It is concluded that intestinal dopaminergic tonus in old Fishe r 344 rats is higher than in adult rats and is accompanied by lower basal i ntestinal Na+,K+-ATPase activity. In old rats, HS diet failed to alter the intestinal dopaminergic tonus or Na+,K+-ATPase activity, whereas in adult r ats increases in AADC activity were accompanied by decreases in Na+,K+-ATPa se activity. The association between salt intake, increased dopamine format ion and inhibition of Na+,K+-ATPase at the intestinal level was not as stra ightforward as that described in renal tissues. (C) 2001 Elsevier Science I nc. All rights reserved.