ONTOGENY OF THE CELL OUTWARD DOPAMINE TRANSPORTER IN CANINE RENAL TISSUES

The present work has determined the activities of aromatic L-amino aci d decarboxylase (AAAD) and evaluated the presence of an active transpo rt system for dopamine in renal tissues of developing dogs (newborn pu ppies less than 24 hours after birth, animals at the age of 10 days an d 2 months) and adult animals. AAAD activity (V-max, in pmol/mg protei n/h) as determined in kidney homogenates was found to be in the adult dog kidney (V-max = 3216 +/- 268) higher (p < 0.05) than that occurrin g in the three other groups of animals; no significant difference on A AAD activity was observed between the 10 day-old (V-max = 1139 +/- 185 ) and the 2 month-old dogs (V-max = 783 +/- 23). AAAD activity in newb orn puppies (V-max = 259 +/- 40) was markedly lower than in the three other groups. A considerable amount of the total dopamine formed from added L-DOPA in kidney slices, depending on the age, was found to esca pe into the incubation medium. The application of the Michaelis-Menten equation to the net transport of newly-formed dopamine has allowed th e identification of a saturable (carrier-mediated transfer) and a non- saturable component (diffusion). The V-max (nmol/g/15 min), K-m (mu M) values for the saturable component and diffusion constant mu mol(-1)) were as follows: adult (V-max = 112 +/- 16; K-m = 319 +/- 35; diffusi on constant = 0.0009 +/- 0.0001), 2 month-old (V-max = 19 +/- 5; K-m = 48 +/- 14; diffusion constant = 0.0007 +/- 0.0002), 10 day-old (V-max = 25 +/- 3; K-m = 69 +/- 20; diffusion constant = 0.0033 +/- 0.0007) and newborn (V-max = 6 +/- 1; K-m = 16 +/- 6; diffusion constant = 0.0 095 +/- 0.0010). In conclusion, renal AAAD develops with age, though s ome AAAD activity can already be detected at birth. The dopamine outwa rd transporter appears to be considerably immature at birth, but under goes rapid maturation within the next 10 days; however, development of AAAD and of the renal dopamine outward transporter still proceedes fr om the age of 2 months until adulthood.