Insulin enhances l-dopa renal proximal tubule uptake: a regulatory mechanism impaired in insulin resistance

A stimulatory role for insulin in the uptake of neutral amino acids has been reported for a variety of tissues. Here we examine the effect of insulin on l-dopa uptake by proximal tubule cells (PT cells) isolated from control and fructose-fed rats (FR-rats, 10% w/v fructose solution in tap water), a model of insulin resistance. Insulin (200 U/ml) increased l-dopa uptake into PT cells by about 50% (705±186 vs.1117±140 pmol l-dopa/mg protein per minute) (p<0.05). The higher uptake correlated with a 40% increase in the number of high-affinity l-dopa transport sites (l-dopa 0.2 M) (0.59±0.05 vs. 0.82±0.09 pmol l-dopa/mg protein per minute), without changing their affinity. The effect of insulin was not modified by ouabain (1 mM), nocodazole (1–10 M) or colchicine (50–100 M), whereas it was abolished by cytochalasin D or latrunculin B (both 1 M). This suggests that the process is independent of Na⁺,K⁺-ATPase activity or the microtubule network but that it requires the integrity of the actin cytoskeleton. l-dopa transport by the low-affinity transport sites (l-dopa 5 M) was not affected by insulin, neither was the effect of insulin observed in PT cells isolated from FR-rats. In line with this, FR-rats showed lower renal l-dopa reabsorption as compared to control animals (81±4 vs. 51±9%). Taken together, our results support the involvement of insulin in the multifactorial regulation of renal l-dopa reabsorption.