Hormonal regulation of amino acid transport and cAMP production in monolayer cultures of rat hepatocytes

Abstract

The effects of insulin, glucagon or Dexamethasone (DEX) and of glucagon with insulin or DEX were examined on the uptake of 2‐amino [1‐¹⁴C]isobutyric acid (AIB) and N‐Methyl‐2‐amino [1‐¹⁴C]isobutyric acid (NMe AIB) in monolayer cultures of rat hepatocytes. Insulin and glucagon stimulated the uptake of both the amino acids and DEX inhibited it, showing that all three of these hormones regulate the A system (the sodium‐dependent system that permits the transport of NMe AIB) for amino acid transport in these cultures. Experiments investigating the transport of aminocyclopentane‐1‐carboxylic acid, 1‐ [carboxyl‐¹⁴C] in the presence of excess AIB or in the absence of sodium showed that insulin had no effect on the activity of the L system (the sodium‐independent system that prefers leucine). Experiments on the uptake of AIB in the presence of excess NMe AIB showed insulin had no effect on the transport activity of the ASC system (the sodium‐dependent system that does not transport NEe AIB). Insulin concentrations ranging from 0.1 nM to 100 nM did not antagonize the stimulatory effect of optimum or suboptimum concentrations of glucagon on the uptake of either AIB or NMe AIB. Similarly, glucagon did not antagonize the stimulatory effect of optimum or suboptimum concentrations of insulin on the uptake of both the amino acids. The combined effect of insulin and glucagon was additive on the rate as well as the cumulative uptake of both AIB and NMe AIB. DEX alone inhibited the transport of both AIB and NMe AIB by about 25%, while glucagon caused a 2–3‐fold increase; however, the addition of glucagon to cultures containing DEX caused a 7–8‐fold increase in the uptake of both AIB and NMe AIB when compared to cultures containing DEX alone. The effect of insulin on the levels of cAMP was also investigated. Insulin had no effect on the cAMP levels in cultures treated or untreated with optimum or suboptimum concentrations of glucagon.