Insulin stimulates amino acid transport by alveolar type II epithelial cells in primary culture.

The present experiments were designed to investigate the effect of insulin on amino acid transport by alveolar type II epithelial cells isolated from adult rats. The initial rates of amino acid transport were determined by measuring the uptake of the nonmetabolizable amino acid, alpha-aminoisobutyric acid (AIB). Alveolar type II epithelial cells transported AIB by a sodium-dependent process that displayed saturable kinetics. Insulin stimulated transport of 0.1 mM AIB (82.3 +/- 15.0%, mean +/- SE, n = 4) with a half-maximally effective insulin concentration of 2.2 +/- 1.0 ng/ml (mean +/- SE), which is within the physiologic range of rat serum insulin concentration. Kinetic analysis of AIB transport showed that insulin increased the Vmax from 6.7 +/- 1.2 nmol/10(6) cells/min to 9.3 +/- 1.8 nmol/10(6) cells/min and had no effect on the Km value (5.2 +/- 0.8 mM). Alpha-(methylamino) isobutyric acid, a specific substrate for amino acid transport system A, inhibited both the sodium-dependent and insulin-modulated transport of AIB. Thus, alveolar type II epithelial cells transport amino acids in a sodium-dependent process and insulin stimulates this transport. The present study indicates that insulin may play a physiologic role in regulating amino acid transport in alveolar type II epithelial cells.