Growth hormone stimulation of amino acid transport and utilization by the perfused rat liver

Effects of growth hormone, administered in vivo or added in vitro, on amino acid transport and utilization were studied in perfused livers of normal and hypophysectomized rats. A perfusion system employing a nonrecirculating medium was used in all studies. Two nonmetabolizable amino acid analogs, α aminoisobutyric acid (AIB) and 1 aminocyclopentane carboxylic acid (cycloleucine) were used to study transport. Accumulation of AIB increased linearly over a 60 min perfusion period, reaching distribution ratios of between 1 and 2 for both groups of animals. Treatment of both normal and hypophysectomized rats with growth hormone 60 min prior to the start of perfusion increased AIB distribution ratios by up to 84 and 108%, respectively. Accumulation of cycloleucine was linear for only about 20 min of perfusion and then plateaued. Steady state distribution ratios of this analogue ranged between 1 and 2 for both groups of animals. Growth hormone treatment had no apparent effect on the time necessary to reach these steady state levels, but significantly increased them in livers of both normal and hypophysectomized rats by 16 and 42%, respectively. Studies designed to analyze the kinetic properties of these hormone effects revealed that growth hormone treatment caused by 2 fold increase in the maximum velocities of both the AIB and cycloleucine transport systems. The substrate concentration for half maximal transport velocity was increased slightly for both systems by growth hormone. Direct effects of growth hormone were demonstrated in studies where livers of hypophysectomized rats were perfused under conditions simulating those of experiments in which the hormone was administered in vivo. Following an initial 45 min period of perfusion the medium was switched to one containing [¹⁴C]cycloleucine and accumulation of this amino acid analog was measured over the following 20 min. Growth hormone added to the medium during the entire 65 min perfusion at a concentration of 1 μg/ml caused a 30% increase in the cycloleucine distribution ratio. Under similar experimental conditions growth hormone directly stimulated 3 hepatic pathways of amino acid utilization: incorporation of [¹⁴C]valine into protein, urea formation and conversion of ¹⁴C amino acids to labeled glucose. Intracellular concentrations of 7 amino acids, including threonine, serine, proline, glycine, alanine, lysine, and arginine, were increased significantly in livers perfused with medium containing growth hormone. This finding and the observation that the hormone had no effect in livers perfused with levels of amino acids that were saturating for the pathways of protein synthesis, gluconeogenesis, and ureogenesis suggested that growth hormone increased amino acid utilization as a result of its stimulatory action on the transport of these compounds into liver cells. When urea production was used to monitor the direct action of growth hormone on perfused liver, a lag period of 20 to 25 min occurred before a significant effect of the hormone was manifested.