Fasting and refeeding modulate neutral amino acid transport activity in the basolateral membrane of the rat exocrine pancreatic epithelium: fasting-induced insulin insensitivity

Dietary Protein and Amino Acid Deficiency Inhibit Pancreatic Digestive Enzyme mRNA Translation by Multiple Mechanisms

BACKGROUND & AIMS

Chronic amino acid (AA) deficiency, as in kwashiorkor, reduces the size of the pancreas through an effect on mammalian target of rapamycin complex 1 (mTORC1). Because of the physiological importance of AAs and their role as a substrate, a stimulant of mTORC1, and protein synthesis, we studied the effect of acute protein and AA deficiency on the response to feeding.

METHODS

ICR/CD-1 mice were fasted overnight and refed for 2 hours with 4 different isocaloric diets: control (20% Prot); Protein-free (0% Prot); control (AA-based diet), and a leucine-free (No Leu). Protein synthesis, polysomal profiling, and the activation of several protein translation factors were analyzed in pancreas samples.

RESULTS

All diets stimulated the Protein Kinase-B (Akt)/mTORC1 pathway, increasing the phosphorylation of the kinase Akt, the ribosomal protein S6 (S6) and the formation of the eukaryotic initiation factor 4F (eIF4F) complex. Total protein synthesis and polysome formation were inhibited in the 0% Prot and No Leu groups to a similar extent, compared with the 20% Prot group. The 0% Prot diet partially reduced the Akt/mTORC1 pathway and the activity of the guanine nucleotide exchange factor eIF2B, without affecting eIF2α phosphorylation. The No Leu diet increased the phosphorylation of eIF2α and general control nonderepressible 2, and also inhibited eIF2B activity, without affecting mTORC1. Essential and nonessential AA levels in plasma and pancreas indicated a complex regulation of their cellular transport mechanisms and their specific effect on the synthesis of digestive enzymes.

CONCLUSIONS

These studies show that dietary AAs are important regulators of postprandial digestive enzyme synthesis, and their deficiency could induce pancreatic insufficiency and malnutrition.

Characterization of the Physiological Spaces and Distribution of Tolbutamide in the Perfused Rat Pancreas

PURPOSE

To set up and validate a viable perfused rat pancreas model suitable for pharmacokinetic studies.

MATERIALS AND METHODS

We setup and conducted multiple indicator dilution studies in the single pass perfused rat pancreas. The distribution of the reference markers [99mTc]-red blood cells (RBC), [14C]-sucrose, and [3H]-water, and tolbutamide were analysed using both non-parametric and parametric methods.

RESULTS

The perfusion preparation was observed to be viable by oxygen consumption, outflow perfusate pH, lactate release and insulin release in response to glucose. Parametric analysis of the outflow profiles suggested that the transport of water and tolbutamide from the vascular space was permeability limited. Parametric and nonparametric estimates of Vd for RBC and sucrose were similar and were 0.14+/-0.01, 0.15 0.005 and 0.35+/-0.01 ml/g. The parametric estimate for water, 1.04+/-0.05 ml/g was greater than the nonparametric estimate, 0.89+/-0.02 ml/g. The multiple indicator dilution method Vd of tolbutamide of 0.75+0.08 ml/g was similar to the reported value of 0.73+/-0.04 ml/g estimated by tissue partitioning studies.

CONCLUSIONS

A viable single pass pancreas perfusion model was established and applied to define distribution spaces of reference markers and the distribution kinetics of tolbutamide.

Influx and incorporation into protein of L-phenylalanine in the perfused rat pancreas: effects of amino acid deprivation and carbachol

The rate of protein synthesis in the isolated perfused rat pancreas was measured from the rate of incorporation of L-[3H]phenylalanine into total protein, and was compared with the transport of this amino acid into the epithelium. Unidirectional (15 s) and net (15-30 min) uptake of L-[3H]phenylalanine was measured relative to D-[14C]mannitol (extracellular marker) using a cell loading technique. The fractional rate of protein synthesis in the pancreas was also measured in vivo using a flooding dose technique and found to be 118 +/- 10% day-1 (corresponding to an absolute rate of incorporation of L-Phe into protein of 36.1 +/- 3 nmol min-1 g-1) in overnight fasted rats. Compared with the in vivo rate, the perfused pancreas exhibited a markedly lower rate of protein synthesis which increased significantly when amino acids were added to the perfusate (15.6 +/- 1.9 vs. 22.5 +/- 0.9% day-1 or 4.7 +/- 0.6 vs. 6.9 +/- 0.3 nmol L-Phe min-1 g-1). Carbachol (3 x 10(-7) M) stimulated protein synthesis provided amino acids were also supplied in the perfusate. Protein synthesis rates measured under all conditions in vivo and in vitro were at least an order of magnitude lower than the unidirectional influx (121 +/- 14 nmol min-1 g-1) of L-phenylalanine into the pancreatic epithelium. These results demonstrate that amino acid transport across the basolateral membrane of the epithelium is not rate-limiting for pancreatic protein synthesis.

Dietary regulation of amino acid transport activity in the exocrine pancreatic epithelium

Dietary-induced alterations in exocrine pancreatic amino acid transport were investigated in rats adapted for 14 days to isocaloric diets of varying casein and carbohydrate content. The kinetics of unidirectional (15 s) L-phenylalanine and L-lysine transport were measured relative to D-mannitol (extracellular tracer) in the perfused pancreas isolated from dietary adapted animals. In rats adapted to a 20% casein diet a weighted non-linear regression analysis of phenylalanine transport (1-24 mM) indicated an apparent Kt = 9.4 +/- 1.1 mM and Vmax = 14.8 +/- 0.9 mumol/min per g pancreas (n = 6). Saturation of lysine transport occurred at lower concentrations (0.05-10 mM) with an apparent Kt = 2.40 +/- 0.09 mM and Vmax = 2.44 +/- 0.18 mumol/min per g (n = 6). The characteristics of phenylalanine transport were modified after adaptation to either high (Kt = 3.6 +/- 1 mM, Vmax = 8.2 +/- 0.9 mumol/min per g, n = 3) or low (Kt = 4.2 +/- 0.9 mM, Vmax = 6.8 +/- 0.5 mumol/min per g, n = 3) carbohydrate diets. Increasing the dietary protein content (0-45% casein) led to a linear increase in the Kt for phenylalanine transport whereas Vmax values remained unchanged. Unlike phenylalanine, adaptation to a 0% casein diet significantly elevated the Vmax for lysine transport (4.82 +/- 0.21 mumol/min per g, n = 4) without altering the Kt (2.54 +/- 0.23 mM). The present findings suggest that changes in dietary composition induce select adaptive responses in the transport activities of System L (phenylalanine) and System y+ (lysine) in the basolateral membrane of the exocrine pancreatic epithelium.

Secretagogue-induced changes in system A amino acid transport in the rat exocrine pancreas: stimulation of 2-methylaminoisobutyric acid efflux by carbachol

Secretagogue-induced changes in exocrine pancreatic amino acid transport are poorly understood. In this study uptake of the specific non-metabolized System A amino acid analogue 2-methylaminoisobutyric acid (2-MeAIB) was measured in the isolated perfused rat pancreas during 60 min loading with D-[3H]mannitol (extracellular tracer) and 2-[14C]MeAIB. Tracer 2-MeAIB reached a maximal uptake of 37 +/- 4% (n = 4) after 3 min of loading and gradually decreased to a steady-state uptake of 13 +/- 1%. Infusion of carbachol (3.10(-7) M) during the tracer loading period abolished net tracer 2-MeAIB uptake, and reperfusion in the absence of carbachol restored net uptake to the prestimulus value. Less than 41% of the arterial 2-[14C]MeAIB or D-[3H]mannitol activity appeared in the basal pancreatic secretion. Carbachol evoked a 4.8-fold increase in pancreatic juice flow and appeared to reduce the activity of both tracers in the exocrine secretion. During washout of the pancreas with an isotope-free medium 2-[14C]MeAIB cleared from a rapidly exchanging pool with a time constant (tau 1) of 1.4 +/- 0.3 min (n = 4) and a more slowly exchanging pool with a time constant (tau 2) of 20.7 +/- 1.1 min. Carbachol accelerated efflux of 2-[14C]MeAIB from the epithelium but had no effect on the slow phase of D-[3H]mannitol washout. Our findings suggest that activation of cholinergic receptors modifies Na+-dependent System A amino acid transport in the basolateral membrane of the exocrine pancreatic epithelium.

Ethanol-induced alterations in exocrine pancreatic amino acid transport and secretion

The effects of ethanol on exocrine pancreatic amino acid transport and secretion were investigated during perfusion of the isolated rat pancreas with ethanol concentrations ranging from 0.06% to 4.1%. Amino acid transport was quantitated using a rapid dual isotope dilution technique in which unidirectional substrate uptake (15-20 sec) is assessed relative to an extracellular tracer. Pancreatic secretion evoked by 0.3 microM carbachol was abolished during perfusion with 0.32% ethanol. Influx of L-lysine, L-serine and methylaminoisobutyric acid (MeAIB) was marginally increased by 0.32% ethanol but significantly inhibited during subsequent perfusion with 1.28-4.1% ethanol. Pancreatic oxygen consumption and effluent PCO2 levels decreased with increasing ethanol concentration, and the control venous pH (7.21 +/- 0.01, n = 8) gradually approached arterial pH values (7.46 +/- 0.02, n = 9). These results indicate that low concentrations of ethanol readily inhibit secretagogue-induced pancreatic secretion. Amino acid transport at the basolateral membrane of the exocrine pancreatic epithelium appears only to be inhibited after acute exposure to high ethanol concentrations.