Control of amino acid transport in the mammary gland of the pregnant mouse

Delineating potential control mechanisms of mammary protein synthesis utilizing atropine

Amino acids (AA) and/or somatotropin (ST) were infused into a jugular vein of lactating cows receiving atropine to determine the effect on milk protein secretion. Atropine decreased milk protein yield by 35%. Plasma alpha-amino nitrogen, which was decreased by 31% due to atropine, was restored to control levels upon AA infusion. Plasma ST concentration was not affected by atropine and, upon ST infusion, increased by 236% in treated animals. Amino acids, ST, or the combination of the two were unable to return milk protein to control levels in atropine-treated animals. Plasma insulin concentration decreased by 37% in atropine-treated animals and was not significantly different for cows receiving atropine as well as ST, AA, or both ST and AA. Similarly, glucagon decreased by 36% in atropine-treated animals but was partially restored in cows receiving AA. Overall, the insulin-to-glucagon ratio was not significantly affected by treatment. Plasma glucose concentration was not affected by treatment. These data lend support to the importance of the insulin-to-glucagon ratio in terms of whole-body metabolism, with the exception of the mammary glands, which are glucagon-insensitive and, therefore, sensitive to the observed decrease in circulating insulin concentration.

Regulation of rat erythrocyte l-glutamine, 1-glutamate and l-lysine uptake by short term starvation

1. The kinetic parameters (Km, Vmax and Kd) of L-glutamine, L-glutamate and L-lysine uptake by isolated red blood cells in fed and 24 hr starved rats have been determined. 2. L-Lysine and L-glutamine uptake was best fitted by a two transport component: a saturable component and a diffusion one. 3. Starvation brought about important decreases in the Km and Vmax for both L-lysine and L-glutamine uptake. 4. The Kd for L-glutamine showed a significant increase whereas that corresponding to L-lysine did not change by starvation. 5. L-Glutamate uptake adjusted to diffusion kinetics, with a Kd which did not change due to starvation. 6. It is concluded that the amino acid uptake showed specific regulation by starvation. 7. The mechanism involved is not dependent on protein synthesis--given the unnucleated nature of mammal red cells. 8. The magnitude of the changes observed in the uptake kinetic parameters may account for the extent of the blood amino acid pool changes as those produced in vivo over physiological limits.

Ionic dependence of amino-acid transport in the exocrine pancreatic epithelium: calcium dependence of insulin action

Rapid unidirectional transport (15 sec) of L-serine and 2-methylaminoisobutyric acid (MeAIB) was studied in the isolated perfused rat pancreas using a dual-tracer dilution technique. Time-course experiments in the presence of normal cation gradients revealed a time-dependent transstimulation of L-serine influx and transinhibition of MeAIB influx. Transport of the model nonmetabolized System A analog MeAIB was Na+ dependent and significantly inhibited during perfusion with 1 mM ouabain. Although transport of L-serine was largely Na+ independent, ouabain caused a time-dependent inhibition of transport. Influx of both amino acids appeared to be inhibited by the ionophore monensin but unaffected by a lowered extracellular potassium concentration. Removal of extracellular calcium had no effect on influx of the natural substrate L-serine, whereas stimulation of transport by exogenous insulin (100 microU/ml) was entirely dependent upon extracellular calcium and unaffected by ouabain. Paradoxically, exogenous insulin had no effect on the time-course of MeAIB influx. The characteristics of L-serine influx described in earlier studies together with our present findings suggest that insulin may modulate the activity of System asc in the exocrine pancreatic epithelium by a calcium-dependent mechanism.

Amino acid transport systems in bovine mammary tissue

Nutrient provision to the lactating mammary gland involves three factors: blood nutrient concentration, blood flow, and cellular uptake. This paper reviews uptake of amino acids by bovine mammary tissue relative to interorgan blood flows, red blood cell contribution, arteriovenous differences, specific mammary amino acid transport systems, and glutathione and gamma-glutamyl transpeptidase. Recent studies with ruminant amino acid blood fluxes and the role of the red blood cell in providing nitrogenous substrates to tissues have brought to light new considerations of nutrient availability to mammary tissue. Previous studies measured arteriovenous differences to quantitate net amino acid uptake. These studies are considered relative to seven specific and separate amino acid transport systems, some of which have been identified in bovine mammary tissue. Uptake of sulfur amino acids by mammary tissue has been of interest because it appeared that insufficient quantities were provided during lactation. Glutathione, a tripeptide, may be a principle source of cysteine to mammary tissue via mechanisms involving gamma-glutamyl transpeptidase, glutathione, and red blood cells. The paper considers these mechanisms in relationship to amino acid transport systems.

Hormonal regulation of the System A amino acid transport adaptive response mechanism in a kidney epithelial cell line (MDCK)

When mammalian cells are starved for amino acids, the activity of the A amino acid transport system increases, a phenomenon called adaptive regulation. We have examined the effects of those factors which support Madin-Darby canine kidney (MDCK) cell growth in a defined medium on the derepression of System A activity. Of the five factors which supported MDCK cell growth, insulin was found to be an absolute requirement for derepression. In contrast, PGE1 was a negative controlling factor for the transport system. Growth of MDCK cells in the absence of PGE1 resulted in elevated System A activity which derepressed poorly upon amino acid starvation. Kinetic analysis of alpha-(methylamino) isobutyric acid (mAIB) uptake as a function of substrate concentration showed that the elevated A activity observed when cells were grown in the absence of PGE1 was kinetically similar to the activity induced by starvation for amino acids. Transport of mAIB by amino-acid-fed cells grown in the presence of PGE1 was characterized by a linear Eadie-Hofstee graph and by a relatively low Vmax. Transport by cells starved for amino acids or by cells grown in the absence of PGE1 was characterized by biphasic kinetics for mAIB transport and by elevated Vmax values. An influence of growth factors on the inactivation of derepressed A activity was also observed. In the presence of cycloheximide the rate of loss of A activity in amino-acid-starved cells was 1/4-1/2 that of amino-acid-fed cells. Insulin slowed inactivation in the absence of most amino acids in a protein-synthesis-independent manner, but insulin did not influence the more rapid inactivation observed in amino-acid-fed cells. These results indicate that the level of System A activity observed in response to regulation by amino acids represents a balance between carrier synthesis and inactivation, which can be positively or negatively influenced by growth factors.

Metabolic energy and cytoskeletal requirements for synthesis and secretion by acini from rat mammary gland-I. Ultrastructural and biochemical aspects of synthesis and release of milk proteins

1. Incubation of acini (alveoli) from lactating rat mammary gland with metabolic and cytoskeletal inhibitors produced a variety of effects on cell function. Cell viability was maintained during incubation as determined by the measurement of lactate dehydrogenase (LDH) activity in media and by light and electron microscopic examination. Caseins and whey proteins were found to be secreted by acini. 2. Addition of iodoacetate, 2,4-dinitrophenol, cyanide, cycloheximide, vinblastine or cytochalasin B inhibited both synthesis and secretion of milk proteins. Colchicine had no effect on synthesis but specifically inhibited protein secretion. Characteristic ultrastructural changes were produced by each inhibitor. 3. Uptake of 2-amino-isobutyric acid was reduced after incubation with all inhibitors except iodoacetate and dinitrophenol. Uridine incorporation was inhibited by colchicine, vinblastine, cytochalasin B and, at high concentrations, 2,4-dinitrophenol; cyanide and cycloheximide stimulated uridine incorporation. 4. Based on these results, milk protein secretion appeared to depend on continued protein synthesis and both processes were energy coupled. Microtubules and microfilaments also appeared to be involved in milk protein secretion.

Dietary fat effects on blood insulin, glucose utilization, and milk protein content of lactating cows

Relationships among dietary fat, glucose and insulin in blood plasma, and milk protein were investigated to determine mechanisms by which high fat diets depress milk protein percentage of lactating cows. Glucose clearance, determined by intravenous glucose infusion tolerance tests, of cows fed high fat diets was lower and insulin release higher than of control cows. Negative relationship between glucose utilization rate and insulin release was linear (correlation -.85), suggesting that fat feeding induced insulin resistance. Feeding lipid in a protected supplement did not change production of milk or milk protein but depressed milk protein percentage and increased quantity and concentration of milk fat. Concentrations of total lipids in blood plasma were increased whereas glucose and insulin were reduced by protected lipid supplement. Dietary fat may impair amino acid transport into the mammary gland and milk protein synthesis by inducing insulin resistance.

Influence of proliferative rates and A system substrate availability on proline transport in primary cell cultures of the R3230AC mammary tumor

Regulation of A system amino acid transport was studied in primary cultures of the R3230AC mammary adenocarcinoma. Higher rates of carrier-mediated Na+-dependent proline transport, vc, was decreased and was attributed to a two-fold decrease in Vmax and a two-fold increase in Km. When compared to cells grown in standard media (Eagle's minimal essential medium, MEM), cells grown in media supplemented with A system substrates (alanine, serine, glycine, and proline) demonstrated adaptive decreases in proline transport; the decrease was due to two-fold reduction in Vmax, with no change in Km for proline. Even in the presence of preferred substrates for the A system, a density-dependent decrease in proline transport was manifested. Both fast- and slow-growing cultures maintained in MEM exhibited rapid increases in proline transport when switched to buffers devoid of amino acids; two-fold increases in Vmax were seen within 4 hr, but Km was unchanged. This starvation-induced adaptation was completely prevented by inclusion in the buffer of 10 mM proline, 0.1 mM alpha-(methylamino)-isobutyric acid (MetAIB) or 10 mM serine, whereas inclusion of the poorer A system substrate, phenylalanine (10 mM), had no effect. The effects of MetAIB to prevent starvation-induced increases in proline transport were dose-related, rapid, and reversible. Amino acid starvation-induced increases in proline transport were partially blocked by cycloheximide or actinomycin D. Data were obtained demonstrating a temporal relationship between increasing intracellular [proline] and decreasing vc for proline uptake. In addition, efflux of proline from preloaded cells preceded the increase in initial rates of proline entry. Taken together, we concluded that: 1) A system transport in primary cultures of this mammary adenocarcinoma is regulated by cell density as well as by availability of A system substrates, but these two types of regulation are kinetically distinct; and 2) starvation-induced enhancement of proline transport appears to be due to release from transinhibition, but may also involve a derepression-repression type of mechanism.

Amino acid uptake by bovine mammary slices

A method which measures amino acid uptake by bovine mammary slices is described. Slices of approximately 200 mg were preincubated for 1 h in a modified Krebs-Ringers bicarbonate buffer containing a physiological mixture of amino acids. After preincubation, slices were incubated with radioactive amino acid for 10 min. Slices then were homogenized in perchloric acid, centrifuged, and a sample of the supernate was counted for radioactivity. In parallel experiments, the percent tissue water was determined by drying slices to constant weight. Extra-cellular space of slices was measured with inulin. These measurements allowed uptake to be expressed in terms of amino acid concentration in the cell water. The in vitro uptake pattern of nine essential amino acids was compared to the known in vivo uptake pattern of these amino acids by the cow udder. The correlation between in vitro and in vivo uptake patterns was .91, suggesting that the tissue slice technique can be useful in further study of bovine mammary amino acid transport.