Alpha-aminoisobutyric acid uptake in isolated mouse fat cells

Effect of theophylline on the transport of tyrosine in cultured B-16 mouse melanoma cells

The effect of theophylline, an inhibitor of phosphodiesterase, on tyrosine (Tyr) transport across cell membrane was studied using cultured B-16 mouse melanoma cells. 1.5 mM Theophylline in culture medium increased Tyr uptake velocity in linear fashion up to 36 h, after which it reached a plateau at which the cells showed about a 60% increase in Tyr transport velocity. This increase was independent of extracellular Na, essentially unaffected by cell density, and partially inhibited by concomitantly added cycloheximide. These results suggested that biosynthesis of macromolecules, probably acting as System L transporter, was induced by theophylline treatment.

Influence of serum and amino acids on the accumulation of aminoisobutyrate by rat hepatoma cells. A dedifferentiation of transport routes?

Rat hepatoma cells accumulate considerably less 2-aminoisobutyrate after cultivating in the absence of serum--the change in rate of aminoisobutyrate uptake takes place within 1 h of serum starvation. Starvation of amino acids by contrast raises aminoisobutyrate uptake in the presence or absence of serum, but the cells are much less responsive to amino acid supply than to availability of serum. Phosphate (10 mM) reduced aminoisobutyrate uptake by cells grown in serum to that exhibited by serum-starved cells. Aminoisobutyrate uptake by cells grown in serum was reduced by glycine, proline, alanine, serine, glutamine, methylaminoisobutyrate and 2-aminonorbornane-2-carboxylate, the effects of methylaminoisobutyrate and 2-aminonorbornane-2-carboxylate being additive. However, similar inhibition phenomena were not seen for cells deprived of serum where aminoisobutyrate uptake tended to a relatively constant level insensitive to inhibitory influences, yet substantially greater than that arising by simple diffusion. The comparative insensitivity of our hepatoma line when starved of serum to competition and repression phenomena is in contrast to findings of others. Our results also suggest a lack of clear delineation of specificities for the A and L transport systems as usually defined.

Characteristics of proline transport into R3230AC mammary tumor cells

Cells separated by enzyme treatment of the R3230AC mammary carcinoma were used to characterize the entry of proline. These cells showed minimal changes in cell viability and intracellular volume and were found to be suitable for transport studies, since the vi of proline was maintained for at least 4 h when cells were stored at 37 or 4 degrees C, or when transport was measured in the presence or absence of Na+. Proline was acitvely transported by these tumor cells, reaching a distribution ratio ([proline] intracellular/[proline] extracellular) of 20 after 2 h. Proline entry consisted of two processes, one saturable (carrier mediated) and the other, non-saturable. The carrier-mediated entry, Km - 0.83 mM and V = 151.10(-5) mumol/min per 5.10(6) cells, was Na+-dependent, sensitive to pH and metabolic inhibitors, and completely inhibited by alpha-(methylamino)-isobutyric acid (Ki = 0.34 mM). Proline entry in the absence of Na+ was 20% that in the presence of Na+ and was found to be due to a non-saturable process, since (a) vi of proline uptake in the absence of Na+ increases linearly with increasing proline concentration and (b) was not suppressed by either 20 mM alpha-(methyl-amino)-isobutyric acid, 50 mM glycine +20 mM phenylalanine, or 50 mM serine +20 mM phenylalanine when proline uptake was measured in the presence or absence of Na+. Therefore, under the conditions studied, we conclude that proline transport appears to be restricted to the A (alanine-preferring) system. Furthermore, these cells should provide a suitable model to study the effect of hormonal manipulations on the amino acid transport process.

The effects of bioregulators upon amino acid transport and protein synthesis in isolated rat hepatocytes

Isolated rat hepatocytes prepared by an enzyme perfusion technique possess a functional amino acid transport system and retain the capacity to synthesize protein. Amino acid transport was studied using the non-metabolizable amino acid analog alpha-aminoisobutyric acid. The transport process was time, temperature and concentration dependent. Similarly, leucine incorporation into protein was time and temperature dependent being optimal at 3m degrees C. Amino acid, fetal calf serum, growth hormone and glucose all produced small, reproducible increases in protein synthesis rates. Bovine serum albumin diminished the uptake of alpha-aminoisobutyric acid and leucine incorporation into protein. The amino acid content on either side of the cell membrane was found to affect transport into or out of the cellular compartment (transconcentration effects). High cell concentrations decreased transport and protein synthesis as a result of isotopic dilution of labelled amino acids with those released by the hepatocytes. This was consistent with the capacity of naturally occurring amino aicds to compete with alpha-aminoisobutyric acid for uptake into the hepatocyte. In order to define more precisely the effects of bioregulators on transport and protein synthesis it will be necessary to define and subfractionate cellular compartments and proteins which are the specific targets of cellular regulation.

Uptake and metabolism of fatty acids and analogues by cultured cardiac cells from chick embryo

Cultured heart cells from chick embryo, present as a confluent monolayer in a minimal medium, take up labelled fatty acids (from C6 to C22) bound to serum albumin with a rapid incorporation into neutral and complex lipids (visible at 15 s), while beta-oxidation is delayed (visible at 15 min). Fatty acids enter the cardiac cells through two mechanisms, one involving a readily saturable process, the other resembling passive diffusion. The saturable transport system recognizes fatty acids with a high affinity, Km from 4.5 to 16 muM. The V is from 0.03 to 0.3 nmol of fatty acid incorporated min-1 (10(6) cells)-1. Competitive inhibition occurs between fatty acids while glucose, aminoacids and lactate do not compete with palmitate for the entry into the cells. The analysis of unesterified fatty acids from the usual culture medium and from the cells show a gradient of concentration up to 54-fold; moreover experiments performed in minimal medium at 20 degrees C show that fatty acids accumulate up to 28-fold in the cells. Efflux rates can be measured after loading the cells at 20 degrees C; the curves are bimodal and the pseudo first-order rate constants thus determined are in the order of 0.5 min-1, as opposed to 2.5 to 5.0 min-1 for the passive-diffusion component calculated for the influx rates. Studies on the role of serum albumin at constant and at variable fatty acid-albumin molar ratios indicate that the rate of uptake is controlled by the total fatty acid concentration and not solely by the concentration of unbound fatty acids. No labelled fatty acid was found to be bound to the cytoplasmic fraction supposed to contain the fatty acids-binding protein; our results are in favour of the presence of a specific transport system for fatty acids either at the level of subcellular organelles or more likely at the surface of cardiac cells.

Structure-function relationships in the adipose cell. 3. Effects of bovine serum albumin on the metabolism of glucose and the release of nonesterified fatty acids and glycerol by the isolated adipose cell

Serum albumin stimulates the uptake of U-glucose-(14)C and the incorporation of (14)C-counts into triglyceride glycerol and inhibits the incorporation of (14)C-counts into triglyceride fatty acids by isolated adipose cells; insulin and epinephrine enhance these effects. In the absence of hormones, these responses to albumin increase with increasing albumin concentration. In the presence of insulin, a qualitatively similar pattern of increasing responses to albumin is observed; the enhancement of each response by insulin is, however, only slightly potentiated by higher albumin concentrations. In contrast, in the presence of epinephrine, these responses to albumin are maximal at the lowest albumin concentration tested, 0.1%; the enhancement of each response by epinephrine is similarly maximal at 0.1% albumin, but decreases rapidly as the albumin concentration is raised. Increasing serum albumin concentrations do, however, stimulate the release of fatty acids and glycerol by epinephrine-treated cells increasingly until a plateau, determined by the epinephrine dose, is reached. These data support the suggestion that intracellular fatty acid levels function in the regulation of adipose cell activity, and further suggest that serum albumin plays a role in determining the metabolic fate of these fatty acids.

Structure-function relationships in the adipose cell. II. Pinocytosis and factors influencing its activity in the isolated adipose cell

Pinocytic activity in the adipose cell has been examined by measuring the uptake of colloidal gold. Pinocytic activity occurs in the isolated adipose cell under all experimental conditions; a portion of the vesicular elements of the cell can be identified by electron microscopy as pinocytic in origin. The isolated adipose cell appears to take up serum albumin by pinocytosis. Pinocytic activity in the isolated adipose cell is enhanced by epinephrine, but not by insulin. The relationship between pinocytosis and the metabolic activity of the adipose cell has been studied by measuring simultaneously the uptake of radioactive colloidal gold, the incorporation of (14)C-counts from U-glucose-(14)C into CO(2), total lipid, triglyceride glycerol and triglyceride fatty acids, and the release of nonesterified fatty acids in the absence of hormones and in the presence of insulin or epinephrine. Correlations between hormone-produced alterations in lipid metabolism and in pinocytic activity suggest that intracellular nonesterified fatty acid levels are a factor in the regulation of both the cell's pinocytic activity and its metabolism and that pinocytosis in the adipose cell functions in the extracellular-intracellular transport of nonesterified fatty acids.