Diacylglycerol and calcium induce rapid enhancement of A-system amino acid transport by independent mechanisms in human T-lymphocytes

Effect of theophylline on l-alanine absorption in the rat jejunum

The effect of theophylline on the accumulation of L-alanine (Ala) by the rat jejunum and the mechanism involved has been investigated. Ala is rapidly accumulated by the jejunal strips in vitro and saturation is reached between 10 and 15 min. An In/Out ratio of 2.55 reflects the presence of an active component in the overall transport mechanism. Ala accumulation shows a tendency toward saturation as cellular Ala concentration increases. In the absence of Na+, Ala accumulation is reduced and a direct relationship is observed between alanine concentration in the incubation medium and its intracellular concentration. Alanine accumulation is inhibited when theophylline (TH) concentration in the incubation medium is greater than 0.5 mM. A maximum inhibition of approximately 50% in the presence of 10 mM theophylline is observed. Further inhibition (57-65%) is observed when the jejunal strips are incubated in a Na+-free medium containing 10 mM theophylline. Single-pass perfusion of the rat jejunum shows that the presence of 0.5 mM TH in the perfusate, simulating therapeutic doses, did not affect Ala absorption. However, about 55% inhibition of Ala absorption was observed when 10 mM TH was included in the perfusate. In conclusion, it could be stated that in both in vitro and in vivo studies high toxic but not therapeutic doses of TH inhibit intestinal Ala uptake. The mechanism of inhibition may be attributed to a non-carrier mediated mechanism with a minor effect noticed on Ala carrier system.

Regulation of L-alanine transport systems A and ASC by cyclic AMP and calcium in a reptilian duodenal model

The regulation of neutral amino acid transport by cyclic AMP (cAMP) and calcium across the isolated duodenum of the lizard Gallotia galloti has been studied under short-circuit conditions. Active L-alanine transport was stimulated by forskolin, theophylline and dibutyryl cyclic AMP (db-cAMP). All these agents increased transmural potential difference (PD) and short-circuit current (I(sc)) in a manner consistent with the activation of a chloride secretory pathway. Both forskolin and theophylline increased intracellular cAMP levels in the lizard duodenal mucosa. Addition of calcium ionophore A23187 rapidly reduced mucosa-to-serosa L-alanine fluxes and diminished net L-alanine transport. Despite the reduction of alanine fluxes by A23187, transepithelial PD and I(sc) values were increased by the ionophore. Analyses of the responses of isolated transport pathways indicated that the Na(+)-independent L-alanine transport system was unaffected by db-cAMP or calcium ionophore. By contrast, Na(+)-dependent transport activities were profoundly modified by these agents. Thus, while system A [alpha-methylamino-isobutiric acid (MeAIB)-transporting pathway] was stimulated by increased calcium, system ASC activity was nearly abolished. Calcium ionophore also potentiated the electrogenic response of system A. Forskolin strongly stimulated system ASC activity but left system A activity unchanged. Activation of system ASC by forskolin was clearly electroneutral, as pre-incubation of the tissues with the chloride channel blocker diphenylamine-2-carboxilic acid (DPC) completely prevented forskolin-induced transepithelial electrical responses. It is concluded that intracellular messengers cAMP and calcium oppositely modulate active Na(+)-dependent (L)-alanine transport in the lizard intestine. The different sensitivity exhibited by individual transport pathways may well account for the changes observed in overall alanine transport.

The role of system A for neutral amino acid transport in the regulation of cell volume

System A is a secondary active, sodium dependent transport system for neutral amino acids. Strictly coupled with Na,K-ATPase, its activity determines the size of the intracellular amino acid pool, through a complex network of metabolic reaction and exchange fluxes. Many hormones and drugs affect system A activity in specific cell models or tissues. In all the cell models tested thus far the activity of the system is stimulated by amino acid starvation, cell cycle progression, and the incubation under hypertonic conditions. These three conditions produce marked alterations of cell volume. The stimulation of system A activity plays an important role in cell volume restoration, through an expansion of the intracellular amino acid pool. Under normal conditions, system A substrates represent a major fraction of cell compatible osmolytes, organic compounds that exert a protein stabilizing effect. It is, therefore, likely that the activation of system A represents a portion of a more complex response triggered by exposure to stresses of various nature. Since system A transporters have been recently cloned, the molecular bases of these regulatory mechanisms will probably be elucidated in a short time.

Biologically active factors in bovine milk and dairy byproducts: influence on cell culture

Substantial progress has been made in our knowledge of the biological properties of mammal milks. Many nutritional, biochemical, immunological, or other biological properties have been studied in mature or industrially processed bovine milk as well as in human milk and colostrum. This article is a critical review of selected publications covering (1) the use of bovine milk or dairy byproducts (processed acid and enzymatic whey fractions) as a serum substitute for cell cultures, (2) specific factors in bovine milk and industrially processed milk the affect cell proliferation, and (3) the known functional and biological roles of two whey proteins: beta-lactoglobulin and the PP3 component.

The role of protein phosphorylation in renal amino acid transport

Changes in tubular reabsorption of amino acids and other solutes are characteristic of the immature renal tubule and of various hereditary nephropathies. The cellular mechanisms governing these aberrations in renal amino acid transport have not been established. Calcium (Ca2+)-dependent protein kinases are known to phosphorylate membrane-bound carrier proteins, thereby modulating transport of various solutes by the proximal tubule. The role of these enzymes in regulating renal tubular amino acid transport, particularly during kidney development, is unknown. We investigated: (1) the effect of Ca(2+)- and phospholipid-dependent protein kinase [protein kinase C (PKC)] and Ca2+/calmodulin-dependent protein kinase II (CaMKII) on sodium chloride (NaCl)-linked proline transport by renal brush border membrane vesicles (BBMV) from adult rats using the "hypoosmotic shock" technique (lysis of vesicles); (2) the activity, expression and subcellular distribution (cytosol, particulate, BBM) of Ca(2+)-dependent protein kinases in kidneys from 7-day-old and adult rats using MBP 4-14 and autocamtide II phosphorylation assays for PKC and CaMKII, respectively, endogenous protein phosphorylation (using gel electrophoresis and autoradiography) and Western immunoblot analysis to detect PKC and CaMKII. The studies showed: (1) endogenous (membrane-bound) CaMKII and PKC as well as exogenous, highly purified PKC inhibit proline uptake by phosphorylated, lyzed/resealed BBMV when compared with control vesicles; the voltage-clamped, nonelectrogenic component of proline transport was inhibited by PKC- but not CaMKII-mediated phosphorylation; (2) a Ca(2+)-dependent activity of both kinases was evident in all subcellular fractions tested in immature and adult kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Phorbol esters stimulate the transport of anionic amino acids in cultured human fibroblasts

The effect of phorbol esters on the transport of amino acids has been evaluated in cultured human fibroblasts. The activity of the Na(+)-dependent system XAG- for anionic amino acids is selectively and markedly stimulated by phorbol esters. The effect is maximal within 15 min; it is attributable to an increase in transport maximum (Vmax) and not prevented by inhibitors of protein synthesis. The half-maximal stimulation is observed at concentrations of phorbol 12,13-dibutyrate lower than 100 nM. Prolonged incubations in the presence of 1 microM phorbol 12,13-dibutyrate lower the binding of the ligand to its receptor with a loss of the stimulatory effect on transport. The results presented indicate that the stimulation of amino acid transport through system XAG- by phorbol esters requires the activation of protein kinase C.

Lactate production and amino acid incorporation in interleukin 3-dependent, factor-deprived hemopoietic murine cell lines

Interleukin 3 (IL-3) stimulates the growth of various types of hemopoietic progenitors. In vitro, survival of a series of murine cell lines derived from either neoplastic or nonneoplastic hemopoietic tissue shows a strict IL-3 dependence. In order to test the implication of energy metabolism in this dependence as claimed in several studies, intracellular ATP levels as well as accumulative lactate release were measured in the murine hemopoietic lines FDC-P1, 32Dcl.23, DA-1, DA-3, NFS-60, and NFS-78. ATP levels showed little or no changes within 4-6 h of IL-3 starvation. In the absence of IL-3 the accumulative lactate release ranged from 1.4 to 2.6 mM, and in its presence values between 1.5 and 3.4 mM were recorded within 7 h. Only 32Dcl.23 showed an almost complete suppression of lactate release upon IL-3 withdrawal. The cell cycle times of these cell lines determined by flow cytometry ranged between 9 (DA-3) and 24 h (NFS-78). In the presence of IL-3 there was a significant inverse relationship between cell cycle times and lactate production. It is concluded that neither ATP generation nor the metabolic pathway of lactate production, although the latter correlated with proliferative activity in the studied cell lines, is controlled by IL-3. Furthermore, no control by IL-3 of essential amino acid incorporation into proteins was detected in cell lines 32Dcl.23 and NFS-60.

Epinephrine regulation of amino acid transport in rat hepatocytes isolated during development

The effect of epinephrine on the amino acid transport mediated by system A was investigated by determining the uptake of 2-amino [1-14C]isobutyric acid (AIB) in rat hepatocytes, freshly isolated at different stages of pre- and postnatal development. The data obtained show that the hormone increased AIB uptake, enhancing the Vmax, while Km was unchanged. This effect was evident in cells from adult, 18- to 20-day-old fetus, and neonate rat. Actinomycin D or cycloheximide abolished the hormone dependent increase. Experiments carried out with alpha- and beta-antagonists showed that the effect of epinephrine was beta-mediated in fetal life and alpha-mediated in adult life. Membrane binding experiments showed a higher value for epinephrine and beta-agonist dihydroalprenolol in the fetus versus the adult. The calcium depletion obtained after cell incubation with EGTA or calcium ionophore A23187 reduced the hormonal stimulation in the adult, and was ineffective in the prenatal period. An involvement of cAMP was present in the epinephrine modulation of AIB transport, both in adult and in fetal life.