Transepithelial transport in cell culture: bioenergetics of Na-, D-glucose-coupled transport

Inhibition of glycogenolysis prolongs action potential repriming period and impairs muscle function in rat skeletal muscle

KEY POINTS

Muscle glycogen content is associated with muscle function, but the physiological link between the two is poorly understood. This study investigated the effects of inhibiting glycogenolysis, while maintaining high overall energy status, on different aspects of muscle function. We demonstrate here that Na⁺ ,K⁺ -ATPase activity depends on glycogenolytically derived ATP regardless of high global ATP, with a decrease in activity leading to reduced force production and accelerated fatigue development. The results support the concept of compartmentalized energy transfer with glycogen metabolism playing a crucial role in intramuscular ATP resynthesis and ion regulation. This study gives specific insights into muscular function and may help towards a better understanding of glycogen storage diseases and muscle fatigue.

ABSTRACT

Skeletal muscle glycogen content is associated with muscle function and fatigability. However, little is known about the physiological link between glycogen content and muscle function. Here we aimed to investigate the importance of glycogenolytically derived ATP per se on muscle force and action potential (AP) repriming period, i.e. the time before a second AP can be produced (indicative of Na⁺ ,K⁺ -ATPase activity). Single fibres from rat extensor digitorum longus muscles were isolated and mechanically skinned in order to investigate force production and the AP repriming period while global ATP and PCr concentrations were kept high. The importance of glycogenolytically derived ATP was studied by inhibition of glycogen phosphorylase (1,4-dideoxy-1,4-imino-d-arabinitol (DAB; 2 mm) or CP-316,819 (CP; 10 µm)) or glycogen removal (amyloglucosidase, 20 U ml^-1 ). Tetanic force decreased by (mean (SD)) 21 (15)% (P < 0.001) and 76 (28)% (DAB) or 94 (6)% (CP, P < 0.001) in well-polarized and partially depolarized fibres, respectively. In depolarized fibres, twitch force decreased by 16 (10)% and 55 (26)% with DAB and CP, respectively, with no effect in well-polarized fibres (84 (10)%, P = 0.14). There was no effect of glycogen phosphorylase inhibition on repriming period in well-polarized fibres (median (25th, 75th percentile): 5 (4, 5) vs. 4 (4, 5) ms, P = 0.26), while the repriming period was prolonged from 6 (5, 7) to 8 (7, 10) ms (P = 0.01) in partially depolarized fibres. In line with this, glycogen removal increased repriming period from 5 (5, 6) to 6 (5, 7) ms (P = 0.003) in depolarized fibres. Together, these data strongly indicate that blocking glycogenolysis attenuates Na⁺ ,K⁺ -ATPase activity, which in turn increases the repriming period and reduces force, demonstrating a functional link between glycogenolytically derived ATP and force production.

Vanadate reduces sodium-dependent glucose transport and increases glycolytic activity in LLC-PK1 epithelia

The effect of vanadate pentoxide on apical sodium-dependent glucose transport in LLC-PK1 epithelia was examined. Epithelia grown in the presence or absence of 1 microM vanadate formed confluent monolayers and exhibited no differences in DNA, protein, or ultrastructure. Vanadate-supplemented epithelia demonstrated a lower steady-state alpha-methyl-D-glucopyranoside (AMG) concentrating capacity and a twofold reduction in apical AMG uptake Jmax. This decreased AMG transport occurred as a consequence of a reduction in the number of transport carriers and was not associated with a change in the sodium electrochemical gradient. The vanadate-induced reduction in apical glucose carrier functional activity and expression was accompanied by a stimulation of intracellular glycolytic flux activity, as evidenced by increased glucose consumption, lactate production, PFK-1 activity, and intracellular ATP. There was no difference in intracellular cAMP levels between vanadate-supplemented and non-supplemented epithelia. These results demonstrate an association between stimulation of glycolytic pathway activity and an adaptive response in the form of a reduction in the function and expression of the sodium-dependent apical glucose transporter in LLC-PK1 epithelia.

Functional properties of proximal tubule cell lines derived from transgenic mice harboring L-pyruvate kinase-SV40 (T) antigen hybrid gene

This study describes the functional characterization of two cell lines derived from the proximal convoluted (PKSV-PCT cells) and proximal straight (PKSV-PR) tubules microdissected out from kidneys of transgenic mice harboring the simian virus 40 (SV40) large T and small t antigens placed under the control of the rat L-type pyruvate kinase (L-PK) 5′ regulatory sequence. Both cell lines exhibited cellular cyclic AMP stimulated by parathormone (PTH) and calcitonin (CT) and a sodium-dependent glucose transporter. Uptake of the fluid-phase marker [3H]inulin showed that both cell lines grown on filters exhibited biphasic apical and basolateral endocytic rates. Results from Northern blot analysis indicate that the expression of the T antigen gene (Tag) is dependent on the concentration of D-glucose in the medium and show that the L-PK construct has maintained its capacity for up- or down-regulation by carbohydrates. Replacement of D-glucose by neoglucogenic substrates (lactate, oxaloacetate) blunted the expression of Tag transcripts and induced arrest of cell growth. Compared to cell grown in D-glucose-enriched medium, the hormonal sensitivities to PTH and CT and the sodium-dependent glucose uptake were unchanged whereas quiescent cells exhibited increased hydrolase content. Thus the proximal function has been preserved in these cultured cells derived from tissue-specific targeted oncogenesis in transgenic mice. As the expression of Tag transcripts is controlled by D-glucose, the structural and physiological characteristics of these cell lines can be studied in either quiescent or active growth conditions.

Renal cells in culture as a model for cystinosis

The established renal cell line LLC-PK1 was used as a model to investigate the mechanism underlying kidney malfunction observed in cystinosis patients. In this disease lysosomal accumulation of cystine impairs kidney function, and glycosuria is an early clinical manifestation. The linkage between lysosomal accumulation of cystine and impairment of kidney function is still unclear, and no animal model is available. In an attempt to gain a better insight into this relationship, we studied the effects of lysosomal loading with cystine on the survival and functions of normal noncystinotic renal epithelial cells (LLC-PK1), nonrenal fibroblasts (NIH-3T3), and cystinotic fibroblasts (GM2837). Incubation of the cells with cystine dimethylester (CDME) resulted in time- and dose-dependent accumulation of cystine, with 80% of the cystine in the lysosomal fraction. The lysosomal concentration of cystine increased in the three cell lines after 3 hours of incubation and declined significantly after 48 hours in the normal, but not cystinotic, cells. The accumulation of cystine in the lysosomes caused dose- and time-dependent cell mortality, assessed by measuring the activity of the cytosolic enzyme, lactic dehydrogenase, in the medium. Survival of fibroblasts and renal cells was similar in all three cell lines. The concentrating capacity (the ratio fo the intra- and extracellular concentrations) of the nonmetabolized sugar analog, alpha methyl glucoside (AMG), was used to assess the function of the kidney cells. The sugar concentrating capacity of LLC-PK1 cells was reduced after incubation with CDME in a dose- and time-dependent manner. Since there was no change in sugar efflux between the untreated and treated cells, we conclude that an impairment of the uptake of AMG is responsible for the reduction in the sugar-concentrating capacity in LLC-PK1 cells. In the absence of a genetically impaired animal model, LLC-PK1 cells treated with CDME can be used to investigate the cellular mechanisms responsible for the impairment of kidney function in cystinotic patients.

Glycolytic and oxidative metabolism in primary renal proximal tubule cultures

Cultured cells often exhibit alterations in energy metabolism (increased glycolytic activity and decreased oxidative metabolism) during adaptation to the culture environment. The role of hypoxia as a mediator of these effects was examined by comparison of metabolism in primary rabbit renal proximal tubule (RPT) cultures maintained in stationary culture dishes (DISH), shaking Erlenmeyer flasks (SHAKE), and DISH cultures transferred back to SHAKE conditions (RESHAKE). Both oxidative metabolism and transport capacity were fully preserved in SHAKE cultures over a 24-h period. In contrast, within 6 h, DISH cultures exhibited a continuous decline in transport-dependent and -independent oxygen consumption, respiratory capacity, and ATP and K+ contents. The loss of oxidative metabolism in DISH cultures was accompanied by stimulation of lactate production, detectable within 1 h after plating. Comparison of metabolic properties of DISH cultures to those of RPT exposed to graded levels of hypoxia suggested that medium oxygen tensions may be as low as 1-3% in DISH cultures. RESHAKE cultures exhibited metabolic properties comparable to those of SHAKE cultures, indicating reversibility of DISH culture metabolism on reoxygenation. We concluded that DISH cultures rapidly become hypoxic as a consequence of static culture conditions. Shaking suspension cultures may provide a more metabolically appropriate model for long-term in vitro studies.

Coupling of aerobic glycolysis and Na+-K+-ATPase in renal cell line MDCK

The relation between the activity of the Na+-K+-ATPase and the metabolic source of ATP was investigated in suspensions of MDCK cells. The pump activity of Na+-K+-ATPase was estimated from the initial rate of ouabain-sensitive K+ uptake into K+-depleted cells. Uptake was initiated by the reintroduction of K+ to the medium in which the cells were suspended. The metabolic source of ATP was varied by changing the substrates supplied to the suspension. Cells respiring on glutamine produced ATP from oxidative metabolism alone, whereas cells incubated with glucose and glutamine produced ATP via glycolysis and oxidative phosphorylation. Over a wide range of extracellular K+ concentrations, the initial rate of K+ uptake was faster in cells incubated with glucose and glutamine when compared with cells incubated with glutamine alone. Kinetic analysis together with ouabain-binding data demonstrated that this increase in K+ uptake was due to an increase in maximal velocity (Vmax) at a constant number of Na+-K+-ATPase transport sites. In addition, steady-state studies revealed that the addition of glucose to K+-depleted cells respiring on glutamine alone resulted in a net ouabain-sensitive influx of K+. These data demonstrate that in MDCK cells the maximal capacity for transport via the Na+-K+-ATPase is greater when ATP is produced from both glycolysis and oxidative phosphorylation than when ATP is produced from oxidative phosphorylation alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Energy metabolism of renal cell lines, A6 and MDCK: regulation by Na-K-ATPase

The energy metabolism of two continuous cell lines of renal origin, MDCK (Madin-Darby dog kidney) and A6 (toad kidney), was investigated by measuring the oxygen consumption (QO2) and lactate production (Jlac) by cells taken into suspension from monolayer cultures. Cells suspended from fully differentiated monolayers produce approximately 80% of their ATP requirements from oxidative metabolism. The interrelationship between ion transport and metabolism was determined by analyzing the ouabain sensitive components of intermediary metabolism under control conditions and after the stimulation of active Na-K transport with nystatin. In both cell lines, approximately 25% of the net rate of ATP production was inhibited by ouabain. Ouabain inhibited Jlac by 40% in MDCK and 45% in A6 cells, whereas QO2 was decreased by only 20% in both cell lines. In the presence of 0.05 mg nystatin/mg cell protein, ouabain sensitive Jlac increased by 75% in MDCK and was more than doubled in A6, whereas the ouabain-sensitive QO2 was not statistically different than control. This preferential stimulation of glycolysis with nystatin was not due to a limited capacity of mitochondrial oxidative phosphorylation since nystatin treatment of cells incubated without glucose (no glycolysis) significantly elevated the rate of QO2. These data demonstrate that aerobic glycolysis is more sensitive than is QO2 to changes in hydrolytic activity of the Na-K-adenosine triphosphatase (ATPase), in both cell lines.

Transport and metabolic functions in cultured renal tubule cells

The study tool of cultured tubule epithelia has been applied to new areas in nephron cell biology, such as the evolution of epithelial membrane asymmetry. Studies utilizing monoclonal antibodies against plasma membrane glycoproteins in MDCK revealed that the development of surface cell polarity is a continuous process requiring intact tight junctions and their electrical resistor function [101]. The role of the junctional complex to establish and maintain distinct membrane protein domains had been suggested earlier from work utilizing the apical aminopeptidase [102] and fluorescent membrane probes [103]. Cultured tubule epithelia lend themselves for the evaluation of cell-specific membrane protein synthesis [104] and antigenic determinants [105]. Human renal epithelia, from normal [106, 107] and defined abnormal kidney [108], have been maintained functional in primary and passage culture [106]. Pathophysiological mechanisms may be examined in cultured tubule epithelia, as shown first [109] by studies on the recovery from ischemic failure, where anoxia and substrate deprivation resulted in cell swelling which was prevented in culture by an oncotic agent. This article has not attempted to give an exhaustive account of the studies in which cultured tubule cells have served as a tool. Instead, the investigations quoted herein represent some principal lines of study, as seen from renal physiology, which may disclose details in culture of complex in vivo phenomena. It was Bernard [110] who, in 1865, suggested that "physiological events must be isolated outside the organism . . . to better understand the deepest associations of the phenomena."

Oxygen consumption of mammalian myocardial cells in culture: measurements in beating cells attached to the substrate of the culture dish

A Lucite attachment which permitted the measurement of oxygen consumption in cells in culture without manipulating the cells was constructed. The attachment fit over commercially available dishes for cell culture and had an oxygen electrode built into it. Oxygen uptake of cells in culture was thus measured. Cells were attached to the substrate of the culture dish during the measurements and could be observed in an inverted phase microscope. Cells did not show any morphological changes, e.g., cell shapes or beating rate in case of myocardial cells, before and after the measurements of oxygen consumption. Using this method the rate of oxygen consumption was determined in rat myocardial and heart non-muscle cells in culture and also in HeLa and L6 cell lines. Myocardial cells in culture had an approximately four times higher rate of oxygen uptake compared with heart non-muscle, HeLa, and L6 cells. The oxygen uptake of beating myocardial cells was higher by about 50% compared with quiescent myocardial cells.

Studies on the relationship between glycolysis and (Na+ + K+)-ATPase in cultured cells

In several tissues a coupling between glycolysis and (Na+ + K+)-ATPase has been observed. We report here studies on the coupling of glycolysis and (Na+ + K+)-ATPase in Rous-transformed hamster cells and Ehrlich ascites tumor cells. The rate of (Na+ + K+)-ATPase was estimated by the initial rate of ouabain-sensitive K+ influx after K+ reintroduction to K+-depleted cells. Experiments were performed with cells producing ATP via oxidative phosphorylation alone (i.e., lactate sole substrate), glycolysis alone (i.e., glucose as substrate in the absence of oxygen or with antimycin A), or glycolysis and oxidative phosphorylation (i.e., glucose as substrate in the presence of oxygen). The cells produced ATP at approximately the same rate under all of these conditions, but the initial rate of K+-influx was approx. 2-fold higher when AtP was produced from glycolysis. Changes in cell Na+ due to other transport processes related to glycolysis, such as Na+-H+ exchange, Na+-glucose cotransport, and K+-H+ exchange were ruled out as mediators of this effect on (Na+ + K+)-ATPase. These data suggest that glycolysis is more effective than oxidative phosphorylation in providing ATP to (Na+ + K+)-ATPase to these cultured cells.

Hexose regulation of sodium-hexose transport in LLC-PK1 epithelia: The nature of the signal

We have shown previously that the concentration of glucose in the growth medium regulates sodium-coupled hexose transport in epithelia formed by the porcine renal cell line LLC-PK1. Assayed in physiological salt solution, the ratio of the concentration of alpha-methyl glucoside (AMG) accumulated inside the cell at steady state to its concentration outside, and the number of glucose transporters, as measured by phlorizin binding, was inversely related to the glucose concentration in the growth medium. In this study, using a cloned line of LLC-PK1 cells, we provide evidence that the difference in AMG concentrating capacity is the result of a regulatory signal and not simply due to a selection process where the growth of cells with enhanced glucose transport is favored by low glucose medium or vice-versa. By adding glucose to conditioned medium (collected after 48 hr incubation with cells and therefore containing less than 0.1 mM glucose), we demonstrate that the signal in the growth medium is indeed the concentration of glucose rather than another factor secreted into or depleted from the medium. Fructose and mannose, two sugars not transported by the sodium-dependent glucose transporter, can substitute for glucose as a carbohydrate source in the growth medium and have a modest glucose-like effect on the transporter. Growth in medium containing AMG does not affect the transporter, indicating that the regulatory signal is not a direct effect of the hexose on its carrier but involves hexose metabolism.