Insulin regulation of sugar transport in giant muscle fibres of the barnacle

The Role of Insulin-like Peptide in Maintaining Hemolymph Glucose Homeostasis in the Pacific White Shrimp Litopenaeus vannamei

Insulin-like peptide (ILP) has been identified in various crustaceans, but whether it has a similar function in regulating hemolymph glucose as vertebrate insulin is unclear. We analyzed the components of hemolymph sugar in the Pacific white shrimp, Litopenaeus vannamei, and investigated the changes of hemolymph glucose concentration and the expressions of ILP and glucose metabolism genes under different treatments. We found glucose was a major component of hemolymph sugar in shrimp. Starvation caused hemolymph glucose to rise first and then decline, and the raised hemolymph glucose after exogenous glucose injection returned to basal levels within a short time, indicating that shrimp have a regulatory mechanism to maintain hemolymph glucose homeostasis. In addition, injections of bovine insulin and recombinant LvILP protein both resulted in a fast decline in hemolymph glucose. Notably, RNA interference of LvILP did not significantly affect hemolymph glucose levels, but it inhibited exogenous glucose clearance. Based on the detection of glucose metabolism genes, we found LvILP might maintain hemolymph glucose stability by regulating the expression of these genes. These results suggest that ILP has a conserved function in shrimp similar to insulin in vertebrates and plays an important role in maintaining hemolymph glucose homeostasis.

Regulation of GLUT1-mediated sugar transport by an antiport/uniport switch mechanism

Avian erythrocyte sugar transport is stimulated during anoxia and during exposure to inhibitors of oxidative phosphorylation. This stimulation results from catalytic desuppression of the cell surface glucose transporter GLUT1 [Diamond, D., & Carruthers, A. (1993) J. Biol. Chem. 268, 6437-6444]. The present study was undertaken to investigate the mechanisms of GLUT1 suppression/desuppression. Sugar uniport (sugar uptake or exit in the absence of sugar at the opposite side of the membrane) is absent in normoxic avian erythrocytes, but sugar antiport (sugar uptake coupled to sugar exit) is present. Exposure to cyanide and/or to FCCP (mitochondrial inhibitors) stimulates erythrocyte sugar uniport but not sugar antiport. K(m)(app) for 3-O-methylglucose uniport and antiport are unaffected by metabolic poisoning. Ki(app) for inhibitions of 3-O-methylglucose uniport by cytochalasin B and forskolin (sugar export site ligands) are unaffected by progressive stimulation of sugar uniport. Cyanide and FCCP stimulation of 3-O-methylglucose uniport are associated with increased AMP-activated protein kinase activity. Purified human GLUT1 is not phosphorylated by exposure to cytosol extracted from poisoned avian erythrocytes. FCCP does not stimulate GLUT1-mediated 3-O-methylglucose uptake in K562 cells but does increase K562 AMP-activated protein kinase activity. FCCP stimulation of 3-O-methylglucose uniport in resealed erythrocyte ghosts requires cytosolic ATP and/or glutathione. The nonmetabolizable ATP analog AMP-PNP cannot be substituted for ATP in this action. These results are contrasted with allosteric regulation of human erythrocyte sugar transport and suggest that avian erythrocyte sugar transport suppression results from inhibition of carrier uniport function. Uniport suppression is not mediated by interaction with cytosolic molecular species that bind to the sugar export site. The antiport to uniport switch mechanism requires ATP hydrolysis, is associated with elevated AMP-activated kinase function, and, if triggered by this kinase, is mediated by factors absent in K562 cells and downstream from the kinase.

Mechanisms of hyperpolarization induced by two cytokines, hTNF alpha and hIL-1 alpha in neurons of the mollusc, Onchidium

The voltage and current responses induced by extracellular tumor necrosis factor (hTNF alpha) or interleukin-1 (hIL-1 alpha) on the Be-1 and Es-1 neurons of the Onchidium ganglia were examined. Pressure-ejected hTNF alpha or hIL-1 alpha produced an inhibitory, hyperpolarized effect in unclamped neurons. In the same neurons voltage-clamped at their resting potential levels, the same hTNF alpha or hIL-1 alpha elicited an outward current having a time course similar to that of the hyperpolarization, associated with a decreased membrane conductance. The hTNF alpha- or hIL-1 alpha-induced outward current did not reverse even at positive membrane potentials considerably above + 100 mV in the absence ouabain (a specific blocker of Na-pump). In the presence of ouabain, the hTNF alpha- or hIL-1 alpha-induced current was reduced over a wide range of membrane potential, so that the current reversed at about + 20 mV. Lowering the external Na+ concentration from 450 to 200 mM in the presence of ouabain, shifted the reversal potential from + 20 to 0 mV, to near the shift value of 20.8 mV predicted by the Nernst equation for a Na(+)-selective conductance. Neither an increase nor a decrease of extracellular K+, Cl- or Ca2+, however, significantly altered the current induced by hTNF alpha or hIL-1 alpha. These suggest that the hTNF alpha- or hIL-1 alpha-induced hyperpolarization or outward current response is mediated by two mechanisms, a decrease in Na+ conductance and activation of the Na-pump.

Barnacle muscle: Ca2+, activation and mechanics

In this review, aspects of the ways in which Ca2+ is transported and regulated within muscle cells have been considered, with particular reference to crustacean muscle fibres. The large size of these fibres permits easy access to the internal environment of the cell, allowing it to be altered by microinjection or microperfusion. At rest, Ca2+ is not in equilibrium across the cell membrane, it enters the cell down a steep electrochemical gradient. The free [Ca2+] at rest is maintained at a value close to 200 nM by a combination of internal buffering systems, mainly the SR, mitochondria, and the fixed and diffusible Ca(2+)-binding proteins, as well as by an energy-dependent extrusion system operating across the external cell membrane. This system relies upon the inward movement of Na+ down its own electrochemical gradient to provide the energy for the extrusion of Ca2+ ions. As a result of electrical excitation, voltage-sensitive channels for Ca2+ are activated and permit Ca2+ to enter the cell more rapidly than at rest. It has been possible to determine both the amount of Ca2+ entering by this step, and what part this externally derived Ca2+ plays in the development of force as well as in the free Ca2+ change. The latter can be determined directly by Ca(2+)-sensitive indicators introduced into the cell sarcoplasm. A combination of techniques, allowing both the total and free Ca2+ changes to be assessed during electrical excitation, has provided valuable information as to how muscle cells buffer their Ca2+ in order to regulate the extent of the change in the free Ca2+ concentration. The data indicate that the entering Ca2+ can only make a small direct contribution to the force developed by the cell. The implication here is that the major source of Ca2+ for contraction must be derived from the internal Ca2+ storage sites within the SR system, a view reinforced by caged Ca2+ methods. The ability to measure the free Ca2+ concentration changes within a single cell during activation has also provided the opportunity to analyse, in detail, the likely relations between free Ca2+ and the process of force development in muscle. The fact that the free Ca2+ change precedes the development of force implies that there are delays in the mechanism, either at the site of Ca2+ attachment on the myofibril, or at some later stage in the process of force development that were not previously anticipated.(ABSTRACT TRUNCATED AT 400 WORDS)

Protection by GTP from the effects of aluminum on the sodium efflux in barnacle muscle fibers

The idea that guanine nucleotides act as chelators of Al3+ and that Al interrupts the mechanism by which GTP or Gpp(NH)p stimulates the Na efflux in single muscle fibers from the barnacle Balanus nubilus has been tested. As a rule, injection of GTP or Gpp(NH)p into unpoisoned and ouabain-poisoned fibers produces a rise in the 22Na efflux that is usually transitory in nature. Fibers preinjected with GTP show a fall in the Na efflux following the injection of AlCl3 in an equimolar concentration. If, however, the concentration of Al for injection is halved, then GTP is found to be fully protective. Fibers preinjected with AlCl3 show little or no response to the injection of GTP. This is also the case with ouabain-poisoned fibers. Ouabain-poisoned fibers preinjected with GTP also show little or no response to the injection of AlCl3. The stimulatory response to the injection of AlCl3 into fibers preinjected with 0.5 M GTP is dose-dependent. A graded response is also found when 0.5 M AlCl3 is injected into fibers preinjected with GTP in varying concentrations. Gpp(NH)p is fully protective against the inhibitory effect of Al injection in unpoisoned fibers. Further, Gpp(NH)p abolishes the biphasic effect of Al injection on the ouabain-insensitive Na efflux. To strengthen the argument that GTP acts as a chelator of Al, a solution mixture of 0.5 M GTP/0.5 M AlCl3 (pH 1-2) was injected into unpoisoned fibers. This is found to lead to a smaller fall in the resting Na efflux than that obtained by injecting AlCl3 alone or injecting AlCl3 after GTP. It is thus quite clear that the barnacle muscle fiber is a useful preparation for studies of this type.

High affinity insulin binding and insulin receptor-effector coupling: modulation by Ca2+

Insulin binding and insulin stimulated amino acid and glucose uptake were determined in cultured HTC hepatoma cells in the presence of Ca2+ and ruthenium red (RR) in order to further characterise the putative calcium binding site on the receptor. These ions increased insulin receptor high affinity binding and the sensitivity of these responses to insulin. The insulin concentration required to half-maximally stimulate amino acid uptake decreased significantly from 26.9 +/- 5.8 ng/ml to 6.0 +/- 1.3 ng/ml in the presence of 10 mM Ca2+ and to 1.3 +/- 0.5 ng/ml in the presence of RR. The effect of Ca2+ and RR was more pronounced on insulin stimulated glucose uptake. These agents also increased receptor-effector coupling, reducing the percentage of occupied receptors required for maximal insulin stimulation of amino acid uptake from 10.8% in control cells to 3.4 and 1.4% in the presence of Ca2+ and RR respectively. The receptor occupancy required to produce maximal insulin responses on glucose uptake decreased from 20% (control) to 3.8% (Ca2+ and RR). We hypothesize that since Ca2+ and RR have similar effects, that occupation of Ca2+ binding sites on the receptor produces a conformational change in the insulin receptor which increases insulin receptor affinity, insulin sensitivity and acts on an early post-receptor event responsible for coupling binding to insulin action.

A study of the ouabain-insensitive sodium efflux in barnacle muscle fibres using phorbol dibutyrate as a probe

1. The resting ouabain-insensitive Na+ efflux in muscle fibres isolated from the barnacle, Balanus nubilus, is stimulated by external or internal application of phorbol 12,13-dibutyrate (PD). The response occurs fairly promptly and may not decay at all, or more commonly, decay rather slowly. The magnitude of the response to external or internal application of PD is dose-dependent, the minimum effective concentration being about 10(-8) M. 2. The response to PD fails to occur in the nominal absence of external Ca2+. Sudden removal of external Ca subsequent to peak stimulation by PD leads to almost complete reversal of the response. The response to PD of fibres suspended in Li(+)-ASW (artificial sea water) is similar in magnitude to that of fibres suspended in Na(+)-ASW. However, it differs in that it is of a sustained nature. 3. Calcium channel blockers, e.g. verapamil, completely prevent the response to PD from occurring. Both Cd2+ and Co2+ are less effective than verapamil. 4. Pre- but not post-injection of EGTA reduces the response to PD. Pre- or post-injection of Mg2+ reduces the response considerably. 5. Fibres pre-injected with GTP show a reduced response to PD. Fibres pre-injected with PD show a reduced response to GTP. Pre-injection of protein kinase inhibitor is without effect on the response to PD. 6. Furosemide, piretanide and bumetanide are without effect on the response to PD. 7. DIDS (4,4'-diisothiocyanostilbene-2,2-disulphonic acid) is a potent inhibitor of the response to PD but not amiloride. Pyridoxal 5-phosphate and benzolamide are also powerful inhibitors. Pyridoxal 5-phosphate in combination with benzolamide fails to completely abolish or reverse the response to PD. 8. Luminescence from aequorin is promptly increased by PD in a dose-dependent manner, the minimal effective concentration being in the nanomolar range. The signal is monophasic or multiphasic in shape, and is often less than 5 min in duration. Not infrequently, however, the aequorin response fails to completely decay and the new level of resting glow remains above the original baseline level. 9. Collectively, these observations accord with a tentative general hypothesis stating that the stimulatory response of the ouabain-insensitive Na+ efflux to PD is triggered by two mechanisms. One involves a rise in myoplasmic free [Ca2+] resulting from the entry of external Ca2+ via opened Ca2+ channels which is followed by the operation of the Na(+)-Ca2+ exchanger in the reverse mode.(ABSTRACT TRUNCATED AT 400 WORDS)

Glucose transport in skeletal muscle membrane vesicles from control and exercised rats

Skeletal muscle responds to exercise by increasing the rate of glucose uptake. Recent studies have indicated that these changes occur via mechanisms modulating the number of transporters in the plasma membrane and/or transporter intrinsic activity. In the present study, a protocol was developed for measuring the initial rate of glucose uptake by rat hindlimb skeletal muscle plasma membrane vesicles. Membranes were isolated from sedentary (control) and acutely exercised rats, and the initial rates of D- and L-glucose influx were assayed under equilibrium exchange conditions to obtain the kinetic constants for carrier-mediated transport. These values were compared with the values for transporter number measured by cytochalasin B binding, and the carrier turnover numbers were calculated. The maximum velocity (Vmax) for carrier-mediated glucose influx was increased 3.7-fold by exercise, from 3.5 nmol.mg protein-1.s-1 for the membranes from control rats to 13 nmol.mg protein-1.s-1 for the membranes from exercised animals. The mean affinity constant (K0.5; approximately 20 mM) was not different between the two groups. The number of transporters in the plasma membrane was increased to a lesser degree, 5.4 to 9.4 pmol/mg protein. As a result, the average carrier turnover number was increased almost twofold by exercise, 719 s-1 in the controls vs. 1,380 s-1 in the exercised rats. These data indicate that the response of glucose transport to exercise involves an increase in the average carrier intrinsic activity as well as a recruitment of transporters to the plasma membrane. Whether the increase in carrier turnover number is due to activation of the transporters or recruitment of a more "active" form of the carrier is unknown.

Effects of insulin on glucose transport and glucose transporters in rat heart

The effect of insulin on glucose transport and glucose transporters was studied in perfused rat heart. Glucose transport was measured by the efflux of labelled 3-O-methylglucose from hearts preloaded with this hexose. Insulin stimulated 3-O-methylglucose transport by: (a) doubling the maximal velocity (Vmax); (b) decreasing the Kd from 6.9 to 2.7 mM; (c) increasing the Hill coefficient toward 3-O-methylglucose from 1.9 to 3.1; (d) increasing the efficiency of the transport process (k constant). Glucose transporters in enriched plasma and microsomal membranes from heart were quantified by the [3H]cytochalasin-B-binding assay. When added to normal hearts, insulin produced the following changes in the glucose transporters: (a) it increased the translocation of transporters from an intracellular pool to the plasma membranes; (b) it increased (from 1.6 to 2.7) the Hill coefficient of the transporters translocated into the plasma membranes toward cytochalasin B, suggesting the existence of a positive co-operativity among the transporters appearing in these membranes; (c) it increased the affinity of the transporters (and hence, possibly, of glucose) for cytochalasin B. The data provide evidence that the stimulatory effect of insulin on glucose transport may be due not to the sole translocation of intracellular glucose transporters to the plasma membrane, but to changes in the functional properties thereof.

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.

Interactions of insulin, catecholamines and adenosine in the regulation of glucose transport in isolated rat cardiac myocytes

The regulation of the glucose transport system by catecholamines and insulin has been studied in isolated rat cardiomyocytes. In the basal state, 1-isoproterenol exhibited a biphasic concentration-dependent regulation of 3-O-methylglucose transport. At low concentrations (less than 10 nM), isoproterenol induced a maximal inhibition of 65-70% of the basal rates, while at higher concentrations (greater than 10 nM) a 25-70% stimulation of transport was observed. In the presence of adenosine deaminase, the inhibition of isoproterenol at low doses was attenuated. No effect of adenosine deaminase was observed on the stimulation of transport at high doses of isoproterenol. The inhibitory effect of isoproterenol returned when N6-phenylisopropyladenosine (a non-metabolizable analog of adenosine) was included along with adenosine deaminase. Dibutyryl cAMP and forskolin both inhibited basal transport rates. In the presence of maximally stimulating concentrations of insulin, cardiomyocyte 3-O-methylglucose transport was generally elevated 200-300% above basal levels. In the presence of isoproterenol, insulin stimulation was inhibited at both high and low concentrations of catecholamine, with maximum inhibition occurring at the lowest concentrations tested. When cells were incubated with both adenosine deaminase and isoproterenol, the inhibition of the insulin response was greater at all concentrations of catecholamine and was almost completely blocked at isoproterenol concentrations of 10 nM or less. Dibutyryl cAMP inhibited the insulin response to within 10% of basal transport levels, while forskolin completely inhibited all transport activity in the presence of insulin. These results suggest that catecholamines regulate basal and insulin-stimulated glucose transport via both cAMP-dependent and cAMP-independent mechanisms and that this regulation is modulated in the presence of extracellular adenosine.

Evidence that insulin and guanosine triphosphate regulate dephosphorylation of the beta-subunit of the insulin receptor in sarcolemma membranes isolated from skeletal muscle

When sarcolemma membranes isolated from rat skeletal muscle were incubated with [gamma-32P]ATP, a membrane protein of apparent Mr 95,000 was rapidly phosphorylated, with the 32P content reaching a maximum within 2 s. On the basis of immunoprecipitation with anti-insulin-receptor antiserum, phosphoamino acid analysis and Mr, this protein probably represents the beta-subunit of the insulin receptor. Similarly, on incubation of the membrane with adenosine 5'-[gamma-[35S]thio] triphosphate the 95 kDa protein was thiophosphorylated, indicating thiophosphorylation of the beta-subunit of the insulin receptor on the basis of immunoprecipitation studies. The effect of insulin on the phosphorylation of this protein in the membrane was studied. Insulin induced a 20% decrease in the 32P labelling of the protein when the membranes were phosphorylated for 10 s. This insulin effect was dose-dependent, with half-maximal effect obtained at 2-3 nM-insulin. Addition of GTP, but not GDP or guanosine 5'-[beta, gamma-imido]triphosphate, enhanced the effect to 35% inhibition, with half-maximal effect of GTP obtained at 0.5 microM. GTP had no effect on the phosphorylation of the protein in the absence of insulin. Analysis of this insulin effect showed that insulin increased the rate of dephosphorylation of the 95 kDa protein in the membrane. In contrast, insulin had no effect on thiophosphorylation of the 95 kDa membrane protein after incubation with adenosine 5'-[gamma-[35S]thio]triphosphate. Since thiophosphorylated proteins are less sensitive to phosphatase action, these investigations suggest that insulin stimulated a protein phosphatase activity in a GTP-dependent manner. The possibility that GTP-regulatory proteins are involved in the action of insulin on the phosphorylation of the insulin receptor and other membrane proteins is discussed.

Na+-sensitive component of 3-O-methylglucose uptake in frog skeletal muscle

A Na+-sensitive uptake of 3-O-methylglucose (3-O-MG), a nonmetabolized sugar, was characterized in frog skeletal muscle. A removal of Na+ from the bathing solution reduced 3-O-MG uptake, depending on the amount of Na+ removed. At a 3-O-MG concentration of 2 mM, the Na+-sensitive component of uptake in Ringer's solution was estimated to be about 26% of the total uptake. The magnitude of Na+-sensitive component sigmoidally increased with an increase of 3-O-MG in bathing solution, whereas in Na+-free Ringer's solution the uptake was proportional to the concentration. The half saturation of the Na+-sensitive component was at a 3-O-MG concentration of about 13 mM, and the Hill coefficient was 1.4 to 1.6. Phlorizin (5 mM), a potent inhibitor specific for Na+-coupled glucose transport, reduced the uptake in a solution containing Na+ to the level in Na+-free Ringer's solution. Glucose of concentrations higher than 20 mM suppressed 3-O-MG uptake to a level slightly lower than that in Na+-free Ringer's solution. These observations indicate that there are Na+-coupled sugar transport systems in frog skeletal muscle which are shared by both glucose and 3-O-MG.

Modulation of uncoupler-induced sugar uptake in isolated adult rat heart cells by isoproterenol

When cells (2.4 mg/ml) in the presence of glucose were exposed to 0.15 microM p-trifluoromethoxyphenylhydrazone (FCCP), the time until 50% of the rod-shaped cells had undergone contracture was more than twice as long for cells without isoproterenol as for cells with isoproterenol. The cause of this large effect was revealed in experiments without glucose where 3-O-methylglucose entry, ATP levels, and cellular configuration were measured simultaneously. It was found that the onset of contracture was almost coincident with the decline in total measured ATP, suggesting that, in any cell, contracture was accompanied by a sudden and total ATP loss. In control cells, FCCP stimulated 3-O-methylglucose entry at or before the time this ATP catastrophe occurred. In cells exposed to isoproterenol, however, the stimulation of 3-O-methylglucose entry by FCCP did not occur until after the ATP catastrophe, and the extent of stimulation was reduced. This suggests that, when glucose was present, the FCCP-induced glucose influx was sufficient to significantly delay the onset of contracture in control cells but not in cells treated with isoproterenol. This conclusion was borne out by the observation that the effect of isoproterenol on contracture could be overcome with insulin.

Effect of insulin on intracellular pH in frog skeletal muscle fibers

The effect of insulin on intracellular pH (pHi) and membrane potential was studied in frog semitendinosus muscle fibers, using recessed-tip pH-sensitive glass microelectrodes and conventional 3 M KCl-filled microelectrodes. After a lag period of approximately 20 min, insulin [1 mU/ml, 0.1% bovine serum albumin (BSA)] produced a slow hyperpolarization of 2-5 mV and an alkalinization of 0.05-0.10 pH unit, which were both completed within 1 h and were not reversed by washing in insulin-free solution for 1 h. The effect of insulin on the pHi recovery rate from CO2-induced acidification was examined at various membrane voltages. At normal membrane voltage, insulin (400 mU/ml, no BSA) slightly increased the slow pHi recovery (from 0.01 to 0.04 delta pH/h). In fibers depolarized in 15 mM K to about -50 mV, insulin nearly tripled the recovery rate (from 0.05 to 0.13 delta pH/h). This insulin-induced recovery was abolished by 1 mM amiloride, a Na-H exchange inhibitor. The increased pHi recovery in 15 mM K thus represents an increased Na-H exchange, which may be due to an interaction between insulin and either membrane depolarization, per se, or increased intracellular Ca. In fibers depolarized in 50 mM K to about -25 mV, insulin did not affect recovery (0.28 delta pH/h). This lack of insulin effect might be due to fiber swelling or to the difference in the time course of elevation of intracellular Ca at -25 and -50 mV. These results are consistent with an alkalinizing effect of insulin in frog muscle mediated by Na-H exchange.

Hormones and the barnacle muscle fiber as a preparation

This minireview discusses the use of single barnacle muscle fibers as a model system for studying hormonal actions. The response of barnacle muscle fibers to serotonin, proctolin, octopamine, aldosterone and insulin is described. Recent data relating to the actions of these hormones on other invertebrate and vertebrate preparations is touched upon. The use of the barnacle muscle fiber as a preparation to investigate hormone-stimulated protein phosphorylation is emphasized.

Role of calcium ions in insulin action on hexose transport in L6 muscle cells

It has been proposed that Ca2+ ions mediate the stimulation by insulin of glucose uptake in muscle (Clausen, T., Cell Calcium 1:311-325, 1980). However, absolute measurements of the concentration of cytosolic free Ca2+, [Ca2+]i, during the course of insulin action have not been made. The stimulation of hexose uptake by insulin was studied in an in vitro model system of muscle cells, the L6 cell line. The following evidence suggests that Ca2+ ions are not likely to fulfill the purported role. 1) Insulin in Ca2+-free media induced stimulation of 2-deoxy-D-glucose uptake. 2) Elevation of [Ca2+]i with the ionophore A23187 did not enhance hexose uptake. 3) Insulin action was not diminished when the hormone was added to Ca2+-depleted cells in Ca2+-free media with A23187. 4) Hexose uptake was not affected by a number of agents thought to modify [Ca2+]i including epinephrine, caffeine, 2,4-dinitrophenol, hyperosmolar mannitol, salicylate, vanadate, veratrine, and trypsin. 5) Direct determinations of [Ca2+]i by fluorescence of the novel indicator Quin-2 did not show differences between basal and insulin-stimulated cells; under identical conditions hexose uptake was stimulated by the hormone. 6) Chelation of [Ca2+]i with Quin-2 in Ca2+-free media did not affect the response to insulin. 7) Low concentrations of trypsin (7.5 micrograms/ml) elevated [Ca2+]i but did not increase the rate of hexose uptake.

A paired-tracer dilution method for characterizing membrane transport in the perfused rat hindlimb. Effects of insulin, feeding and fasting on the kinetics of sugar transport

We have applied the paired-tracer dilution method to the study of transport processes in a mixed mammalian muscle preparation, the perfused rat hindlimb. The method is suitable for the characterization of the kinetic parameters of sugar and amino acid transport and its regulation by hormones, contractile activity, hypoxia, etc. Insulin stimulates sugar transport by increasing the Vmax. of the process 2-3 fold, but its affinity is unchanged. Starvation increases the affinity of sugar transport in perfused skeletal muscle.

Sugar transport in giant barnacle muscle fibres

The kinetics of 3-O-methylglucose transport in the giant muscle cells of Balanus nubilus have been studied both in intact fibres and in fibres subjected to intracellular solute control using internal dialysis. 3-O-methylglucose is not metabolized by barnacle muscle and at equilibrium the 3-O-methylglucose space of the tissue does not differ significantly from the water content of the muscle. These results indicate that 3-O-methylglucose transfer in barnacle muscle is mediated by a passive process. 3-O-methylglucose transport is facilitated by a saturable, symmetric transfer mechanism inhibited by cis but not trans sugars and by low concentrations of phloretin and cytochalasin B. The kinetic constants for uptake and exit are identical. These features indicate that sugar transport in barnacle muscle is mediated by a limited number of membrane transport sites. The number of sugar-displaceable cytochalasin B binding sites in barnacle muscle is 3 X 10(13) cm-2. Indirect kinetic estimates indicate that the number of sugar transport sites is in the order of 1.6 X 10(12) cm-2. This passive, facilitated, selective, saturable transport system is consistent with both symmetric mobile carrier (one-site) and symmetric simultaneous carrier (two-site) models for transport.

Asymmetric or symmetric? Cytosolic modulation of human erythrocyte hexose transfer

(1) The Michaelis-Menten parameters for hexose transfer in erythrocytes, erythrocyte ghosts and inside-out vesicles at 20 degrees C were determined using the light scattering method of Sen and Widdas ((1962) J. Physiol. 160, 392-403). (2) The external Km for infinite-cis exit of D-glucose in cells and ghosts is 3.6 +/- 0.5 mM. (3) Dilution of cellular solute (up to X 90 dilution) by lysing and resealing cells in varying volumes of lysate is without effect on the Vm for net D-glucose exit. The Km for net exit, however, falls from 32.4 +/- 3.7 mM in intact cells to 12.9 +/- 2.3 mM in ghosts. This effect is reversible. (4) Infinite-cis net D-glucose uptake measurements in cells and ghosts reveal the presence of a low Km, high affinity internal site of 5.9 +/- 0.8 mM. The Vm for net glucose entry increases from 23.2 +/- 3.7 mmol/1 per min in intact cells to 55.4 +/- 6.3 mmol/l per min in ghosts. (5) The external Km for infinite-cis D-glucose exit in inside-out vesicles is 6.8 +/- 2.7 mM. The kinetics of zero-trans D-glucose exit from inside-out vesicles are changed markedly when cellular solute (obtained by lysis of intact cells) is applied to either surface of inside-out vesicles. When solute is present externally, the Km and Vmax for zero-trans exit are decreased by up to 10-fold. When solute is present at the interior of inside-out vesicles, Vmax for zero-trans exit is reduced; Km for exit is unaffected. In the nominal absence of cell solute, transfer is symmetric in inside-out vesicles. The orientation of transporter in the bilayer is unaffected by the vesiculation procedure. (6) External application of cellular solute to ghosts reduces Vmax for D-glucose exit but is without effect on the external Km for infinite-cis exit. (7) The inhibitory potency of cell lysate on hexose transfer is lost following dialysis indicating that the factors responsible for transfer modulation are low molecular weight species. (8) We consider the hexose transfer in human erythrocytes is intrinsically symmetric and that asymmetry of transfer is conferred by interaction of the system with low molecular weight cytosolic factors.

Some observations on the behaviour of the sodium efflux in barnacle muscle fibres towards lithium ions

The behaviour of the Na efflux towards Li+ was studied using single barnacle muscle fibres as a preparation. It is found that the Na efflux into Li+-ASW (artificial seawater) is reduced and that this effect is not fully reversed by returning back to Na+-ASW. Preinjection of 100 mM-EGTA reduces the magnitude of the fall of the Na efflux into Li+-ASW. The remaining Na efflux into Li+-ASW is further reduced by external application of 10(-4) M-ouabain. The remaining Na efflux in ouabain-poisoned fibres is reduced by replacing Nae by Li+. However, some fibres show a rise rather than a fall. Fibres loaded with NaCl (by injection) show a prompt and sustained stimulation of the Na efflux when Nae is replaced by Li+. A similar but less pronounced response is often seen with ouabain-poisoned fibres. Injection of LiCl (e.g. a 2 M-solution), causes a 20% fall in Na efflux. Subsequent replacement of Nae by Li+ fails to bring about a fall in the remaining efflux. It is concluded that the Na efflux in these fibres consists of a Na-Na exchange diffusion component which is not mediated by the Na-K pump and that its operation is interrupted by injecting Li+. The relative size of this component is about one-fifth and not one-half of the Na efflux.

Some aspects of sodium efflux from single barnacle muscle fibres

This mini-review attempts to summarize information about the efflux of 22Na from single barnacle muscle fibres, based on the use of the microinjection technique. The view is put forward that the Na efflux consists of three components: an ouabain-sensitive component, an ouabain-insensitive component (representing secondary active transport), and an Na-Na exchange diffusion component. Evidence is brought forward which supports the view that the ouabain-insensitive Na efflux is divisible operationally into 3 phases: (i) the cyclic nucleotide-sensitive phase, (ii) the Cai-sensitive phase, and (iii) the pHe-sensitive phase. It is shown how the barnacle muscle fibre preparation has yielded information about the validity of the cAMP-protein kinase hypothesis and how it can be used to shed some light on the post-translational mechanism of aldosterone action.