Dietary subacute zinc deficiency and potassium metabolism

In a controlled animal experiment the effects of dietary subacute Zn deficiency on growth, Zn concentration, and tissue 42-K distribution were studied. Growth retardation caused lower body weight because both skeletal and heart muscle showed a reduction in cell mass. Zn concentrations were reduced in most tissues, however, they remained unaltered in heart muscle. 42-K activity increased in skeletal muscle and pancreas. We hypothesize the latter reflects the organs rate of metabolism, inducing the exocrine pancreas to increase Zn absorption; in skeletal muscle it may induce also alterations in cell potentiation, causing restless behavior. As suggested by the calculated specific K activity (Bq/mol), the K uptake was highest in liver and bone, high in pancreas and skeletal muscle and low in heart muscle. The latter suggests K retention in heart muscle. Specific activity in plasma and jejunum remained unaltered: K status and absorption seem unaffected. Zn deficiency causes different 42-K activities in the various tissues, that respond by alterations in K metabolism without the induction of K deficiency.     

Potassium depletion increases potassium clearance capacity in skeletal muscles in vivo during acute repletion

Muscular K uptake depends onskeletal muscle Na-K-ATPase concentration and activity. ReducedK uptake is observed in vitro in K-depleted rats. We evaluated skeletalmuscle K clearance capacity in vivo in rats K depleted for 14 days.[³H]ouabain binding, ₁ and₂ Na-K-ATPase isoform abundance, and K, Na, and Mgcontent were measured in skeletal muscles. Skeletal muscle K, Na, andMg and plasma K were measured in relation to intravenous KCl infusionthat continued until animals died, i.e., maximum KCl dose wasadministered. In soleus, extensor digitorum longus (EDL), andgastrocnemius muscles K depletion significantly reduced K content by18%, 15%, and 19%, [³H]ouabain binding by 36%, 41%,and 68%, and ₂ isoform abundance by 34%, 44%, and70%, respectively. No significant change was observed in₁ isoform abundance. In EDL and gastrocnemius muscles Kdepletion significantly increased Na (48% and 59%) and Mg (10% and17%) content, but only tendencies to increase were observed in soleusmuscle. K-depleted rats tolerated up to a fourfold higher KCl dose.This was associated with a reduced rate of increase in plasma K andincreases in soleus, EDL, and gastrocnemius muscle K of 56%, 42%, and41%, respectively, but only tendencies to increase in controls.However, whereas K uptake was highest in K-depleted animals, the Kuptake rate was highest in controls. In vivo K depletion is associatedwith markedly increased K tolerance and K clearance despitesignificantly reduced skeletal muscle Na-K-ATPase concentration. Theconcern of an increased risk for K intoxication during K repletionseems unwarranted.

     

Cation-coupled chloride influx in squid axon. Role of potassium and stoichiometry of the transport process

Evidence is presented showing that the Cl- uptake process in the squid giant axon is tightly coupled not only to Na+ uptake but also to K+ uptake. Thus, removal of external K+ causes both Cl- and Na+ influxes to be reduced, particularly when [Cl-]i is low, that is, under conditions previously shown to be optimal for Cl-/Na+-coupled influx. In addition, there exists a ouabain-insensitive K+ influx, which depends on the presence of external Cl- and Na+, is inversely proportional to [Cl-]i, and is blocked by furosemide/bumetanide. Finally, this ouabain-insensitive K+ influx appears to require the presence of cellular ATP. The stoichiometry of the coupled transport process was measured using a double-labeling technique combining in the same axon either 36Cl and 42K or 22Na and 42K. The stoichiometry of the flux changes occurring in response either to varying [Cl-]i between 150 and 0 mM or to treatment with 0.3 mM furosemide is, in both cases, approximately 3:2:1 (Cl-/Na+/K+). Although these fluxes require ATP, they are not inhibited by 3 mM vanadate. In addition, treatment with DIDS has no effect on the fluxes.     

Sodium selenite attenuated cisplatin-induced toxicity in rats: role of electrolytes homeostasis

Sodium selenite (1 mg/kg body weight, ip) for 10 consecutive days treatment showed marked increase in intra-erythrocytes K+ and plasma Na+ level while slight increase in Na+ K+ ATPase level. No mortality was observed at this dose of sodium selenite. However, sodium selenite pretreatment partially restored the Na+ K+ ATPase and intra-erythorcytes and plasma sodium level, while completely restored the intra-erythrocytes K+ and plasma Mg2+ level. No change was observed in plasma Ca2+ level. Thus sodium selenite successively attenuated the Cisplatin-induced electrolytes alterations and toxicity by exerting the stress response of sodium.     

Decreased sodium-potassium pump activity in isolated hypertrophied feline ventricular myocytes

The activity of the Na-K pump was assessed in normal and hypertrophied isolated feline myocytes by measuring ouabain-sensitive 42-K uptake. Right ventricular hypertrophy was produced in feline myocardium by placing a constricting band around the pulmonary artery of adult cats. High yields of calcium tolerant myocytes were isolated from the right and left ventricle of banded and sham operated animals. Intracellular sodium (Na) and potassium (K) concentrations (mM) were not significantly different (P greater than 0.5) in normal (Na: 13.2; K: 133.4) and hypertrophied (Na: 12.3; K: 127.5) myocytes. Morphometric analysis demonstrated a 26% increase in width and a 42% increase in volume of hypertrophied myocytes, however, the sarcomere length (1.9 mu) was not different in both cell types. The rate constant (k, min-1) describing 42-K uptake and the calculated total K influx (I, pmol/cm2/sec) were not significantly different (P greater than 0.5) in normal (k = 0.059; I = 15.9) and hypertrophied (k = 0.062; I = 15.3) cells. Ouabain-sensitive (active) K influx, a measure of Na-K pump activity, was maximally inhibited at 10(-4)M ouabain in both cell types. At this concentration, ouabain-sensitive K uptake was decreased 23.5% in hypertrophied myocytes compared to control. The decrease in active K influx may be due to a decrease in the activity of the Na-K ATPase and/or to a reduction in the passive movement of sodium and potassium down their electrochemical gradients.     

Changes in the muscle ultrastructure and electrolyte composition of rats in embryogenesis. II. X-ray spectrum microanalysis of the intracellular concentrations of potassium, sodium and phosphorus in skeletal and heart muscle

Concentrations of K, P and Na were determined in skeletal and cardiac muscle cells of rat embryos. High K and P levels--133 and 166 mmole/kg wet weight, resp.,--were found in skeletal muscles of 13 day old embryos, the concentration of Na in these cells being 81 mmole/kg w. w. On the 18th day of development, K and P in skeletal muscle cells decreased down to 79 and 118 mmole/kg w. w., resp., while the concentration of Na increased to 165 mmole/kg w. w. In 19 day old embryos, the concentrations of K and P increased, although they did not reach the level typical of skeletal muscles of adult rats. The concentrations of K and P in cardiac muscle cells of 13 day embryos were found equal to 100 and 108 mmole/kg w. w., resp., on the 19th day of development these concentrations reached the level typical of the cardiac muscle cells of adult rats.     

Acute alterations in sodium flux in vitro lead to decreased myofibrillar protein breakdown in rat skeletal muscle.

Myofibrillar protein breakdown was evaluated by measuring the release of N tau-methylhistidine by isolated rat skeletal muscles or perfused rat muscles in the presence of a variety of agents known to affect Na+ flux. Total cell proteolysis was evaluated simultaneously by measuring tyrosine release by muscles after the inhibition of protein synthesis with cycloheximide. Treatment of muscles with the Na+ ionophore monensin or inhibitors of Na+-K+ ATPase (ouabain, digoxin or vanadate) decreased N tau-methylhistidine release by muscles by 21-35%. A phorbol ester (phorbol 12-myristate 13-acetate) as well as a synthetic diacylglycerol known to activate protein kinase C and a Na+/H+ antiport also decreased N tau-methylhistidine release by muscles. Removal of extracellular Na+ blocked the ability of these agents to attenuate N tau-methylhistidine release by muscles, suggesting that their effectiveness required a change in Na+ flux. In contrast with N tau-methylhistidine release by muscles, these agents, except for monensin, did not effect the release of tyrosine, suggesting that they attenuate specifically the breakdown of myofibrillar proteins. Overall these results indicate a link between Na+ and the regulation of protein breakdown in rat skeletal muscle, whereby an influx of Na+ can result in a decrease in myofibrillar proteolysis. Left unresolved is whether phospholipid hydrolysis is involved in this scheme.     

Role of skeletal muscle Na+-K+ ATPase activity in increased lactate production in sub-acute sepsis

Bacterial sepsis is frequently accompanied by increased blood concentration of lactic acid, which traditionally is attributed to poor tissue perfusion, hypoxia and anaerobic glycolysis. Therapy aimed at improving oxygen delivery to tissues often does not correct the hyperlactatemia, suggesting that high blood lactate in sepsis is not due to hypoxia. Various tissues, including skeletal muscle, demonstrate increased lactate production under well-oxygenated conditions when the activity of the Na+-K+ ATPase is stimulated. Although both muscle Na+-K+ ATPase activity and muscle plasma membrane content of Na+, K+-ATPase subunits are increased in sepsis, no studies in vivo have demonstrated correlation between lactate production and changes in intracellular Na+ and K+ resulting from increased Na+-K+ pump activity in sepsis. Plasma concentrations of lactate and epinephrine, a known stimulator of the Na+-K+ pump, were increased in rats made septic by E. coli injection. Muscle lactate content was significantly increased in septic rats, although muscle ATP and phosphocreatine remained normal, suggesting oxygen delivery remained adequate for mitochondrial energy metabolism. In septic rats, muscle intracellular ratio of Na+:K+ was significantly reduced, indicating increased Na+-K+ pump activity. These data thus demonstrate that increased muscle lactate during sepsis correlates with evidence of elevated muscle Na+-K+ ATPase activity, but not with evidence of impaired oxidative metabolism. This study also further supports a role for epinephrine in this process.     

Enhancement (by ATP, insulin, and lack of divalent cations) of ouabain inhibition of cation transport and ouabain binding in frog skeletal muscle; effect of insulin and ouabain on sarcolemmal (Na + K)MgATPase.

Using small, intact frog muscles, the basic properties of Na+ and K+ transport were shown to resemble those of the (Na+ + K+)Mg2+ATPase (EC 3.6.1.3) isolated from skeletal muscle. (a) External K+ is essential for Na+ exit and K+ entry after the muscles are Na+-loaded and K+-depleted; (b) the ouabain concentration causing maximum inhibition of recovery is the same for transport as for the inhibition of the isolated enzyme. Ouabain causes a decrease in the sorbitol space and causes muscle fibre swelling. Absence of Ca2+ and Mg2+ inhibits recovery of normal Na+ and K+ concentrations and increases the sorbitol space. Insulin stimulates K+ uptake and Na+ loss in intact muscles but has no effect on the isolated sarcolemmal (Na+ + K+)Mg2+ATPase. Absence of divalent cations, addition of external ATP and of insulin enhance the ouabain inhibition of recovery. Bound ouabain was measured using [3H]ouabain and [14C]sorbitol (to measure the extracellular space). The process of binding was slowly reversible and was saturable within a range of ouabain concentrations from 1.48 X 10(-7) to 5.96 X 10(-7) M. From the nonexchangeable ouabain bound, the density of glycoside receptors was estimated to be 650 molecules per square micrometre of membrane surface. The absence of divalent cations, addition of external ATP and of insulin significantly enhanced the amount of ouabain bound. Substitution of Na+ and K+ by choline greatly reduced the bound ouabain.     

Myo1c regulates glucose uptake in mouse skeletal muscle

Contraction and insulin promote glucose uptake in skeletal muscle through GLUT4 translocation to cell surface membranes. Although the signaling mechanisms leading to GLUT4 translocation have been extensively studied in muscle, the cellular transport machinery is poorly understood. Myo1c is an actin-based motor protein implicated in GLUT4 translocation in adipocytes; however, the expression profile and role of Myo1c in skeletal muscle have not been investigated. Myo1c protein abundance was higher in more oxidative skeletal muscles and heart. Voluntary wheel exercise (4 weeks, 8.2 ± 0.8 km/day), which increased the oxidative profile of the triceps muscle, significantly increased Myo1c protein levels by ～2-fold versus sedentary controls. In contrast, high fat feeding (9 weeks, 60% fat) significantly reduced Myo1c by 17% in tibialis anterior muscle. To study Myo1c regulation of glucose uptake, we expressed wild-type Myo1c or Myo1c mutated at the ATPase catalytic site (K111A-Myo1c) in mouse tibialis anterior muscles in vivo and assessed glucose uptake in vivo in the basal state, in response to 15 min of in situ contraction, and 15 min following maximal insulin injection (16.6 units/kg of body weight). Expression of wild-type Myo1c or K111A-Myo1c had no effect on basal glucose uptake. However, expression of wild-type Myo1c significantly increased contraction- and insulin-stimulated glucose uptake, whereas expression of K111A-Myo1c decreased both contraction-stimulated and insulin-stimulated glucose uptake. Neither wild-type nor K111A-Myo1c expression altered GLUT4 expression, and neither affected contraction- or insulin-stimulated signaling proteins. Myo1c is a novel mediator of both insulin-stimulated and contraction-stimulated glucose uptake in skeletal muscle.     

Effect of Lithium and of Other Drugs Used in the Treatment of Manic Illness on the Cation-Transporting Properties of Na+, K+-ATPase in Mouse Brain Synaptosomes

We have developed and used a novel technique to investigate the effects of lithium and other psychotropic drugs on the cation-transporting properties of the sodium- and potassium-activated ATPase enzyme (Na+,K+-ATPase) in intact synaptosomes. Rubidium-86 uptake into intact synaptosomes is an active process and is inhibited by approximately 75% in the presence of the Na+,K+-ATPase inhibitor acetylstrophanthidin. In vitro addition of lithium to synaptosomes prepared from untreated mice causes a progressive inhibition of acetylstrophanthidin-sensitive 86Rb uptake, but only at concentrations higher than the clinical therapeutic range. However, pretreatment of mice for 14 days in vivo with lithium, carbamazepine, and haloperidol, but not phenytoin, causes a significant stimulation of 86Rb uptake into synaptosomes via Na+,K+-ATPase.     

K+ transport in resting rat hind-limb skeletal muscle in response to paraxanthine, a caffeine metabolite

This study tested the hypothesis that paraxanthine, a caffeine metabolite, stimulates skeletal muscle potassium (K+) transport by an increase in Na+ -K+ ATPase activity. The unidirectional transport of K+ into muscle (J(in)K) was studied using a perfused rat hind limb technique. Using 12 hind limbs, we examined the response to 20 min of paraxanthine perfusion (0.1 mM), followed by 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 5) to irreversibly inhibit Na+ -K+ ATPase activity. Paraxanthine stimulated J(in)K by 23+/-5% within 20 min. Ouabain abolished the paraxanthine-induced stimulation of J(in)K, suggesting the increase in K+ uptake was due to activation of the Na+ -K+ ATPase. To confirm the role of the Na+ -K+ ATPase, 14 hind limbs were perfused for 20 min with 5 mM ouabain prior to 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 6). Ouabain alone resulted in a 41+/-7% decrease in J(in)K within 15 min. Inhibition of ouabain-sensitive J(in)K prevented the paraxanthine-induced increase in J(in)K. Hind limbs (n = 3) were also perfused with 0.1 mM paraxanthine for 60 min to examine the response to longer duration paraxanthine perfusion. The paraxanthine-induced increase in J(in)K continued for the entire 60 min. In another series, hind limbs were perfused with 0.01 (n = 9), 0.1 (n = 9), or 0.5 (n = 6) mM paraxanthine for 15 min. There was no concentration-dependent relationship between J(in)K and paraxanthine concentration, and 0.01, 0.1, and 0.5 mM paraxanthine increased J(in)K similarly (25+/-5, 22+/-4, and 27+/-6%, respectively). The effect of paraxanthine on J(in)K could not be reversed by subsequent perfusion with paraxanthine-free perfusate. Caffeine (0.05-1.0 mM) had no effect on K+ transport. It is concluded that paraxanthine increases J(in)K in resting skeletal muscle by stimulating ouabain-sensitive Na+ -K+ ATPase activity.     

ERK1/2 mediates insulin stimulation of Na(+),K(+)-ATPase by phosphorylation of the alpha-subunit in human skeletal muscle cells

Insulin stimulates Na(+),K(+)-ATPase activity and induces translocation of Na(+),K(+)-ATPase molecules to the plasma membrane in skeletal muscle. We determined the molecular mechanism by which insulin regulates Na(+),K(+)-ATPase in differentiated primary human skeletal muscle cells (HSMCs). Insulin action on Na(+),K(+)-ATPase was dependent on ERK1/2 in HSMCs. Sequence analysis of Na(+),K(+)-ATPase alpha-subunits revealed several potential ERK phosphorylation sites. Insulin increased ouabain-sensitive (86)Rb(+) uptake and [(3)H]ouabain binding in intact cells. Insulin also increased phosphorylation and plasma membrane content of the Na(+),K(+)-ATPase alpha(1)- and alpha(2)-subunits. Insulin-stimulated Na(+),K(+)-ATPase activation, phosphorylation, and translocation of alpha-subunits to the plasma membrane were abolished by 20 microm PD98059, which is an inhibitor of MEK1/2, an upstream kinase of ERK1/2. Furthermore, inhibitors of phosphatidylinositol 3-kinase (100 nm wortmannin) and protein kinase C (10 microm GF109203X) had similar effects. Notably, insulin-stimulated ERK1/2 phosphorylation was abolished by wortmannin and GF109203X in HSMCs. Insulin also stimulated phosphorylation of alpha(1)- and alpha(2)-subunits on Thr-Pro amino acid motifs, which form specific ERK substrates. Furthermore, recombinant ERK1 and -2 kinases were able to phosphorylate alpha-subunit of purified human Na(+),K(+)-ATPase in vitro. In conclusion, insulin stimulates Na(+),K(+)-ATPase activity and translocation to plasma membrane in HSMCs via phosphorylation of the alpha-subunits by ERK1/2 mitogen-activated protein kinase.     

Adenosine triphosphatases during maturation of cultured human skeletal muscle cells and in adult human muscle.

Na+/K(+)-ATPase, Mg(2+)-ATPase and sarcoplasmic reticulum (SR) Ca(2+)-ATPase are examined in cultured human skeletal muscle cells of different maturation grade and in human skeletal muscle. Na+/K(+)-ATPase is investigated by measuring ouabain binding and the activities of Na+/K(+)-ATPase and K(+)-dependent 3-O-methylfluorescein phosphatase (3-O-MFPase). SR Ca(2+)-ATPase is examined by ELISA, Ca(2+)-dependent phosphorylation and its activities on ATP and 3-O-methylfluorescein phosphate. Na+/K(+)-ATPase and SR Ca(2+)-ATPase are localized by immunocytochemistry. The activities of Na+/K(+)-ATPase and SR Ca(2+)-ATPase show a good correlation with the other assayed parameters of these ion pumps. All ATPase parameters investigated increase with the maturation grade of the cultured muscle cells. The number of ouabain-binding sites and the activities of Na+/K(+)-ATPase and K(+)-dependent 3-O-MFPase are significantly higher in cultured muscle cells than in muscle. The Mg(2+)-ATPase activity, the content of SR Ca(2+)-ATPase and the activities of SR Ca(2+)-ATPase and Ca(2+)-dependent 3-O-MFPase remain significantly lower in cultured cells than in muscle. The ouabain-binding constant and the molecular activities of Na+/K(+)-ATPase and SR Ca(2+)-ATPase are equal in muscle and cultured cells. During ageing of human muscle the activity as well as the concentration of SR Ca(2+)-ATPase decrease. Thus the changes of the activities of the ATPases are caused by variations of the number of their molecules. Na+/K(+)-ATPase is localized in the periphery of fast- and slow-twitch muscle fibers and at the sarcomeric I-band. SR Ca(2+)-ATPase is predominantly confined to the I-band, whereas fast-twitch fibers are much more immunoreactive than slow-twitch fibers. The presence of cross-striation for Na+/K(+)-ATPase and SR Ca(2+)-ATPase in highly matured cultured muscle cells indicate the development and subcellular organization of a transverse tubular system and SR, respectively, which resembles the in vivo situation.     

Inhibition of Na+/Ca2+ exchanger by peroxynitrite in microsomes of pulmonary smooth muscle: role of matrix metalloproteinase-2

Treatment of bovine pulmonary artery smooth muscle microsomes with peroxynitrite (ONOO-) (100 microM) markedly stimulated matrix metalloproteinase-2 (MMP-2) activity and also enhanced Ca2+ATPase activity and ATP-dependent Ca2+ uptake. Pretreatment of the microsomes with vitamin E (1 mM) and TIMP-2 (50 microg/ml) preserved the increase in MMP-2 activity, Ca2+ATPase activity and also ATP-dependent Ca2+ uptake in the microsomes. In contrast, Na(+)-dependent Ca2+ uptake in the microsomes was inhibited by ONOO- and this was found to be reversed by vitamin E (1 mM) and TIMP-2 (50 microg/ml). However, changes caused by ONOO- in MMP-2 activity, ATP-dependent Ca2+ uptake and Na(+)-dependent Ca2+ uptake were not reversed upon pretreatment of the microsomes with a low concentration of 5 microg/ml of TIMP-2 which, on the contrary, reversed MMP-2 (1 microg/ml)-mediated alteration on these parameters. The inhibition of Na(+)-dependent Ca2+ uptake by ONOO- and MMP-2 overpowered the stimulation of ATP-dependent Ca2+ uptake in the microsomes. Treatment with ONOO- abolished the inhibitory effect of TIMP-2 (5 microg/ml) on MMP-2 (1 microg/ml) causing 14C-gelatin degradation. Overall, the present study suggests that ONOO- inactivated TIMP-2, the ambient inhibitor of MMP-2, leading to activation of the ambient proteinase, MMP-2, and subsequently stimulated Ca2+ATPase activity and ATP-dependent Ca2+ uptake, but inhibited Na(+)-dependent Ca2+ uptake, resulting in a marked decrease in Ca2+ uptake in microsomes of bovine pulmonary artery smooth muscle.     

Increase of (Na+ + K+)-activated ATPase activity of basolateral plasma membranes from intestinal mucosa of diabetic rats

A significant increase of the (Na+ + K+)-activated ATPase was found in mucosal homogenates of rat small intestine under conditions of alloxan and streptozotocin diabetes. From studies with isolated plasma membranes it has been shown that the activity changes were caused by that part of the (Na+ + K+)-activated ATPase only which is localized in the basolateral plasma membranes, whereas the enzyme activity in the brush border region remains unchanged. In connection with the enhanced capacity of ion, nonelectrolyte and water absorption in experimental diabetes, our findings support a concept of intestinal transport mechanism which suggest that the basolateral part of the (Na+ + K+)-activated ATPase is responsible for metabolic energy supply. The luminal part of the enzyme may be involved in regulation of passive Na+ influx.     

In vivo monitoring of norepinephrine and its metabolites in skeletal muscle

Although skeletal muscle sympathetic nerve activity plays an important role in the regulation of vascular tone and glucose metabolism, relatively little is known about regional norepinephrine (NE) kinetics in the skeletal muscle. With use of the dialysis technique, we implanted dialysis probes in the adductor muscle of anesthetized rabbits and examined whether dialysate NE and its metabolites were influenced by local administration of pharmacological agents through the dialysis probes. Dialysate dihydroxyphenylglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG) were measured as two major metabolites of NE. The skeletal muscle dialysate NE, DHPG and MHPG were 11.7+/-1.2, 38.1+/-3.2, and 266.1+/-28.7 pg/ml, respectively. Basal dialysate NE levels were suppressed by tetrodotoxin (Na(+) channel blocker, 10 microM) (5.1+/-0.6 pg/ml), and augmented by desipramine (NE uptake blocker, 100 microM) (25.8+/-3.2 pg/ml). Basal dialysate DHPG levels were suppressed by pargyline (monoamine oxidase blocker, 1mM) (24.3+/-4.6 pg/ml) and augmented by reserpine (vesicle NE transport blocker, 10 microM) (75.8+/-2.7 pg/ml). Basal dialysate MHPG levels were not affected by pargyline, reserpine, or desipramine. Addition of tyramine (sympathomimetic amine, 600 microM), KCl (100 mM), and ouabain (Na(+)-K(+) ATPase blocker, 100 microM) caused brisk increases in dialysate NE levels (200.9+/-14.2, 90.6+/-25.7, 285.3+/-46.8 pg/ml, respectively). Furthermore, increases in basal dialysate NE levels were correlated with locally administered desipramine (10, 100 microM). Thus, dialysate NE and its metabolite were affected by local administration of pharmacological agents that modified sympathetic nerve endings function in the skeletal muscle. Skeletal muscle microdialysis with local administration of a pharmacological agent provides information about NE release, uptake, vesicle uptake and degradation at skeletal muscle sympathetic nerve endings.     

Cation regulation in the smooth muscle of frog stomach

Cation composition of frog smooth muscle cells was investigated. Fresh stomach muscle rings resembled skeletal muscle, but marked Na gain and K loss followed immersion. Mean Na (49.8–79.7 mM/kg tissue) and K (61.8–80.1 mM/kg tissue) varied between batches, but were stable for long periods in vitro. Exchange of 6–30 mM Na/kg tissue with ²²Na was extremely slow and distinct. Extracellular water was estimated from sucrose-¹⁴C uptake. Calculated exchangeable intracellular Na was 9 mM/kg cell water, and varied little. Thus steady-state transmembrane cation gradients appeared to be steep. K-free solution had only slight effects. Ouabain (10^-4 M) caused marked Na gain and reciprocal K loss; at 30°C, Na and K varied linearly with time over a wide range of contents, indicating constant net fluxes. Net fluxes decreased with temperature decrease. ²²Na exchange in ouabain-treated tissue at 20–30°C was rapid and difficult to analyze. The best minimum estimates of unidirectional Na fluxes at 30°C were 10–12 times the constant net flux; constant pump efflux may explain these findings. The rapidity of Na exchange may not reflect very high permeability, but it does require a high rate of transport work.     

Increased flow rate and papaverine increase K+ exchange in perfused rat hind-limb skeletal muscle.

This study tested the hypothesis that increases in perfusate flow rate result in increased rates of unidirectional and net K+ transport in rat hind-limb skeletal muscle at rest. Ten neurally and vascularly isolated hind limbs, with arterial and venous catheters placed proximal to the popliteal region, were perfused for 10-min periods at flow rates (presented in a random order) of 0.27, 0.42, 0.63, 0.84, or 1.05 mL x min(-1) x g(-1). Potassium extraction and unidirectional K+ influx were determined using 42K, and arterial perfusion pressure was measured continuously. Increases in flow rate resulted in decreases in K+ extraction and increases in unidirectional K+ influx, unidirectional K+ efflux, and net K+ efflux. The increases in K+ flux were associated with increases in oxygen uptake, glucose uptake, and lactate release. In separate experiments (n = 5), the vasodilator papaverine (10(-4) M) did not further vasodilate the vasculature of resting hind limbs, suggesting that the hind limbs in this preparation were fully vasodilated. Papaverine, at constant flow, resulted in a nearly 1.5-fold increase in K+ extraction, a doubling of unidirectional K+ influx, and increases in unidirectional K+ efflux and net K+ efflux. It is concluded that physiological increases in flow rate result in increases in K+ transport in isolated, perfused rat hind-limb skeletal muscle. Furthermore, papaverine appeared to induce an increase in skeletal muscle membrane permeability to K+.     

Changes in erythrocyte sodium, sodium transport and 3H ouabain binding capacity during digoxin administration in the pig

Time course of the changes in erythrocyte sodium content, sodium transport, 3H ouabain binding capacity and Na+, K+-ATPase activity were measured for 14 weeks, in 6 young pigs treated with digoxin and in 6 control pigs. After one week of treatment the erythrocyte sodium content increased from 5.4 mmol/kg cells to 6.9 mmol/kg cells and the efflux rate constant of sodium decreased. With prolonged treatment the erythrocyte sodium content returned to normal and the 3H ouabain binding capacity increased by week 5. The plasma digoxin concentration decreased from 1.1 ng/ml at week 5 to 0.6 ng/ml at week 8 probably due to the decline in dose (microgram/kg) of digoxin with age. The efflux rate constant of sodium and Na+, K+-ATPase activity were higher towards the end of treatment. It is concluded that with prolonged administration of digoxin there is an increase in erythrocyte sodium pump units.