Intracellular sodium modulates the state of protein kinase C phosphorylation of rat proximal tubule Na+,K+‐ATPase

The natriuretic hormone dopamine and the antinatriuretic hormone noradrenaline, acting on α‐adrenergic receptors, have been shown to bidirectionally modulate the activity of renal tubular Na⁺,K⁺‐adenosine triphosphate (ATPase). Here we have examined whether intracellular sodium concentration influences the effects of these bidirectional forces on the state of phosphorylation of Na⁺,K⁺‐ATPase. Proximal tubules dissected from rat kidney were incubated with dopamine or the α‐adrenergic agonist, oxymetazoline, and transiently permeabilized in a medium where sodium concentration ranged between 5 and 70 mM . The variations of sodium concentration in the medium had a proportional effect on intracellular sodium. Dopamine and protein kinase C (PKC) phosphorylate the catalytic subunit of rat Na⁺,K⁺‐ATPase on the Ser23 residue. The level of PKC induced Na⁺,K⁺‐ATPase phosphorylation was determined using an antibody that only recognizes Na⁺,K⁺‐ATPase, which is not phosphorylated on its PKC site. Under basal conditions Na⁺,K⁺‐ATPase was predominantly in its phosphorylated state. When intracellular sodium was increased, Na⁺,K⁺‐ATPase was predominantly in its dephosphorylated state. Phosphorylation of Na⁺,K⁺‐ATPase by dopamine was most pronounced when intracellular sodium was high, and dephosphorylation by oxymetazoline was most pronounced when intracellular sodium was low. The oxymetazoline effect was mimicked by the calcium ionophore A23187. An inhibitor of the calcium‐dependent protein phosphatase, calcineurin, increased the state of Na⁺,K⁺‐ATPase phosphorylation. The results imply that phosphorylation of renal Na⁺,K⁺‐ATPase activity is modulated by the level of intracellular sodium and that this effect involves PKC and calcium signalling pathways. The findings may have implication for the regulation of salt excretion and sodium homeostasis.     

Na^＋，K^＋—ATPase研究进展

Na+ ,K+ ATPase广泛分布于多种细胞的细胞膜上 ,是维持细胞内外Na+ ,K+ 浓度梯度的关键酶。Na+ ,K+ ATPase由α,β和γ亚基组成 ,在不同组织 ,不同发育阶段表达不同的亚型。激素等通过蛋白激酶A、蛋白激酶C、酪氨酸激酶等调节Na+ ,K+ ATPase的活性。小的跨膜蛋白结合Na+ ,K+ ATPase的特异亚基 ,调节其活性。Na+ ,K+ ATPase不仅参与离子转运、蛋白转运、维持离子自稳平衡 ,而且在脊椎动物胚胎发育、神经元导向、中枢神经系统发育、细胞形态维持、细胞粘附等方面发挥作用 ,甚至可作为喹巴因的受体 ,参与信号传递。     

Gentamicin inhibition of Na+, K+-ATPase in rat kidney cells

Na,K+-ATPase activity is decreased in homogenized renal tissue from GM-treated rats. This study examines whether the site of the active effect of GM on Na,K-ATPase activity in the kidney can be localized to the proximal convoluted tubules (PCT) where the drug is taken up and where it will produce necrosis. In rats treated with gentamicin (50 μg. kg^-1.day^-1 i.m.) for 7 days, PCT Na,K-ATPase activity was reduced as compared to vehicle-treated rats but returned to control levels 7 days after treatment withdrawal. In another nephron segment, the medullary thick ascending limb of Henle (mTAL), where GM induced lesions are uncommon, Na,K-ATPase activity was the same in GM- and vehicle-treated rats treatment. To study the in vitro effect of GM, dissected PCT and mTAL segments from untreated rats were preincubated for 30 min with GM 10^-3m , a dose similar to the tissue concentration in chronically treated rats. In tubule segments that were permeabilized to allow the drug to enter the cells, GM 10^-3m significantly inhibited Na,K-ATPase activity both in PCT and mTAL. In non-permeabilized mTAL segments GM did not inhibit Na,K-ATPase activity. GM inhibition of Na,K-ATPase activity in permeabilized PCT segments persisted after the tubules were rinsed in GM free medium. GM does not inhibit Na,K-ATPase partly purified from the renal cortex. Conclusion. Gentamicin inhibits Na,K-ATPase activity in renal tubule cells when it has access to the cytoplasm. Treatment with GM will therefore cause a selective inhibition of Na,K-ATPase in the proximal tubule cells.     

Amiloride-inhibitable Na+ conductance in rat proximal tubule

 Previous single-channel recordings from the luminal membrane of the rabbit proximal tubule have revealed amiloride-inhibitable Na⁺ channels of a characteristic conductance range. The present study aimed to pursue this issue in rat proximal tubule. Control rats were compared to those put on a low-Na⁺ diet or pretreated by triamcinolone injections (s.c.). Stimulation of Na⁺ absorption by glucocorticoids was verified by examining the transepithelial voltage in Ussing chamber studies of the distal colon. The membrane voltage (V _m) of isolated, in-vitro-perfused proximal tubule segments was measured in patch-clamp and impalement studies. It was found that amiloride hyperpolarized V _m significantly by 2.1 ± 0.9 mV (n = 26) in tubules of control rats, by 3.9 ± 0.7 mV (n = 12) in rats put on a low-Na⁺ diet and by 3.7 ± 1.0 mV (n = 17) in rats treated with glucocorticoids. The effect of amiloride was concentration dependent with a half-maximal effect at < 1 μmol/l. RT-PCR techniques were used to search for the presence of the α-, β- and γ-subunits of the epithelial Na⁺ channel in isolated oximal tubule segments. The presence of the respective mRNAs was verified. These data indicate that: (1) amiloride-inhibitable Na⁺ channels are present in rat proximal tubules; (2) the Na⁺ conductance may be upregulated by Na⁺ deprivation but is still very limited when compared to total cell conductance; (3) therefore, the contribution of Na⁺-channel-mediated absorption to total proximal Na⁺ absorption is probably small. Received: 5 August 1996 / Received after revision: 22 January 1997 / Accepted: 28 January 1997     

Age differences in hormonal regulation of Na+, K+-ATPase activity in the rat renal cortex

Laboratory of Physiological Genetics, Institute of Cytology and Genetics, Siberian Brach, Russian Academy of Sciences, Novosibirsk. (Presented by Academician of the Russian Academy of Medical SciencesV. P. Lozov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 114, No. 8, pp. 150–153, August, 1992.     

Protein kinase C activity in rat renal proximal tubule cells

The presence of protein kinase C (PKC) in proximal tubule cells of the rat kidney is established by means of immunodetection and by the demonstration of calcium-and phospholipid-dependent, staurosporine-inhibitable histone phosphorylation. The calcium-dependence of renal PKC is described. Maximal activation of the enzyme (178.2 and 258.8 pmol P₁ mg^-1 min^-1 for cytosol and membrane respectively) was achieved with 5 μM of Ca²⁺. Phorbol 12,13 dibutyrate (PDBu) translocated PKC from cytosol to membrane in a dose- and time-dependent fashion, while 4α-phorbol 12,13-didecanoate produced no significant effect on translocation. Cytosolic PKC activity was compared in immature and mature tissues (10- and 40-day-old kidneys). Basal activity was found to be significantly higher (P < 0.05) in immature cells (272.8 vs. 157.5 pmol P, mg^-1 min^-1). PDBu at 10^-6 M for 15 min reduced immunoreactivity in the soluble fraction of both groups, which was accompanied by a significant decrease in kinase activity. We speculate that the high PKC activity in the infant kidney plays a role in cell growth.     

Substructure of membrane-bound Na+−K+-ATPase protein

Purified membrane-bound Na⁺–K⁺-ATPase from rat kidney outer medulla was studied by freeze-fracturing, by freeze-etching and by negative staining. Freeze-fracturing of purified Na⁺–K⁺-ATPase membranes shows intramembraneous particles with a diameter of about 100 Å. The frequency of these intramembraneous particles — as estimated from the particle densities on the two fracture faces — lies between 4700 and 5600 particles per m². Applying rotary shadowing a four partite substructure could be detected in these intramembraneous particles observed on the fracture planes. The same four partite substructure was detected in particles observed on freeze-fractured and rotary shadowed intact baso-lateral plasma membranes of the thick ascending limb of Henle's loop. Particles could be also detected on both membrane surfaces of the purified Na⁺–K⁺-ATPase. These surface particles have about the same diameter and are present at about the same frequency as those observed within the freeze-fractured membranes. Negative staining of isolated Na⁺–K⁺-ATPase membranes showed particles on both membrane surfaces with a diameter between 30 and 50 Å, at a frequency of about 19,000 per m². On aspects of membrane edges we observed structures which suggest a transmembraneous connection of the negatively stained particles on both membrane surfaces.Our results suggest that the Na⁺–K⁺-ATPase protein is composed of four units and that each unit spans the cell membrane. The native enzyme structure of the Na⁺–K⁺-ATPase protein seems to be preserved during freeze-fracturing and freeze-etching. It is proposed that the four enzyme units of the Na⁺–K⁺-ATPase complex are dissociated during the negative staining procedure.Part of this work was presented at the Frühjahrstagung of the Deutsche Physiologische Gesellschaft [6]     

Digitalis-like compounds and Na+, K+-ATPase activity in bovine lens

 Digitalis-like compounds in bovine lens capsule, cortex and nucleus were determined quantitatively, following extraction, by their ability to inhibit [³H]ouabain binding to red blood cells. These compounds were found to be highly concentrated in the epithelium capsule and were significantly diminished in the cortex and nucleus. Na⁺, K⁺-ATPase density in the different regions was determined by [³H]ouabain binding to membranes and by autoradiography of lens slices. The highest concentration of [³H]ouabain-binding sites was observed to occur in membranes prepared from the epithelial cells of the capsule, and was almost 100- and 200-fold higher than the concentrations observed in membranes prepared from fiber cells of the cortex and nucleus, respectively. In the autoradiography studies, strong labeling of [³H]ouabain appeared in the epithelial cell zone, and only weak specific labeling appeared in the lens cortex and nucleus. Almost all (99%) of the Na⁺,K⁺-ATPase specific activity was found to be in the capsule epithelium and only 0.5% was measured in the cortex and no activity was detected in the nucleus. These results indicate that the digitalis-like compounds and Na⁺, K⁺-ATPase are concentrated in the lens capsule epithelium and are present only at low levels in the cortex and nucleus, thus implying that the lens capsular epithelial layer is the major region of the lens responsible for the homeostasis of ions and water in this tissue. Received: 20 September 1996 / Received after revision and accepted: 17 October 1996     

High salt diet down-regulates proximal tubule Na+, K+-ATPase activity in Dahl salt-resistant but not in Dahl salt-sensitive rats: evidence of defective dopamine regulation

Nishi , A., Bertorello , A. M. & Aperia , A. 1992. High salt diet down-regulates proximal tubule Na⁺, K⁺-ATPase activity in Dahl salt-resistant but not in Dahl salt-sensitive rats; evidence of defective dopamine regulation. Acta Ptiysiol Scand 144 , 263–267. Received 26 July 1991, accepted 25 October 1991. ISSN 0001–6772. Department of Paediatrics, Karolinska Institute, Sweden We examined the regulation of Na⁺, K⁺-ATPase activity in proximal tubule segments during a high salt diet in prehypertensive Dahl salt-sensitive and salt-resistant rats. Rats were placed on normal salt or high salt diets (0.9% saline as drinking water). During the normal salt diet, Na⁺, K⁺-ATPase activity was not different between Dahl salt-sensitive and salt-resistant rats. After 2 days and 10 days on a high salt diet, Na⁺, K⁺-ATPase activity in Dahl salt-resistant rats significantly decreased when compared to Dahl salt-resistant rats on a normal salt diet (P < 0.01). The decreased Na⁺, K⁺-ATPase activity in Dahl salt-resistant rats during a high salt diet was reversed by treatment with an inhibitor of aromatic l -amino acid decarboxylase (dopamine synthesizing enzyme), benserazide. In contrast, Na⁺, K⁺-ATPase activity did not decrease during the high salt diet and benserazide had no effect on Na⁺, K⁺-ATPase activity in Dahl salt-sensitive rats. These results indicate that Dahl salt-sensitive rats do not have the capacity to down-regulate the proximal tubule Na⁺, K⁺-ATPase activity during a high salt diet. Indirect evidence suggests that the regulation of Na⁺, K⁺-ATPase activity by locally produced dopamine is absent in Dahl salt-sensitive rats.     

Decreased expression of apical Na+ channels and basolateral Na+, K+-ATPase in ulcerative colitis

Impaired absorption of sodium (Na+) and water is a major factor in the pathogenesis of diarrhoea in ulcerative colitis (UC). Electrogenic Na+ absorption, present mainly in human distal colon and rectum, is defective in UC, but the molecular basis for this is unclear. The effect of UC on the expression of apical Na+ channels (ENaC) and basolateral Na+, K+-ATPase, the critical determinants of electrogenic Na+ transport, was therefore investigated in this study. Sigmoid colonic and/or proximal rectal mucosal biopsies were obtained from patients with mild to moderate UC, and patients with functional abdominal pain (controls). ENaC subunit expression was studied by immunohistochemistry, western blot analysis, and in situ hybridization, and Na+, K+-ATPase isoform expression was studied by immunohistochemistry, western blotting, and northern blot analysis. UC was associated with substantial decreases in the expression of the ENaC beta- and gamma-subunit proteins and mRNAs, whereas the decrease in ENaC alpha-subunit protein detected by immunolocalization was less marked. The levels of expression of Na+, K+-ATPase alpha1- and beta1-isoform proteins were also lower in UC patients than in controls, although there were no differences in Na+, K+-ATPase alpha1- and beta1-isoform mRNA levels between the two groups. Taken together, these results show that UC results mainly in decreased expression of the apical ENaC beta- and gamma-subunits, as well as the basolateral Na+, K+-ATPase alpha1- and beta1-isoforms. In conclusion, these changes provide a basis for the low/negligible levels of electrogenic Na+ absorption seen in the distal colon and rectum of UC patients, which contribute to the pathogenesis of diarrhoea in this disease.     

Training in inhibitory avoidance causes a reduction of Na+,K+-ATPase activity in rat hippocampus

Compelling evidence has indicated the involvement of Na(+),K(+)-ATPase in the mechanisms of synaptic plasticity. In the present study, we investigated the effect of inhibitory avoidance training on Na(+),K(+)-ATPase activity, at different times after training, in the rat hippocampus. Male adult Wistar rats were trained in a step-down inhibitory avoidance task and compared to those submitted to isolated footshock (0.4 mA) or placed directly onto the platform. Na(+),K(+)-ATPase activity decreased, by 60%, in hippocampus of rats sacrificed immediately after the isolated footshock, as well as immediately (0 min) and 6 h after training; this effect was not present 24 h after training. We also verified that enzyme activity was not altered in rats killed after just being on the platform. These findings suggest that Na(+),K(+)-ATPase activity may be involved in the memory consolidation of step-down inhibitory avoidance in the hippocampus.     

Modulation of Na+,K+-ATPase activity is of importance for RVD

AIM: This study was performed to examine the role of Na+,K+-ATPase activity for the adaptive response to cell swelling induced by hypoosmoticity, i.e. the regulatory volume decrease (RVD). METHODS: The studies were performed on COS-7 cells transfected with rat Na+,K+-ATPase. To study changes in cell volume, cells were loaded with the fluorescent dye calcein and the intensity of the dye, following exposure to a hypoosmotic medium, was recorded with confocal microscopy. RESULTS: Ouabain-mediated inhibition of Na+,K+-ATPase resulted in a dose dependent decrease in the rate of RVD. Total 86Rb+ uptake as well as ouabain dependent 86Rb+ uptake, used as an index of Na+,K+-ATPase dependent K+ uptake, was significantly increased during the first 2 min following exposure to hypoosmoticity. Since protein kinase C (PKC) plays an important role in the modulation of RVD, a study was carried out on COS-7 cells expressing rat Na+,K+-ATPase, where Ser23 in the catalytic alpha1 subunit of rat Na+,K+-ATPase had been mutated to Ala (S23A), abolishing a known PKC phosphorylation site. Cells expressing S23A rat Na+,K+-ATPase exhibited a significantly lower rate of RVD and showed no increase in 86Rb+ uptake during RVD. CONCLUSION: Taken together, these results suggest that a PKC-mediated transient increase in Na+,K+-ATPase activity plays an important role in RVD.     

Renal Na+, K+-ATPase in Dahl salt-sensitive rats: K+ dependence,effect of cell environment and protein kinases

Na⁺, K⁺-ATPase in renal epithelial cells plays an important role in the regulation of Na⁺ balance, extracellular volume and blood pressure. The function of renal Na⁺, K⁺-ATPase in Dahl salt-sensitive (DS) rats, an animal model for salt-sensitive hypertension, and Dahl salt-resistant (DR) rats has been studied. In Na⁺, K⁺-ATPase partially purified from renal cortex, affinities and the Hill coefficients for Na⁺ and K⁺ activation were similar in DS and DR rats. Only one component of low ouabain affinity site was found in both strains, indicating the presence of the al isoform. Protein kinase C and cAMP-dependent protein kinase phosphorylated Na⁺, K⁺-ATPase α subunit in DS and DR rats, and the phosphorylation by protein kinase C was associated with an inhibition of enzyme activity. The kinetic parameters for K⁺ activation were also studied in a preparation of basolateral membranes and were found to be similar in DS and DR rats. In a preparation of cortical tubule cells, Na⁺, K⁺-ATPase activity was determined as ouabain-sensitive oxygen consumption (OS Qo₂). Maximal OS Qo₂, measured in Na⁺ loaded cells, was the same in DS and DR rats. The K₀₆ for K⁺ was significantly lower in DS than DR rats (0.163 ±0.042 vs. 0.447 + 0.061 mM, P < 0.05), indicating that factors regulating Na⁺, K⁺-ATPase activity in intact cells are altered in DS rats. Kinetic parameters for Na⁺ activation in cells were the same in both strains. In summary, the function of renal Na⁺, K⁺-ATPase molecule is not altered in DS rats. The intracellular systems that regulate renal Na⁺, K⁺-ATPase activity might be different in DS and DR rats.     

On the functional interaction between nicotinic acetylcholine receptor and Na+,K+-ATPase

Previous studies have shown that nanomolar acetylcholine (ACh) produces a 2 to 4-mV hyperpolarization of skeletal muscle fibers putatively due to Na⁺,K⁺-ATPase activation. The present study elucidates the involvement of the nicotinic ACh receptor (nAChR) and of Na⁺,K⁺-ATPase isoform(s) in ACh-induced hyperpolarization of rat diaphragm muscle fibers. A variety of ligands of specific binding sites of nAChR and Na⁺,K⁺-ATPase were used. Dose–response curves for ouabain, a specific Na⁺,K⁺-ATPase inhibitor, were obtained to ascertain which Na⁺,K⁺-ATPase isoform(s) is involved. The ACh dose–response relationship for the hyperpolarization was also determined. The functional relationship between these two proteins was also studied in a less complex system, a membrane preparation from Torpedo electric organ. The possibility of a direct ACh effect on Na⁺,K⁺-ATPase was studied in purified lamb kidney Na⁺,K⁺-ATPase and in rat red blood cells, systems where no nAChR is present. The results indicate that binding of nAChR agonists to their specific sites results in modulation of ouabain-sensitive (most probably α2) isoform of Na⁺,K⁺-ATPase, leading to muscle membrane hyperpolarization. In the Torpedo preparation, ouabain modulates dansyl-C6-choline binding to nAChR, and vice versa. These results provide the first evidence of a functional interaction between nAChR and Na⁺,K⁺-ATPase. Possible interaction mechanisms are discussed.     

Protein kinase C activity in rat renal proximal tubule cells.

The presence of protein kinase C (PKC) in proximal tubule cells of the rat kidney is established by means of immunodetection and by the demonstration of calcium- and phospholipid-dependent, staurosporine-inhibitable histone phosphorylation. The calcium-dependence of renal PKC is described. Maximal activation of the enzyme (178.2 and 258.8 pmol P1 mg-1 min-1 for cytosol and membrane respectively) was achieved with 5 microM of Ca2+. Phorbol 12, 13 dibutyrate (PDBu) translocated PKC from cytosol to membrane in a dose- and time-dependent fashion, while 4 alpha-phorbol 12,13-didecanoate produced no significant effect on translocation. Cytosolic PKC activity was compared in immature and mature tissues (10- and 40-day-old kidneys). Basal activity was found to be significantly higher (P less than 0.05) in immature cells (272.8 vs. 157.5 pmol Pi mg-1 min-1). PDBu at 10(-6) M for 15 min reduced immunoreactivity in the soluble fraction of both groups, which was accompanied by a significant decrease in kinase activity. We speculate that the high PKC activity in the infant kidney plays a role in cell growth.     

Gentamicin inhibition of Na+,K(+)-ATPase in rat kidney cells.

Na,K(+)-ATPase activity is decreased in homogenized renal tissue from GM-treated rats. This study examines whether the site of the active effect of GM on Na,K(+)-ATPase activity in the kidney can be localized to the proximal convoluted tubules (PCT) where the drug is taken up and where it will produce necrosis. In rats treated with gentamicin (50 micrograms.kg-1.day-1 i.m.) for 7 days, PCT Na,K(+)-ATPase activity was reduced as compared to vehicle-treated rats but returned to control levels 7 days after treatment withdrawal. In another nephron segment, the medullary thick ascending limb of Henle (mTAL), where GM induced lesions are uncommon, Na,K(+)-ATPase activity was the same in GM- and vehicle-treated rats treatment. To study the in vitro effect of GM, dissected PCT and mTAL segments from untreated rats were preincubated for 30 min with GM 10(-3) M, a dose similar to the tissue concentration in chronically treated rats. In tubule segments that were permeabilized to allow the drug to enter the cells, GM 10(-3) M significantly inhibited Na,K(+)-ATPase activity both in PCT and mTAL. In non-permeabilized mTAL segments GM did not inhibit Na,K(+)-ATPase activity. GM inhibition of Na,K(+)-ATPase activity in permeabilized PCT segments persisted after the tubules were rinsed in GM free medium. GM does not inhibit Na,K(+)-ATPase partly purified from the renal cortex. Conclusion. Gentamicin inhibits Na,K(+)-ATPase activity in renal tubule cells when it has access to the cytoplasm. Treatment with GM will therefore cause a selective inhibition of Na,K(+)-ATPase in the proximal tubule cells.     

Androgens stimulate preoptic area Na+,K+-ATPase activity in male rats

This paper describes the effects of castration and testosterone replacement on the Na+,K+-ATPase activity levels of the cerebral cortex (CC), preoptic-suprachiasmatic region (POSC) and mediobasal hypothalamus (MBH) in male rats. Na+,K+-ATPase activity was estimated as the ouabain-sensitive fraction of ADP and AMP generation rate, measured by high-pressure liquid chromatography (HPLC) with UV detection, from a standard incubation mixture containing 3 mM ATP. Orchidectomy, performed 4 weeks before sacrifice, decreased ATPase activity of MBH. Testosterone propionate treatment (50 micrograms/day X 2 days) to castrated animals resulted in a 4-fold increase in enzyme activity in the POSC, an effect that might be related to the behavioral effects of androgens. None of the treatments seemed to influence the enzyme activity of the cerebral cortex.     

Stretch receptor-associated expression of alpha 3 isoform of the Na+, K+-ATPase in rat peripheral nervous system

Expression of the neuronal alpha(3) isoform of the Na(+),K(+)-ATPase (alpha(3) Na(+),K(+)-ATPase) was studied in the rat peripheral nervous system using histological and immunohistochemical techniques. Non-uniform expression of the alpha(3) Na(+),K(+)-ATPase was observed in L5 ventral and dorsal roots, dorsal root ganglion, sciatic nerve and its branches into skeletal muscle. The alpha(3) Na(+),K(+)-ATPase was not detected in nerve fibers in skin, saphenous and sural nerves. In dorsal root ganglion 12+/-2% of neurons were immunopositive for alpha(3) Na(+),K(+)-ATPase and all these neurons were large primary afferents that were not labeled by Griffonia simplicifolia isolectin B4 (marker of small primary sensory neurons). In dorsal and ventral roots 27+/-3% and 40+/-3%, respectively, of myelinated axons displayed immunoreactivity for alpha(3) Na(+),K(+)-ATPase. In contrast to the dorsal roots, strong immunoreactivity in ventral roots was observed only in myelinated axons of small caliber, presumably gamma-efferents. In the mixed sciatic nerve alpha(3) Na(+),K(+)-ATPase was detected in 26+/-5% of myelinated axons (both small and large caliber). In extensor hallicus proprius and lumbricales hind limb muscles alpha(3) Na(+),K(+)-ATPase was detected in some intramuscular axons and axonal terminals on intrafusal muscle fibers in the spindle equatorial and polar regions (regions of afferent and efferent innervation of the muscle stretch receptor, respectively). No alpha(3) Na(+),K(+)-ATPase was found in association with innervation of extrafusal muscle fibers or in tendon-muscle fusion regions. These data demonstrate non-uniform expression of the alpha(3) isoform of the Na(+),K(+)-ATPase in rat peripheral nervous system and suggest that alpha(3) Na(+),K(+)-ATPase is specifically expressed in afferent and efferent axons innervating skeletal muscle stretch receptors.