Ouabain binding kinetics and FXYD7 expression in astrocytes and neurons in primary cultures: implications for cellular contributions to extracellular K+ homeostasis?
Although Na+,K+-ATPase-mediated K+ uptake into astrocytes plays a major role in re-establishing resting extracellular K+ following neuronal excitation little information is available about astrocytic Na+,K+-ATPase function, let alone mechanisms returning K+ to neurons. The catalytic units of the Na+,K+-ATPase are the astrocyte-specific α2, the neuron-specific α3 and the ubiquitously expressed α1. In the present work, Bmax and KD values for α1, α2 and α3 subunits were computed in cultured cerebro-cortical mouse astrocytes and cerebellar granule neurons by non-linear regression as high-affinity (α2, α3) and low-affinity (α1) [3H]ouabain binding sites, which stoichiometrically equal transporter sites. Cellular expression was also determined of the brain- and α1-β1 isoform-specific FDYX7, regulating Na+,K+-ATPase efficiency and K+-sensitivity. From ouabain-sensitive K+ uptake rates published by ourselves (Walz and Hertz, 1982) or others (Atterwill et al., 1985), Na+,K+-ATPase turnover was determined. Subunits α2 and α3 showed Bmax of 15-30 pmol/mg protein, with maximum turnover rates of 70-80/s. Bmax of the α1 subunit was low in neurons but very high in astrocytes (645 pmol/mg protein), where turnover rate was slow, reflecting expression of selectively expressed FXYD7, and binding was increased by K+. The role of these characteristics for K+ homeostasis are discussed.
Peng
Liang
L
Department of Clinical Pharmacology, China Medical University, Shenyang, P.R. China.
Huang
Rong
R
Zhang
Shiquen
S
Hertz
Leif
L
D000818:Animals; D001253:Astrocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002478:Cells, Cultured; D018920:Coculture Techniques; D045604:Extracellular Fluid / Q000378:metabolism*; D006706:Homeostasis / Q000502:physiology*; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D009419:Nerve Tissue Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D009474:Neurons / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D010042:Ouabain / Q000378:metabolism*; D011188:Potassium / Q000502:physiology*; D011485:Protein Binding / Q000502:physiology
C462940:Fxyd7 protein, mouse; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D010042:Ouabain; D011188:Potassium