Freshly isolated astrocytes from rat hippocampus show two distinct current patterns and different [K(+)](o) uptake capabilities.
Whether astrocytes predominantly express ohmic K(+) channels in vivo, and how expression of different K(+) channels affects [K(+)](o) homeostasis in the CNS have been long-standing questions for how astrocytes function. In the present study, we have addressed some of these questions in glial fibrillary acidic protein [GFAP(+)], freshly isolated astrocytes (FIAs) from CA1 and CA3 regions of P7-15 rat hippocampus. As isolated, these astrocytes were uncoupled allowing a higher resolution of electrophysiological study. FIAs showed two distinct ion current profiles, with neither showing a purely linear I-V relationship. One population of astrocytes had a combined expression of outward potassium currents (I(Ka), I(Kd)) and inward sodium currents (I(Na)). We term these outwardly rectifying astrocytes (ORA). Another population of astrocytes is characterized by a relatively symmetric potassium current pattern, comprising outward I(Kdr), I(Ka), and abundant inward potassium currents (I(Kin)), and a larger membrane capacitance (C(m)) and more negative resting membrane potential (RMP) than ORAs. We term these variably rectifying astrocytes (VRA). The I(Kin) in 70% of the VRAs was essentially insensitive to Cs(+), while I(Kin) in the remaining 30% of VRAs was sensitive. The I(Ka) of VRAs was most sensitive to 4-aminopyridine (4-AP), while I(Kdr) of ORAs was more sensitive to tetraethylammonium (TEA). ORAs and VRAs occurred approximately equally in FIAs isolated from the CA1 region (52% ORAs versus 48% VRAs), but ORAs were enriched in FIAs isolated from the CA3 region (71% ORAs versus 29% VRAs), suggesting an anatomical segregation of these two types of astrocytes within the hippocampus. VRAs, but not ORAs, showed robust inward currents in response to an increase in extracellular K(+) from 5 to 10 mM. As VRAs showed a similar current pattern and other passive membrane properties (e.g., RMP, R(in)) to "passive astrocytes"in situ (i.e., these showing linear I-V curves), such passive astrocytes possibly represent VRAs influenced by extensive gap-junction coupling in situ. Thus, our data suggest that, at least in CA1 and CA3 regions from P7-15 rats, there are two classes of GFAP(+) astrocytes which possess different K(+) currents. Only VRAs seem suited to uptake of extracellular K(+) via I(Kin) channels at physiological membrane potentials and increases of [K(+)](o). ORAs show abundant outward potassium currents with more depolarized RMP. Thus VRAs and ORAs may cooperate in vivo for uptake and release of K(+), respectively.
Zhou
M
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Center for Neuropharmacology and Neuroscience and Division of Neurosurgery, Albany Medical College, Albany, New York 12208, USA. kimelbh@mail.amc.edu
Kimelberg
H K
HK
D000818:Animals; D001253:Astrocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002469:Cell Separation; D002478:Cells, Cultured; D002586:Cesium / Q000494:pharmacology; D017097:Electric Impedance; D005904:Glial Fibrillary Acidic Protein / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D016011:Normal Distribution; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism* / Q000493:pharmacokinetics; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D020133:Reverse Transcriptase Polymerase Chain Reaction; D019789:Tetraethylammonium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D005904:Glial Fibrillary Acidic Protein; D007473:Ion Channels; D012333:RNA, Messenger; D002586:Cesium; D013779:Tetrodotoxin; D019789:Tetraethylammonium; D011188:Potassium