Channel-mediated and carrier-mediated uptake of K+ into cultured ovine oligodendrocytes

✍ Scribed by Dr. L. Hertz; B. Soliven; E. Hertz; S. Szuchet; D. J. Nelson

• Publisher: John Wiley and Sons
• Year: 1990
• Tongue: English
• Weight: 906 KB
• Volume: 3
• Category: Article
• ISSN: 0894-1491
• DOI: 10.1002/glia.440030614

✦ Synopsis

Uptake of radioactive K+ by mature ovine oligodendrocytes (OLGs) maintained in primary culture was measured under steady-state conditions, i.e., in cells maintained in a normal tissue culture medium (5.4 mM K+), and in cells after depletion of intracellular K+ to less than 15% of its normal value by pre-incubation in K+-free medium. The latter value is dominated by an active, carrier-mediated uptake (although it may include some diffusional uptake), whereas the former, in addition to active uptake, also reflects passive K+ diffusion through ion selective channels and possible selfexchange between extracellular and intracellular K+, which may be carrier-mediated. The total uptake rate was 144 t 10 nmol/min/mg protein, and the uptake after K+ depletion was 60 ? 2 nmol/min/mg protein, much lower rates than previously observed in astrocytes. The uptake into K+-depleted cells was inhibited by about 80% in tjhe presence of ouabain (1 mM) and about 30% in the presence of furosemide (2 mM). Activators of protein kinase C (phorbol esters) and CAMP-dependent protein kinase (forskolin) have been shown to alter the myelinogenic metabolism as well as outward K' current in cultured OLGs. The present study demonstrates that K' homeostasis in OLGs is modulated through similar second messenger pathways. Active uptake was inhibited by about 60% in the presence of active phorbol esters (100 nM) but was not affected by forskolin (100 nM). Forskolin likewise had no effect on total uptake, whereas phorbol esters caused a much larger inhibition than expected from their effect on carrier-mediated uptake alone, suggesting that channel-mediated uptake was also reduced. The relatively small difference between the carrier-mediated and the total uptake made it difficult to assess accurately the effects of these agents on channel-mediated uptake. Electrophysiological experiments were carried out to assess directly the inhibitory effects of these compounds on inward rectifier K+ channel (KZ) activation. The phorbol ester, phorbol 12-myristate 13-acetate (1.5 to 100 nM), decreased the amplitude of the steady-state K: current at -120 mV by approximately 40%; however, neither the inactive analog 4-a phorbol-12-13 didecanoate nor forskolin (0.1 to 1 FM) decreased current amplitude when extracellular solutions contained normal K+ concentrations. It can be concluded that both the carrier-mediated Kt uptake and passive Kf influx through inwardly relctifying K+ channels in OLGs are modulated by protein kinase C but not by CAMP-dependent protein kinase activation. Furthermore, both carrier-mediated and total K+ uptake in oligodendrocytes are significantly lower than that measured in astrocytes, implying differential physiologcal roles for astrocytes and oligodendrocytes in K ' homeostasis.