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J Biol Chem, Vol. 275, Issue 6, 3749-3754, February 11, 2000
From the Large conductance, calcium-activated potassium
channels (BKCa or maxi-K) are important determinants
of membrane excitability in many cell types. We used patch clamp
techniques to study the biochemical regulation of native
BKCa channel proteins by endogenous Ser/Thr-directed
protein kinases and phosphatases in cell-free membrane patches from rat
pituitary tumor cells (GH4C1). When protein
kinase activity was blocked by removing ATP, endogenous protein
phosphatases slowly increased BKCa channel activity
approximately 3-fold. Dephosphorylated channels could be activated
fully by physiological increases in cytoplasmic calcium or membrane
depolarization. In contrast, endogenous protein kinases inhibited
BKCa channel activity at two functionally distinct sites. A
closely associated, cAMP-dependent protein kinase rapidly
reduced channel activity in a conditional manner that could be overcome
completely by increasing cytoplasmic free calcium 3-fold or 20 mV
further depolarization. Phosphorylation at a pharmacologically distinct
site inhibited channel activity unconditionally by reducing
availability to approximately half that of maximum at all physiological
calcium and voltages. Conditional versus unconditional
inhibition of BKCa channel activity through different
protein kinases provides cells with a powerful computational mechanism
for regulating membrane excitability.
Conditional and Unconditional Inhibition of Calcium-activated
Potassium Channels by Reversible Protein Phosphorylation*
§¶ and
Laboratory of Signal Transduction, NIEHS,
National Institutes of Health, Research Triangle Park, North Carolina
27709 and § Physiology Unit, School of Biosciences, Cardiff
University, Cardiff CF10 3US, Wales, United Kingdom
*
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
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