OXYGEN-SENSITIVE REDUCTION IN CA2+-ACTIVATED K+(CHANNEL OPEN PROBABILITY IN TURTLE CEREBROCORTEX

In response to low ambient oxygen levels the western painted turtle brain undergoes a large depression in metabolic rate which includes a decrease in neuronal action potential frequency. This involves the arrest of N-methyl-D-aspartate receptor (NMDAR) and alpha-amino-3hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) currents and paradoxically an increase in gamma-aminobutyric acid receptor (GABAR) currents in turtle cortical neurons. In a search for other oxygen-sensitive channels we discovered a Ca2+-activated K+ channel (K-ca) that exhibited a decrease in open time in response to anoxia. Single-channel recordings of Kca activity were obtained in cellattached and excised inside-out patch configurations from neurons in cortical brain sheets bathed in either normoxic or anoxic artificial cerebrospinal fluid (aCSF). The channel has a slope conductance of 223 pS, is activated in response to membrane depolarization, and is controlled in a reversible manner by free [Ca2+] at the intracellular membrane surface. In the excised patch configuration anoxia had no effect on Kca channel open probability (P-open) however, in cellattached mode, there was a reversible fivefold reduction in Popen (from 0.5 +/- 0.05 to 0.1 +/- 0.03) in response to 30-min anoxia. The inclusion of the potent protein kinase C (PKC) inhibitor chelerythrine prevented the anoxia-mediated decrease in P-open open while drip application of a phorbol ester PKC activator decreased P open during normoxia (from normoxic 0.4 +/- 0.05 to phorbol-12-myristate-13-acetate (PMA) 0.1 +/- 0.02). Anoxia results in a slight depolarization of turtle pyramidal neurons (similar to 8 my) and an increase in cytosolic [Ca2+]; therefore, Kca arrest is likely important to prevent Ca2+ activation during anoxia and to reduce the energetic cost of maintaining ion gradients. We conclude that turtle pyramidal cell Ca2+-activated K+ channels are oxygen-sensitive channels regulated by cytosolic factors and are likely the reptilian analog of the mammalian large conductance Ca2+-activated K+ channels (BK channels). Crown Copyright (C) 2013 Published by Elsevier All rights reserved.