Oxygen-sensitive reduction in Ca2+-activated K+ channel open probability in turtle cerebrocortex

Abstract

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 α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) currents and paradoxically an increase in γ-aminobutyric acid receptor (GABAR) currents in turtle cortical neurons. In a search for other oxygen-sensitive channels we discovered a Ca2+-activated K+ channel (KCa) that exhibited a decrease in open time in response to anoxia. Single-channel recordings of KCa activity were obtained in cell-attached 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 223pS, 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 (Popen); however, in cell-attached 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 Popen while drip application of a phorbol ester PKC activator decreased Popen 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 (∼8mV) 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).