Potassium Conductance Causing Hyperpolarization of CA1 Hippocampal Neurons During Hypoxia

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Abstract
Erdemli, G., Y. Z. Xu, and K. Krnjević. Potassium conductance causing hyperpolarization of CA1 hippocampal neurons during hypoxia. J. Neurophysiol. 80: 2378–2390, 1998. In experiments on slices (from 100- to 150-g Sprague-Dawley rats) kept at 33°C, we studied the effects of brief hypoxia (2–3 min) on CA1 neurons. In whole cell recordings from submerged slices, with electrodes containing only KMeSO4and N-2-hydroxyethylpiperazine- N′-2-ethanesulfonic acid, and in the presence of kynurenate and bicuculline (to minimize transmitter actions), hypoxia produced the following changes: under current clamp, 36 cells were hyperpolarized by 2.7 ± 0.5 (SE) mV and their input resistance ( Rin) fell by 23 ± 2.7%; in 30 cells under voltage clamp, membrane current increased by 114 ± 22.3 pA and input conductance ( Gin) by 4.9 ± 0.9 nS. These effects are much greater than those seen previously with K gluconate whole cell electrodes, but only half those seen with “sharp” electrodes. The hypoxic hyperpolarizations (or outward currents) were not reduced by intracellular ATP (1–5 mM) or bath-applied glyburide (10 μM): therefore they are unlikely to be mediated by conventional ATP-sensitive K channels. On the other hand, their depression by internally applied ethylene glycol-bis-(β-aminoethyl ether)- N, N, N′, N′-tetraacetic acid (1.1 and 11 mM) and especially 1,2-bis(2-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid (11–33 mM) indicated a significant involvement of Ca-dependent K (KCa) channels. The β-adrenergic agonist isoprenaline (10 μM) reduced hypoxic hyperpolarizations and decreases in Rin( n = 4) (and in another 11 cells corresponding changes in Gin); and comparable but more variable effects were produced by internally applied 3′:5′-adenosine cyclic monophosphate (cAMP, 1 mM, n = 6) and bath-applied 8-bromo-cAMP ( n = 8). Thus afterhyperpolarization-type KCachannels probably take part in the hypoxic response. A major involvement of G proteins is indicated by the near total suppression of the hypoxic response by guanosine 5′- O-(3-thiotriphosphate) (0.1–0.3 mM, n = 23) and especially guanosine 5′- O-(2-thiodiphosphate) (0.3 mM, n = 26), both applied internally. The adenosine antagonist 8-( p-sulfophenyl)theophylline (10–50 μM) significantly reduced hypoxic hyperpolarizations and outward currents in whole cell recordings (with KMeSO4electrodes) from submerged slices but not in intracellular recordings (with KCl electrodes) from slices kept at gas/saline interface. In further intracellular recordings, antagonists of γ-aminobutyric acid-B or serotonin receptors also had no clear effect. In conclusion, these G-protein-dependent hyperpolarizing changes produced in CA1 neurons by hypoxia are probably initiated by Ca2+release from internal stores stimulated by enhanced glycolysis and a variable synergistic action of adenosine.