Mediation by Intracellular Calcium-Dependent Signals of Hypoxic
Hyperpolarization in Rat Hippocampal CA1 Neurons In Vitro.
Yamamoto, S., E. Tanaka, and H. Higashi.
Department of Physiology, Kurume University School of Medicine, 67 Asahi-
machi, Kurume 830, Japan.
APStracts 3:0214N, 1996.
In response to oxygen deprivation, CA1 pyramidal neurons show a
hyperpolarization (hypoxic hyperpolarization), which is associated with a
reduction in neuronal input resistance. The role of extra- and intracellular
Ca 2+ ions in hypoxic hyperpolarization was investigated. The hypoxic hyper-
polarization was significantly depressed by tolbutamide (100 [mu] M);
moreover, the response was reversed in its polarity in medium containing
tolbutamide (100 [mu] M), low Ca 2+ (0.25 mM) and Co 2+ (2 mM), suggesting
that the hypoxic hyperpolarization is mediated by activation of both ATP-
sensitive K + (KATP) channels and Ca 2+ -dependent K + channels. The hypoxic
depolarization in medium containing tolbutamide, low Ca 2+ and Co 2+ is
probably due to inhibition of the electrogenic Na + -K + pump and concomitant
accumulation of interstitial K + . Hypoxic hyperpolarizations were depressed
in either low (0.25 or 1.25 mM) or high (5 or 7.5 mM) Ca 2+ medium (control,
2.5 mM), indicating that there is an optimal extracellular Ca 2+ concentration
required to produce the hypoxic hyperpolarization. BAPTA-AM (50 - 100 [mu] M),
procaine (300 [mu] M) or ryanodine (10 [mu] M) significantly depressed the
hypoxic hyperpolarization, suggesting that Ca 2+ released from intracellular
Ca 2+ stores may have an important role in the generation of hypoxic
hyperpolarization. The high affinity calmodulin inhibitor, W-7 (50 [mu] M),
completely blocked, whereas the low affinity calmodulin inhibitor, W-5 (50
[mu] M), did not affect, the hypoxic hyperpolarization. The calmodulin
inhibitor, trifluoperazine (50 [mu] M), also suppressed the hypoxic
hyperpolarization. In addition, calcium/calmodulin kinase II inhibitor, KN-62
(10 [mu] M), markedly depressed the amplitude and net outward current of the
hypoxic hyperpolarization without affecting the reversal potential. In
contrast, neither the myosin light chain kinase inhibitor, ML-7 (10 [mu] M),
nor the protein kinase A inhibitor, H-89 (1 [mu] M), significantly altered the
hypoxic hyperpolarization. These results suggest that CaM kinase II, which is
activated by CaM, may contribute to the generation of the hypoxic
hyperpolarization. In conclusion, the present study indicates that, in the
majority of hippocampal CA1 neurons, the hypoxic hyperpolarization is due to
activation of both K ATP channels and Ca 2+ -dependent K + channels.
Received 21 June 1996; accepted in final form 12 September 1996.
APS Manuscript Number J493-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996