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.
ABSTRACT
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