Contribution of ATP-Sensitive Potassium Channels to Hypoxic
Hyperpolarization in Rat Hippocampal CA1 Neurons In Vitro.
Fujimura, N., E. Tanaka, S. Yamamoto, M. Shigemoria and H. Higashi.
Departments of Physiology and Neurosurgery, Kurume University School of
Medicine, 67 Asahi-machi, Kurume 830, Japan.
APStracts 3:0204N, 1996.
In order to investigate the mechanism of generation of the hypoxia-induced
hyperpolarization (hypoxic hyperpolarization) in hippocampal CA1 neurons in
rat tissue slices, recordings were made in current-clamp mode and single-
electrode voltage-clamp mode. Superfusion with hypoxic medium produced a
hyperpolarization and corresponding outward current, which were associated
with an increase in membrane conductance. Re-oxygenation produced a further
hyperpolarization, with corresponding outward current, followed by a recovery
to the pre-exposure level. The amplitude of the post-hypoxic hyperpolarization
was always greater than that of the hypoxic hyperpolarization. In single-
electrode voltage-clamp mode, it was difficult to record reproducible outward
currents in response to repeated hypoxic exposure, using electrodes with a
high tip resistance. The current-clamp technique was, therefore, chosen to
study the pharmacological characteristics of the hypoxic hyperpolarization. In
60 - 80 % of hippocampal CA1 neurons, glibenclamide or tolbutamide (3 - 100
mM) reduced the amplitude of the hypoxic hyperpolarization, in a
concentration-dependent manner by up to approximately 70 %. The glibenclamide
or tolbutamide concentrations producing half-maximal inhibition of the hypoxic
hyperpolarization were 6 and 12 mM, respectively. The chord conductance of the
membrane potential between -80 to -90 mV in the absence of glibenclamide (30
mM) or tolbutamide (100 mM) was 2 - 3 times greater than that in the presence
of glibenclamide or tolbutamide. In contrast, the reversal potential of the
hypoxic hyperpolarization was approximately -83 mV in both the absence and
presence of tolbutamide or glibenclamide. In approximately 40 % of CA1
neurons, diazoxide (100 mM) or nicorandil (1 mM) mimicked the hypoxic
hyperpolarization and pretreatment of these drugs occluded the hypoxic
hyperpolarization. When ATP was injected into the impaled neuron, hypoxic
exposure could not produce a hyperpolarization. The intracellular injection of
the non-hydrolysable ATP analogue, AMP-PNP, reduced the amplitude of the
hypoxic hyperpolarization. Furthermore, application of dinitrophenol (10 mM)
mimicked the hypoxic hyperpolarization, and the dinitrophenol-induced
hyperpolarization was inhibited by either pretreatment of tolbutamide or
intracellular injection of ATP, indicating that the hypoxic hyperpolarization
is highly dependent on intracellular ATP. It is, therefore, concluded that in
the majority of hippocampal CA1 neurons, exposure to hypoxic conditions
resulting in a reduction in the intracellular level of ATP, leads to
activation of KATP channels with concomitant hyperpolarization.
Received 21 June 1996; accepted in final form 12 September 1996.
APS Manuscript Number J492-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996