Passive and active membrane properties of isolated rat intracardiac neurons: regulation by H- and M-currents
J. Cuevas, A.A. Harper, C. Trequattrini and D.J. Adams
Department of Molecular and Cellular Pharmacology, University of Miami
School of Medicine, Miami, FL 33101, USA and Department of Physiology and
Pharmacology, University of Queensland, Brisbane, QLD 4072, Australia
APStracts 4:0086N, 1997.
ABSTRACT
The electrical characteristics of isolated neonatal rat intracardiac neurons
were examined at 22øC and 37øC using the perforated patch whole-cell recording technique. The mean resting membrane potential was -52.0 mV at 37øC and
exhibited no temperature dependence. Lowering the temperature from 37øC to
22øC, decreased the mean input resistance from 854 Mê to 345 Mê, respectively, and reduced the membrane time constant approximately three-fold yielding a Q10 of 2.1. Hyperpolarizing current pulses induced time-dependent rectification of the voltage response in all neurons at both temperatures. This behaviour was
7previously not observed in dialyzed neurons, and was reversibly blocked by
external Cs+ (2 mM) but not Ba2+ (1 mM). Voltage-clamp studies of isolated
neurons revealed a hyperpolarization-activated inward current. This inwardly
rectifying conductance was isolated from other membrane currents using
external Cs+. The time- and voltage-dependence of this current is consistent
with Ih and contributes to the passive electrical properties of rat
intracardiac neurons. In >90% of the neurons studied, depolarizing currents
evoked firing of multiple, adapting, action potentials at 22oC. The number of
action potentials increased with current strength producing a mean discharge
of 5.1 (+100 pA, 1 s pulse), which was attenuated at 37oC to a mean of 1.4.
The amplitude and kinetics of the slow, muscarine-sensitive inward and outward currents (IM) were highly temperature dependent. Lowering the temperature from 37 to 22øC reduced the steady-state current amplitude by approximately one-
third and the rate of deactivation of IM by 6- to 9-fold at all voltages
examined. The average Q10 for the time constant of deactivation of IM was 3.7
ñ 0.3. Acetylcholine (ACh) induced tonic discharges in response to
depolarizing currents (+100 pA, 1 s pulse) at both temperatures. This effect
of ACh was inhibited by the muscarinic receptor antagonist, pirenzepine (100
nM). At 37oC, a mean discharge of 1.5 was increased to 23.5 in the presence of ACh. A similar switch from phasic to tonic discharge was also produced by the
potassium channel inhibitors, Ba2+ (1 mM) and uridine triphosphate (UTP; 100
æM), whereas cadmium, 4-aminopyridine, apamin, charybdotoxin and dendrotoxin
did not alter discharge activity. The pharmacological sensitivity profile and
temperature dependence of the active membrane properties are consistent with
the muscarine-sensitive potassium current (IM) regulating the discharge
activity in rat intracardiac neurons.
Received 14 January 1997; accepted in final form 3 June 1997.
APS Manuscript Number J032-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 15 July 1997