Passive and active membrane properties of isolated rat intracardiac neurons: Regulation by H- and M-currents
The electrical characteristics of isolated neonatal rat intracardiac neurons were examined at 22 and 37°C using the perforated-patch whole cell recording technique. The mean resting membrane potential was -52.0 mV at 37°C and exhibited no temperature dependence. Lowering the temperature from 37 to 22°C decreased the mean input resistance from 854 to 345 MΩ, respectively, and reduced the membrane time constant approximately threefold yielding a Q10 of 2.1. Hyperpolarizing current pulses induced time- dependent rectification of the voltage response in all neurons at both temperatures. This behavior was previously 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 I(h) 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 22°C. 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 37°C to a mean of 1.4. The amplitude and kinetics of the slow, muscurine-sensitive inward and outward currents (I(M)) were highly temperature dependent. Lowering the temperature from 37 to 22°C reduced the steady-state current amplitude by approximately one-third and the rate of deactivation of I(M) by six- to ninefold at all voltages examined. The average Q10 for the time constant of deactivation of I(M) was 3.7 ± 0.3 (mean ± SE). 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 antagonists, pirenzepine (100 nM), and mL-toxin (60 nM). At 37°C, 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-5'-triphosphate (UTP; 100 μM), 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 (I(M)) regulating the discharge activity in rat intracardiac neurons.
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