Abstract

The action of halothane was investigated on single pacemaker fibers isolated from the sino-atrial (S-A) node of the rabbit and suspended in Krebs-Henseleit solution at 37°C. Action potentials were recorded from the S-A nodal fibers. During administration of test solutions, equilibrated with 1% and 2% halothane in an O₂-CO₂ mixture, the following changes of the action potential were observed. With 1% halothane the slope of the pacemaker potential decreased from 57 to 36 mV/sec, the other parameters of the membrane potential remaining essentially unchanged. In 2% halothane the slope of the pacemaker potential decreased further to 27 mV/sec; the maximal diastolic potential declined from 65 to 56 mV, the amplitude of the action potential declined from 79.7 to 58.0 mV and the overshoot declined from 14.7 to 2.0 mV. The threshold potential did not change markedly. Preparations of spontaneously beating guinea-pig atria did not show uniform behavior during administration of halothane. In some atria the rate increased while in others it decreased. These effects on atrial rate can be explained by the electrophysiologic findings. However, it is not clear whether they are due to changes in ion permeability or to alteration in active ion transport.