Effects of acetylcholine on single fibers of the atrioventricular node.

The action of acetylcholine on atrioventricular nodal fibers has been studied with the aid of intracellular microelectrodes. It has been found that during total atrioventricular dissociation no block of conduction occurs at the junction between the A-V node and the bundle of His or within fibers of the atrium. Failure of impulse transmission occurs in fibers in the atrial margin of the A-V node. The application of acetylcholine produces characteristic changes in the action potentials of fibers in the atrial margin of the A-V node: depolarization becomes slow, amplitude falls and notching and slurring appear in the upstrokes. These observations support the concept that transmission of excitation through the A-V node is deeremental and that the excitatory process in nodal fibers consists of more than one excitatory event. R ECENT studies employing intracellular recording have provided a new approach to the problem of the conduction of excitation through the atrioventricular node. 1-2 It has been shown that fibers in the A-V node of the rabbit heart have transmembrane action potentials which typically are low in amplitude and slow in rate of depolarization. These fibers are found near the junction of the node with the atrium. The major part of total A-V delay is due to the slow transmission of exci-tation through these fibers. It has been suggested that this slow spread of excitation results from deeremental conduction. 2 The present paper reports results that were obtained by transmembrane recording from fibers of the A-V node during the action of acetylcholine. These results are of interest, in that they support the concept of deeremen-tal conduction and suggest a possible anatomical arrangement of fibers contributing to this phenomenon. METHODS The methods have recently been described in detail. 2 A preparation consisting of part of the right atrium and ventricle of the rabbit heart was employed. This preparation contained the sino-atrial node, the right atrium and interatrial septum, the A-V node, the upper part of the bundle of His and the upper interventricular sep-tum. The tissue was mounted in a bath regulated by thermostat and perfused with oxygenated Tyrode's solution. Records were obtained from single cells by the use of intracellular glass capillary microeleetrodes filled with 3 M potassium chloride and mounted in conventional micromani-pulators. Cathode followers, direct-current coupled amplifiers and a DuMont two-beam oscilloscope were used throughout the experiment. Acetylcho-line chloride was added to the perfusate or directly to the bath in …