SUMMARY Right atria were excised from the hearts of 20 young dogs (15 ± 5 weeks old) and maintained in vitro in order to perform microelectrode impalements. The atria contracted spontaneously for at least 6 hours at a stable rate of 127/min when they were perfused with Krebs-Ringer bicarbonate solution through the sinus node artery. Cells within a small area near the sinus node artery and between tbe superior vena cara and right atrium consistently yielded transmembrane potentials typical of pacemaker cells. This area was verified bistologically to be the midportion of the sinus node.Electrical characteristics of these sinus node pacemaker cells were: maximum diastolic potential, -56 ± 7 mV (mean ± 1 SD); action potential amplitude, 56 ± 8 nrV; overshoot, 0 ± 2 mV. Smooth transitions from phase 4 to phase 0 were always present. From these experiments we conclude that the excised perfused canine right atrium provides a useful new experimental cardiac preparation because it maintains stable spontaneous activity for many hours in vitro and the sinus node within it is readily accessible for study by intracellular microelectrodes.MUCH OF OUR knowledge about initiation and conduction of the excitatory impulse through the right atrial and ventricular conduction systems has been obtained from electrophysiological studies on mammalian hearts. Epicardial electrograms have been used to define the pattern of conduction, 1 " 4 and observations with intracellular microelectrodes have elucidated the electrical behavior of single cells (see Hoffman and Cranefield 5 for review). The different anatomical components of the conducting system (sinus node, internodal pathways, atrioventricular node, and HisPurkinje system) (see James' and James and SherF for review) have been identified histologically in both human and canine hearts and found to correspond topologically with electrophysiological observations. Because the electrical and morphological properties of the canine heart have been particularly well established and found essentially similar to those of man, its usefulness as an experimental analog of the human heart is amply justified.Studies of spontaneous electrical activity in cardiac tissue with intracellular microelectrodes have taken two general directions. The first has focused on cells of the sinus node region which are responsible for genesis of the normal cardiac rhythm, while the second has involved cells elsewhere in the heart which may under abnormal conditions become sites of spontaneous activity. The latter approach has been utilized extensively in the canine heart. Although many studies of the sinus node have been carried out in lapine, feline, and lower vertebrate hearts, we have found no definitive account of electrical activity recorded from individual cells of the canine sinus node. All our current understanding of normal spontaneous impulse initiation by cells within the canine heart has evolved by inference and implication from studies on cells outside the sinus node. The present investigation was undert...

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Mechanical stunning during R is sensitive to MDL-28170. Depressed mechanical function is reflected in a hyposensitization of trabecular myofilaments to Ca2+. Western analysis shows that PKM epsilon is present in R hearts.

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