E. LEPESCHKIN 
651 
difficult to prepare than those from the chicken 
embryo but have the advantage of being more 
similar to human myocardium as far as cell 
structure is concerned. The steep plateau and 
early T M^ave of the rat are not present in the 
rat embryo and fetus,^ and the action potential 
is therefore similar to that of higher mammals 
and man. 
Guinea Pigs, Rabbits 
The guinea pig and the rabbit are the only 
members of the rodent family that show 
the same high and more horizontal plateau 
as man and the carnivora. This w^as at- 
tributed to the fact that they are the only 
grass-eaters among the rodents." The myo- 
cardial electrolyte composition is also more sim- 
ilar to the latter (Table I) . The T v^ave of the 
rabbit and guinea pig is also quite similar in 
its polarity to that of man, usually opposite to 
QRS in direction^ and also shows the same 
changes as the result of drugs and electrolyte 
imbalance as the human T wave.^'" The isolated 
rabbit heart, perfused through the coronary 
arteries and surrounded by a temperature- 
controlled shield, was chosen for such studies 
because it is under more physiological condi- 
tions than a muscle strip, changes in the perfus- 
ing solution reach all parts of the ventricle 
rapidly and unpredictable reactive changes in 
blood composition appearing in a heart in situ 
can be avoided. Although only physically dis- 
solved oxygen can reach the heart, the lower 
viscosity of the solution as compared to blood 
causes an increase in coronary flow which 
makes up for the smaller amount of oxygen 
carried by a unit volume. Better visibility and 
freedom from potassium liberation through 
hemolysis are other advantages of a nutrient 
solution compared to whole blood perfusion. 
Some workers have kept the temperature of 
the heart low (at 24-30°C) in order to reduce 
the oxygen consumption, but such low tempera- 
tures may interfere with the sodium pump. It 
should be mentioned that because of its less 
forceful contraction the guinea pig heart is 
easier to penetrate with the flexibly suspended 
micro-electrode than the rabbit heart. 
It has been mentioned that the rapid ascent 
of the action potential of the guinea pig is fol- 
lowed by a slower ascent to a rounded plateau 
which was attributed to a slower entry of cal- 
cium or to a separate slow sodium channel. ^"'^^ 
The action potential of rabbits, ungulates and 
human myocardial strips shows a slow com- 
ponent at the end of its ascending branch while 
that of the cat and dog always shows a spike 
(Table I). In the guinea pig heart in situ this 
slow portion can be the result of a superposition 
of the potentials caused by activation of distant 
regions of the heart,'^ and the same can be true 
for an initial spike,* which appears when muscle 
near the reference electrode is activated late. It 
is, therefore, not clear whether the spikes of the 
carnivora are the equivalent of phase 1 and 
caused by sodium entry as in the case of the rat 
ventricle and the Purkinje fibers, or whether 
they are caused by superposition of QRS. Their 
narrow form would be in favor of the latter 
interpretation. 
Cats, Dogs 
While the cat and dog have shown little 
difference compared to man with respect to 
the resting and action potential, myocardial 
fiber structure and distribution of Purkinje fi- 
bers (Table I), they do differ from man in the 
direction of the QRS complex and T wave. Dogs 
often show negative T waves not only in lead I 
and/or lead II but also in unipolar leads from 
the regions of the chest which face the surface 
of the left ventricle. Recently^" we have meas- 
ured the myocardial temperatures in the wall of 
both ventricles by pulling a fine thermocouple 
junction through the intact chest, lungs and 
heart in twelve dogs, and have found a good 
positive correlation between the endo-epicardial 
temperature gradient in a given section of the 
left ventricle and amplitude of the T wave in 
unipolar leads from the overlying region of the 
chest. In dogs with inverted T waves this grad- 
ient was small. In addition to these physiological 
differences, the T wave may become negative in 
leads I and II even if it is positive in precordial 
leads, because the mediastinum of the dog is 
very loose, allowing the heart to show consid- 
able clockwise or counterclockwise rotation with 
changes of the body position. Baboons, who are 
dog-like in their chest structure, may also show 
these labile T waves, but chimpanzees show T 
