E. LEPESCHKIN 
649 
of calcium to the contractile elements without 
the aid of tubules.- Another pecularity of the 
birds is that the Purkinje fibers follow the 
coronary vessels and therefore penetrate the 
myocardium from the epicardium toward the 
endocardium." This explains the negative QRS 
complex in ventricular surface leads * and could 
provide an ideal opportunity to study the elec- 
trical properties of Purkinje fibers in situ un- 
der physiological conditions. The fact that birds 
have the tallest U waves among all experimental 
animals * may be related to this distribution, if 
the concept that the U wave is the T wave of the 
Purkinje fibers is correct. 
The Rat 
Because of its low cost and upkeep the rat 
has been a favorite object of experimenta- 
tion concerning the electrical and mechanical 
properties of heart muscle. Its myocardial cells 
have a diameter only slightly smaller than that 
of the larger mammals, and are provided with 
transverse tubules, though of a smaller size.^^ 
The resting potential is of the same magnitude 
but the action potential is lower (Table I) and 
is characterized by prominent rapid initial de- 
polarization (phase 1), a steep, low plateau 
(phase 2) and a terminal phase (phase 3) 
which is less rapid than in other species. Be- 
cause of this configuration the T wave has two 
parts: a prominent initial summit, which cor- 
responds to phase 1, and a low, occasionally 
barely perceptible, terminal summit, which cor- 
responds to phase 3 (Fig. 1). The interval from 
beginning of QRS to the apex of the tallest T 
wave, corrected for the heart rate (Q-aTc) is 
therefore very short. This configuration of ac- 
tion potential and T wave is also present in 
most other rodents, insectivores, marsupials 
and chiropterae, which are mostly hibernators 
and small (a notable exception is the kangaroo).^ 
Coraboeuf 1" found that tetrodotoxin, which 
inhibits the entry of sodium into the cell, elimi- 
nates in the rat the entire initial portion of the 
action potential and reduces its amplitude con- 
siderably. In the guinea pig and other mam- 
mals which have a high and relatively flat 
plateau, the toxin makes the ascent of the 
action potential slower but does not change its 
amplitude. Coraboeuf believes that the high 
plateau, which is believed to be caused by a slow 
calcium current,^'''^^ is absent in the rat because 
of its high intracellular calcium content, and 
the peak of the action potential is due entirely 
to sodium entry. This would also be in keeping 
with the low myocardial sgdium and presum- 
ably a high transmembrane sodium gradient in 
the rat.* Because of these peculiarities, the 
dependence of the myocardial contraction 
strength on the preceding diastolic interval in 
the rat is different from other mammals.^'^" 
Rats and mice therefore cannot be compared to 
man in their T wave behavior. An illustration 
in the "Heart Bulletin" (Vol. 3, No. 3) showing 
F. N. Wilson, the father of modern experi- 
mental electrocardiography, experimenting on 
a rat, is completely imaginary, since he never 
used rats for his electrocardiographic studies. 
The ungulates have about the same myocar- 
dial fiber size and structure as well as resting 
and action potentials as humans, but show a 
more horizontal plateau and a longer, more 
peaked T wave with a later apex (longer Q-aTc 
interval) (Table I and Fig. 1) . This was attrib- 
uted ^ to the high potassium content of the grass 
which they eat and a higher K concentration in 
the serum, resulting in a higher transmembrane 
K gradient, since increasing the K concentra- 
tion of the perfusing fluid causes the same 
changes in configuration. Another reason could 
be that in all these animals the Purkinje fibers, 
which have a very long plateau, extend through- 
out the thickness of the ventricle. This may also 
be the reason that the T wave tends to be di- 
rected opposite the QRS complex, that is, there 
is less of a gradient in ventricular action po- 
tential duration.* However, the main differences 
between man and the ungulates, also caused by 
the wide penetration of the Purkinje fibers, are 
that in the ungulates QRS is much narrower for 
the heart size, that ventricular activation is 
mainly radial and that bundle branch block does 
not result in typical changes of QRS found in 
man or the carnivora. The pig seems to show 
all these characteristics to a smaller degree 
than the other ungulates, as reported by Dr. 
Hamlin. 
The rat embryo has also been used extensively 
for tissue cultures, which in this case are more 
