STINSON, RAHMOELLER, TEOKLENBERG, COLVIN, JONES AND PIERCE 
433 
Denervated animals, in contrast, demon- 
j strated additional and significant elevation of 
heart rate, cardiac output, developed LV pres- 
sure, LV dp/dt, and maximum systolic — dD/dt 
and VcF- 
Maximal mean values in these parameters 
were attained be^^v^^een two and three minutes of 
exercise. At this time differences between de- 
nervated and normal dogs for cardiac output, 
maximum systolic —dD/dt and Vcf, and per- 
cent systolic shortening of transverse diameter 
were statistically insignificant, although mean 
values tended to be higher for normal subjects 
(Table I). Furthermore, end-diastolic diameter 
and LVEDP showed slight decreases in dener- 
vated dogs during late exercise, but these trends 
were of borderline statistical significance. 
Recovery 
Following cessation of exercise, denervated 
dogs showed proportionately more gradual de- 
creases in all parameters, and, in fact, reversal 
of differences between normal and denervated 
subjects was observed one minute after exercise 
for heart rate and cardiac output (P > .05). 
LV diameter tended to increase immediately 
after exercise in normal animals, whereas in de- 
nervated dogs it tended to decrease. By five 
minutes after exercise, return to or near control 
levels was observed for all parameters in both 
groups. 
DISCUSSION/SUMMARY 
The results of these studies confirm the abil- 
ity of the regionally denervated heart to re- 
spond in a directionally appropriate manner to 
the metabolic demands of muscular exercise. 
They also illustrate the primary dependence of 
the denervated heart on the Frank-Starling 
mechanism as a means of increasing output, es- 
pecially in the early stages of exertion. In gen- 
eral, however, the pumping performance (out- 
put) of denervated hearts during running was 
observed to be slightly inferior to that of nor- 
mal hearts, even though comparable levels were 
attained. It is not clear why this discrepancy 
with the data of Donald and Shepard, who 
found no compromise of steady-state cardiac 
output in denervated dogs during light 
exercise,^ was observed. Others, though, have 
noted substantial differences between dener- 
vated and normal animals in cardiac output 
during treadmill exercise.^" Possible factors in 
the present study include the breed of animal 
used and degree of training, differences in oxy- 
gen consumption or actual work performed be- 
tween normal and denervated dogs, and the in- 
fluence of excitement during the control state. 
Although oxygen consumption was not meas- 
ured in the present studies, a consistent attempt 
was made to insure that all subjects ran freely 
on the treadmill without restraint or pulling at 
a constant speed and elevation. Such standardi- 
zation of effort has been shown to result in com- 
parable levels of oxygen consumption during 
work in different animals and from run to run.^ 
The effects of excitement on cardiovascular per- 
formance are difficult to differentiate from phys- 
iological responses to exercise, but have been 
shown by Barger to be associated with higher 
cardiac outputs both at rest and during running 
in normal dogs." The mild tachycardia and rela- 
tively high stroke change in diameter (percent 
systolic shortening) seen in our normal dogs 
immediately prior to running suggest that ex- 
citement may indeed have been present and may 
have influenced the absolute level of cardiac 
output achieved during exercise. Obviously, the 
effects of excitement would tend to be atten- 
uated in dogs deprived of direct neural cardiac 
control. 
In addition to the initial directional changes 
in left ventricular diameter and end-diastolic 
pressure at the onset of exercise, denervated 
dogs differed markedly from normal in the 
early acceleration of myocardial contraction ve- 
locity. Indeed, during the first 30 seconds of 
running minimal increases in the rate of diame- 
ter shortening occurred, and maximal improve- 
ment did not develop until the second or third 
minute of running. In contrast, normal animals 
showed rapid acceleration of this function, as 
well as other rate-dependent parameters, to 
maximal or near maximal levels within the first 
30 seconds. The gradual augmentation of con- 
traction velocity in denervated dogs was asso- 
ciated with parallel elevation of heart rate and 
cardiac output. Such observations suggest a 
