744 
PHARMACOLOGY 
Figure 15. — Histochemical appearance (at the same magnification) of atrial (left) and ventricular (right) myo- 
cardium obtained from a 2-year old sheep (top) and a term. fetal lamb (bottom). The Falck technique was 
employed to produce monoamine fluorescence of the heart's adrenergic nerve fibers. See text for further de- 
scription. 
CONCLUSIONS 
The findings of these investigations illustrate 
that dynamic changes in the anatomic, biochem- 
ical and physiological disposition of cardiac ca- 
tacholamines occur in the perinatal period and 
allow an explanation for age-dependent differ- 
ences in the pharmacological responsiveness of 
ventricular myocardium to norepinephrine, the 
sympathetic neurotransmitter. The hearts of 
the fetuses close to term and youngest neonates 
were found to be supersensitive to norepi- 
nephrine. The histochemical findings supported 
the view that these hearts were partially inner- 
vated with a good deal of the visualized norepi- 
nephrine residing in preterminal nerve trunks 
rather than in terminal nerve endings. Beta re- 
ceptor sensitivity was similar in fetal and adult 
myocardium, indicating that catacholamine re- 
ceptor sites are fully functional before the com- 
plete development of an extrinsic nerve supply. 
The beta adrenergic antagonist, propranolol ex- 
erted a more marked direct depressant action 
on fetal heart when compared to the adult, but 
the degree of beta blockade was similar at both 
ages. At a comparable stage of development, the 
adrenal glands, unlike the heart, were found to 
