Journal of Applied Physiology, Vol 62, Issue 2 673-678, Copyright © 1987 by American Physiological Society
Effects of naloxone on breathing movements during hypercapnia in the fetal lamb
S. A. Joseph, I. C. McMillen and D. W. Walker
To determine whether endogenous opioids influence the fetal breathing
response to CO2 we have investigated the effect of the opiate antagonist,
naloxone on the incidence, rate, and amplitude of breathing movements
during hypercapnia in fetal lambs in utero. In 20 experiments in six
pregnant sheep (130-145 days gestation) hypercapnia was induced by giving
the ewe 4-6% CO2-18% O2 in N2 to breathe for 60 min. After 30 min of
hypercapnia either naloxone (13 experiments) or saline (7 experiments) was
infused intravenously for the remaining 30 min. During hypercapnia breath
amplitude increased from 5.8 +/- 0.5 to 9.1 +/- 1.2 mmHg (P less than
0.001), and infusion of naloxone was associated with a further significant
increase to 15.7 +/- 1.2 mmHg (P less than 0.001). Naloxone had no effect
on the incidence or rate of breathing movements during hypercapnia. After
hypercapnia there was a significant decrease in the incidence of fetal
breathing movements in the naloxone group (14.7 +/- 3.2%). Infusion of
saline during hypercapnia had no effect on incidence, rate, or amplitude of
fetal breathing movements. These results suggest that endogenous opioids
act to suppress or limit breath amplitude during hypercapnia but do not
affect rate or incidence of breathing movements.
