Sex differences in respiratory and cardiovascular effects of beta-endorphin

An attempt was made to determine sex differences in the effects of beta-endorphin (beta-EP) on respiratory and cardiovascular systems using Wistar rats. By the intracerebroventricular administration of beta-EP (0.18 mg/kg) to normal male and female rats, respiratory rate (RR), heart rate (HR) and mean arterial blood pressure (MABP) were depressed significantly (p < 0.01 or 0.001) and CO2 in expired gas increased significantly (p < 0.01). These suppressive effects of the peptide were transiently blocked by the intravenous injection of naloxone (0.2 mg/kg). No differences in the effects of beta-EP between estrous and diestrous female rats could be detected. The effects of the peptide were significantly stronger in RR, HR and MABP for females than for males (0.001 < p < 0.05). Testectomized rats showed suppressive effects of the peptide to the same extent as intact females, but the effects in ovariectomized rats did not differ from those for intact females. In testectomized and ovariectomized rats treated with testosterone, the former showed the same results as intact males, but not the latter. The suppressive effects of beta-EP on the respiratory and cardiovascular systems are thus remarkably relieved by androgen in male rats.