Cardiovascular and respiratory effects of beta-endorphin in anesthetized and conscious rats

This study deals with the effects of beta-endorphin on blood pressure, heart rate, and respiratory frequency in anesthetized and conscious rats after both peripheral and central administration. Intravenous injection of beta-endorphin (40 and 160 micrograms/kg) in urethane-anesthetized rats resulted in a prolonged decrease in blood pressure, which, after the higher dose, was accompanied by bradycardia. In the pentobarbitone-anesthetized animals the same doses of beta-endorphin caused a small rise in blood pressure without affecting heart rate. No effect on respiratory frequency was observed. In urethane-anesthetized rats with hemorrhagic shock intravenous naloxone elicited a small pressor effect. Intracerebroventricular injection (i.c.v.) of beta-endorphin (40 micrograms/rat) in urethane-anesthetized rats resulted in severe respiratory depression and death. With lower doses there was a decrease in all parameters studied without ensuing death. Respiratory depression was even more pronounced in pentobarbitone-anesthetized rats, occurring with much lower doses of the peptide than in the urethane-anesthetized animals. beta-endorphin in the dose of 0.6 micrograms/rat still reduced respiratory frequency as well as heart rate without affecting blood pressure. Naloxone pretreatment antagonized the respiratory depression and death, while the decrease in heart rate was diminished. In conscious rats 10 microgram/rat, i.c.v., of beta-endorphin resulted in a decrease in heart rate and blood pressure without affecting respiratory frequency. It is concluded that beta-endorphin, or related opioid peptides, may be involved in cardiovascular and respiratory regulation. The effects of beta-endorphin, however, are markedly modified by the anesthetic agent used.