Effect of intermediate-dose naloxone on cardiovascular and sympathoneural adjustments to exercise.

The effects of intermediate-dose naloxone on sympathetic nerve activity and cardiovascular adjustments to exercise were examined in two series of experiments. In the first series 10 normal male volunteers, mean age 28 +/- 5 (SD) years received i.v. naloxone, mean 0.28 +/- 0.6 mg/kg in 0.1 mg/kg aliquots 60-90 min after 45 min of submaximum treadmill exercise. Naloxone had no detectable effect on supine blood pressure, heart rate, plasma norepinephrine, or epinephrine concentrations or muscle sympathetic nerve burst frequency at rest or during the strain phase of the Valsalva manoeuvre, but decreased slightly sympathetic burst incidence at rest (p less than 0.05). In the second study, 8 of these subjects repeated the exercise protocol 15 to 20 min after 0.1 mg/kg i.v. naloxone. Supine blood pressure, heart rate, plasma norepinephrine and epinephrine concentrations before and 15 min after naloxone were virtually identical. A comparison of results from both study days did not reveal a naloxone effect on blood pressure during or up to 60 min after exercise, whereas the heart rate response to exercise was attenuated (p less than 0.002). Intermediate-dose naloxone has no apparent effects on blood pressure during and after exercise, attenuates the chronotropic response to exercise, and has only modest inhibitory effects on muscle sympathetic nerve activity.