Endogenous opioids and baroreflex control in humans.

We describe two studies designed to elucidate the role of endogenous opioids in blood pressure control in humans. In the first study, nine normal subjects received infusions of DAMME (a metenkephalin analog), naloxone, or saline, and blood pressure, heart rate, and plasma norepinephrine concentration were determined supine and following 5 minutes of 70 degrees head-up tilt at intervals for 6 hours. Blood pressure following tilt was significantly decreased by DAMME but not influenced by naloxone, the effect being most marked at 3 hours (placebo = 110 +/- 6/78 +/- 7 mm Hg; naloxone = 106 +/- 10/79 +/- 5 mm Hg; DAMME = 96 +/- 16/67 +/- 8 mm Hg (p less than 0.01). However, heart rate and plasma norepinephrine did not rise in response to this hypotension. Heart rates at 3 hours were: placebo = 87 +/- 16 bpm; naloxone = 88 +/- 19 bpm; DAMME = 89 +/- 23 bpm. Plasma norepinephrine levels (nmol/liter) at 3 hours were: placebo = 6.0 +/- 2.2; naloxone = 5.8 +/- 1.9; DAMME = 6.0 +/- 1.9. In the second study, seven normal subjects had blood pressure reduced by incremental infusions of sodium nitroprusside, and the effects of placebo, naloxone, and DAMME on the slope of the heart period/blood pressure relationship investigated. Naloxone significantly increased the slope by 90% and DAMME significantly reduced the slope by 30%. It is concluded that endogenous opioids modulate the baroreflex control of blood pressure in normal humans.