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Circ Res. 1995 Aug;77(2):326-34.

Attenuation of the exercise pressor reflex. Effect of opioid agonist on substance P release in L-7 dorsal horn of cats.

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Department of Internal Medicine, University of Texas Southwestern Medical Center at Dallas, USA.


Using alpha-chloralose-anesthetized cats, we studied blood pressure and heart rate responses to static contraction and passive stretch of the triceps surae muscle before and after microdialyzing the mu-opioid agonist [D-Ala2]-methionine enkephalinamide (DAME, 200 mumol/L) into the L-7 dorsal horn of the spinal cord. In addition, we measured contraction-induced substance P release in the dorsal horn before and after drug delivery. After 92 +/- 3 minutes of dialyzing the opioid agonist, contraction-induced increases in mean arterial pressure and heart rate were attenuated from control values of 58 +/- 7 mm Hg and 17 +/- 3 beats per minute to postdrug values of 27 +/- 7 mm Hg and 10 +/- 2 beats per minute, respectively. A similar attenuation was observed for the passive muscle stretches after 97 +/- 5 minutes of dialysis (control, 38 +/- 4 mm Hg and 8 +/- 2 beats per minute; after drug, 23 +/- 4 mm Hg and 5 +/- 1 beats per minute). Prior microdialysis of naloxone (300 mumol/L), a mu-antagonist, blocked this effect, suggesting that the opioid agonist has a specific receptor action. Naloxone alone had no effect on the pressor or tachycardiac responses. The contraction-induced increase in substance P-like immunoreactivity was reduced from a control value of 0.119 +/- 0.024 to 0.047 +/- 0.010 fmol/100 microL by DAME. Time-control experiments revealed no decrease in the release of substance P-like immunoreactivity. Thus, activation of opioid receptors modulates the transmission of group III and IV muscle afferent nerve activity through the L-7 dorsal horn.(ABSTRACT TRUNCATED AT 250 WORDS).

[Indexed for MEDLINE]

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