Attenuation of reflex pressor and ventilatory responses to static muscular contraction by intrathecal opioids

We have tested the hypothesis that intrathecal injections of opioid peptides attenuate the reflex pressor and ventilatory responses to static contraction of the triceps surae muscles of chloralose-anesthetized cats. We found that before intrathecal injections of [D-Ala²]Met-enkephalinamide (100 μg in 0.2 ml), static contraction increased mean arterial pressure and ventilation by 32 ± 5 (SE) mmHg and 227 ± 61 (SE) ml/min, whereas after injection of this opioid peptide, static contraction increased mean arterial pressure and ventilation by only 15 ± 5 mmHg and 37 ± 33 ml/min, respectively. The attenuation of both the pressor and ventilatory responses to static contraction by [D-Ala²]Met-enkephalinamide were statistically significant (P < 0.05). Moreover, the attenuation was probably not caused by an opioid-induced withdrawal of sympathetic outflow because [D-Ala²]Met-enkephalinamide had no effect on the pressor and ventilatory responses evoked by high-intensity electrical stimulation of the central cut end of the sciatic nerve. In addition, intrathecal injection of peptides that were highly selective agonists for either the opioid μ- or δ-receptor attenuated the reflex response to static contraction. Naloxone (1,000 μg), injected intrathecally, prevented the attenuation of the reflex responses to contraction by opioid peptides. We speculate that the opioid-induced attenuation of the reflex pressor and ventilatory responses to static contraction may have been due to suppression of substance P release from group III and IV muscle afferents.