Selective depression by alfentanil of group III and IV somatosympathetic reflexes in the dog.

The effect of alfentanil on sympathetic reflexes evoked by supramaximal electrical stimulation of the radial nerve has been observed in five dogs anaesthetized with alpha-chloralose, paralysed with suxamethonium and artificially ventilated. In five dogs during the infusion of alfentanil at a rate of 20 micrograms kg-1 min-1 the late long latency sympathetic response evoked by unmyelinated fibres (group IV, C) was abolished at a mean dose of 68.8 (SE 2.85) micrograms kg-1. The infusion rate was then increased to 200 micrograms kg-1 min-1 and the early, short latency response evoked by small myelinated fibres (group III, A delta) was eliminated at mean total dose of 809 (SE 72) micrograms kg-1. When the infusion was stopped the group III reflex returned within 1-5 min and recovery to approximately 50% of control for both reflexes occurred within 15-60 min in different preparations. Mean arterial pressure and mean heart rate decreased from 140 (6) mm Hg and 132 (11) beat min-1 to 100 (7) mm Hg and 70 (6) beat min-1, respectively, by the time the group IV response was eliminated; that is, after a mean infusion time of 3.4 min. Thereafter, there was no further cardiovascular depression. Within 3 min of the administration of naloxone 2 mg i.v., the sympathetic reflexes, arterial pressure and heart rate returned to within control values.