Selective tolerance of group III and IV somatosympathetic reflexes to the effects of alfentanil.

The effect of alfentanil on responses in renal sympathetic nerves evoked by supramaximal electrical stimulation of the radial nerve, has been observed in 6 dogs anaesthesized with alpha-chloralose, paralysed with suxamethonium and ventilated artificially. During an initial infusion of alfentanil the responses of the late group IV (C fibre) and early group III (A delta) were abolished by mean doses of 68 micrograms kg-1 (SEM 3.2 micrograms kg-1) and 797 micrograms kg-1 (SEM 120 micrograms kg-1), respectively. Recovery was allowed to occur to approximately 50% of control values (mean time 76 +/- 14.3 min). The preparations were then conditioned with 7 incremental doses from 7.5 to 120 micrograms kg-1 (i.v.) (total dose 308.5 micrograms kg-1), administered at intervals of 10 min, and subsequently tested with large bolus doses (up to 2000 micrograms kg-1) of alfentanil. In two preparations, the responses of both group IV and group III became completely tolerant to the effects of alfentanil while in the other four the response of the group IV was still eliminated by the drug and the response of group III showed selective tolerance. The heart rate and arterial pressure were reduced by 45 and 29%, respectively during the initial infusion of alfentanil. Thereafter there were no further significant changes in the circulation until the administration of naloxone (2 mg i.v.), which restored the sympathetic responses, heart rate and arterial pressure to control values.