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DUAL EFFECTS OF CLONIDINE ON RAT PREJUNCTIONAL α‐ADRENOCEPTORS
Author(s) -
Medgett I. C.,
Rand M. J.
Publication year - 1981
Publication title -
clinical and experimental pharmacology and physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.752
H-Index - 103
eISSN - 1440-1681
pISSN - 0305-1870
DOI - 10.1111/j.1440-1681.1981.tb00757.x
Subject(s) - clonidine , phentolamine , stimulation , endocrinology , medicine , chemistry
SUMMARY 1. The effects of clonidine (1.0 μmol/l) and phentolamine (3.0 μmol/l) on the stimulation‐induced efflux of radioactivity were assessed in rat isolated atria and tail arteries in which transmitter stores had been labelled with ( 3 H)‐noradrenaline. 2. In atria, clonidine did not affect transmitter release when stimulation was for 10 s periods at frequencies of 1, 2 or 5 Hz, whereas at 10 Hz there was a significant 1.2‐fold enhancement of release. Phentolamine enhanced release with stimulation at 5 Hz and 10 Hz by 1.5‐ and 2.5‐fold respectively. 3. In tail arteries, clonidine reduced release when stimulation was for 30 s periods at a frequency of 1 Hz, but did not affect release at 2 Hz or 5 Hz. Phentolamine in each case produced an approximately 1.5‐fold enhancement of release. 4. In the pithed rat, spinal stimulation of cardioaccelerator fibres produced a tachycardia, the magnitude of which depended on the frequency and duration of stimulation. Clonidine (10jug/kg, i.v.) caused either a reduction or an enhancement of the response depending on the frequency and duration of stimulation; in the presence of phentolamine (2 mg/kg, i.v.), clonidine had no effect. 5. The results are consistent with partial agonistic actions of clonidine at prejunctional a‐adrenoceptors in the rat cardiovascular system; such effects may be of relevance to the mechanism of the clonidine withdrawal syndrome.