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Department of Physiology, The Medical School, University of Birmingham, Birmingham, United Kingdom B15 2TT
In
anesthetized rats, we characterized the contributions of norepinephrine
(NE) and ATP to changes in tail and hindlimb (femoral) vascular
resistances (TVR and FVR, respectively) evoked by three patterns of
sympathetic stimulation: 1) couplets (2 impulses at 20 Hz),
2) short trains (20 impulses at 20 Hz), and 3) a
natural irregular pattern previously recorded from a sympathetic fiber innervating the rat tail artery. All stimuli evoked greater
changes in TVR than FVR. Judging from the effects of the
-adrenoceptor antagonist phentolamine, the purinergic receptor
antagonist suramin, or
,
-methylene ATP (which desensitizes P2X
receptors), we propose that NE has a major role in the constriction
evoked by the couplet, as well as by the short train and by the low-
and high-frequency components of the natural pattern, but that
considerable synergy occurred between the actions of ATP and NE. This
contrasts with previous in vitro studies that indicated that ATP
dominates vascular responses evoked by sympathetic stimulation with a
few impulses at low frequency and that NE dominates responses to longer
trains or at high frequencies.
cotransmission; synergy; cutaneous circulation; skeletal muscle circulation
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