Roles of norepinephrine and ATP in sympathetically evoked vasoconstrictionin rat tail and hindlimb in vivo

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 stimul ation: 1) couplets (2 impulses at 20 Hz), 2) short trains (20 impulses at 2 0 Hz), and 3) a natural irregular pattern previously recorded from a sympat hetic fiber innervating the rat tail artery. All stimuli evoked greater cha nges in TVR than FVR. Judging from the effects of the alpha -adrenoceptor a ntagonist phentolamine, the purinergic receptor antagonist suramin, or alph a,beta -methylene ATP (which desensitizes P2X receptors), we propose that N E 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 natur al pattern, but that considerable synergy occurred between the actions of A TP and NE. This contrasts with previous in vitro studies that indicated tha t ATP dominates vascular responses evoked by sympathetic stimulation with a few impulses at low frequency and that NE dominates responses to longer tr ains or at high frequencies.