Proceedings of The Physiological Society

University of Leeds (2002) J Physiol 544P, S013

Communications

Effects of sympathetic impulse numbers on contributions of noradrenaline and ATP to vasoconstriction in rat tail and femoral arteries

Eamonn Bradley and Christopher D. Johnson

Department of Physiology, Medical Biology Centre, 97 Lisburn Road, Belfast BT9 7BL


In vitro studies reported that responses to sympathetic vasoconstrictor stimulation with single or a few electrical impulses are mainly mediated by ATP (Bao, 1993). When impulse numbers increase to ?ge? 20, noradrenaline (NA) provides the main contribution. However, in vivo, contributions of NA dominated responses of tail and hindlimb vasculature to stimulation with both couplets and short trains (2 and 20 impulses at 20 Hz; Johnson et al. 2001). Here we studied how the number of impulses delivered alters contributions of NA and ATP to responses evoked in tail and femoral arteries, and whether responses gained in vitro are the same as those gained in vivo.

Male Sprague-Dawley rats (250-350 g) were killed humanely. Isometric contractile responses of excised tail and femoral arteries to electrical field stimulation were compared. Batteries of different impulse numbers (1-100 impulses at 20 Hz, 1 ms pulses, supramaximal) were applied to endothelium-denuded vessels in the presence of antagonists for NA or ATP (phentolamine, 2 X 10-6 M and suramin, 10-4 M).

Electrical stimuli evoked responses in tail artery (n = 12) around 16 times greater than in femoral artery (n = 7; significant from ?ge? 4 impulses, P < 0.001, ANOVA and SNK). In both arteries, absolute responses to stimuli were markedly reduced by phentolamine (significant at all impulse numbers in tail, P < 0.001). Relative responses to two impulses were reduced by 82 ± 4 % (mean ± S.E.M.) and 87 ± 13 %, respectively, whilst responses to 20 impulses were reduced by 93 ± 2 and 85 ± 7 %, respectively. When suramin was given first, absolute responses were reduced (in tail at impulse numbers ?ge? 6, P < 0.001 for each; in femoral artery at 100 impulses, P < 0.001). Relative responses to two impulses were reduced by 82 ± 6 and 75 ± 18 % in tail and femoral arteries, respectively, but responses to 20 impulses were reduced by only 31 ± 11 and 58 ± 27 %, respectively. Relative decreases with suramin in response to two impulses were significantly different from decreases in response to 20 impulses in tail arteries (P < 0.001). Responses remaining after the first antagonist were usually abolished after subsequent addition of the second.

We confirm that in tail artery, ATP contributes more to responses to few impulses and NA contributes more to greater numbers. We extend these observations to femoral artery, which shows a greater reliance on ATP at higher numbers of impulses than tail artery. This is in contrast to in vivo studies in tail and hindlimb vascular beds, confirming that conclusions from in vitro studies must be applied to the in vivo situation with caution.

We thank Dr Sean Roe for his help in preparing this abstract.

All procedures accord with current UK legislation.

Where applicable, experiments conform with Society ethical requirements