The object of the present study was to shed some light on the neurotransmitters that mediate the excitatory response of the circular muscle of final branches of mesenteric artery in bovine calves. Mesentery was dissected and the iliac branches were separated and used in the experiment. The final mesenteric arteries of diameter 400 micrometers and less responded strongly to norepinephrine and moderately to ATP.
However, mesenteric branches of wider diameters were gradually less responsive to norepinephrine and those of diameter 700 micrometers were exclusively nonresponsive. These arteries were strongly responsive to ATP (100 μM). Norepinephrine response was sensitive to phentolamine (3 μM) and prazosin (5 μM) indicating that it is mediated by α1 receptor; while ATP response was sensitive to suramin (200 μM), PPADS (50 μM), but not to cibacron blue (100 μM) indicating that it is mediated via P2X receptor.
Confirmative experiments were performed including application of α1 and P2X receptor specific agonists which are methoxamine and α,β-methylene ATP. Methoxamine (1 μM) showed effects similar to norepinephrine in final branches and was without effect in wider branches. α,β-methylene ATP (1 μM), exhibited more pronounced effects on both wide and narrow branches but in parallel manner to that of ATP. Agonists for α2 and P2Y receptors exemplified by clonidine (10 μM) and 2-meThio ATP (10 μM), respectively, were without effect indicating that involvement of these receptors is unlikely. The neuropeptide-Y (200 nM) didn’t have any effects on either the narrow or the wide rings.
These data may imply that in the finest mesenteric arteries a strong pressor power represented by norepinephrine and ATP integration is needed for maintaining peripheral resistance; on the other hand a mild pressor power mediated by only ATP is needed to maintain the vascular tone in the wider mesenteric branches. |
