ARTERIAL SIZE DETERMINES THE ENHANCEMENT OF CONTRACTILE RESPONSES AFTER SUPPRESSION OF ENDOTHELIUM-DERIVED RELAXING FACTOR FORMATION

We studied the effect of endothelium-derived relaxing factor (EDRF) on norepinephrine-induced contractile responses and on the tissue guanosine-3',5'-phosphate (cGMP) concentration of isolated rabbit arteries with an increasing endothelium to smooth muscle cell ratio (aorta, femoral and mesenteric arteries). After suppression of EDRF formation (either by N(G)-nitro-L-arginine or, in mesenteric arteries, by saponin), contractions elicited by cumulative doses of norepinephrine were unaltered in aorta but were enhanced by 22.5% in femoral arteries and by 44.3% in mesenteric arteries (at the highest norepinephrine concentration). The cGMP concentration (pmol/mg protein) of unstimulated, endothelium-intact vessels decreased after suppression of EDRF formation from 1.09 +/- 0.24 to 0.74 +/- 0.28 in aortic, from 2.86 +/- 0.4 to 0.61 +/- 0.19 in femoral and from 6.3 +/- 0.9 to 0.7 +/- 0.15 in mesenteric arterial segments. The basal cGMP concentration did not differ in endothelium-denuded segments of these arteries, suggesting a similar basal activity of soluble guanylate cyclase (sGC). A higher sensitivity of sGC may have contributed to the higher cGMP concentration observed in the smaller arteries, since in the presence of sodium nitroprusside the cGMP concentration of endothelium-denuded segments increased 1.8-fold in aortic, 2.9-fold in femoral and 2.4-fold in mesenteric arterial segments. However, these differences in sGC activation cannot be solely responsible for the high basal cGMP concentration in endothelium-intact mesenteric arteries. The greater ratio of endothelium to smooth muscle cell layers in the smaller arteries might result in a higher EDRF concentration in the vascular wall and subsequently in a higher cGMP concentration. In conclusion, these data support the view of a greater importance of EDRF-mediated vascular control in small arteries than in large conduit arteries.