ROLE OF CONSTITUTIVE AND INDUCIBLE ENDOTH ELIAL NITRIC-OXIDE SYNTHASES IN THE ANGIOTENSIN-II-INDUCED CONTRACTILE RESPONSE IN WKY AND SHR RAT ISOLATED AORTA

Dysfunctions of EDRF/L-arginine-NO pathway have been demonstrated in g enetic and experimental hypertension. NO is produced through the conve rsion of L-arginine to L-citruline by NO synthases (NOS) which exist a t least in two isoforms. The first termed constitutive (NOSc) and loca ted in the endothelium of the vascular wall results in the basal and s timulated NO production. The second termed inducible (NOSi), which pro duces large amounts of NO, can be expressed in both smooth muscle and endothelial cells. The aim of the study was to examine and compare in isolated aortic rings of WKY and SHR rats, the activity of the two iso forms of endothelial NO synthases and their influence on the constrict or response induced by angiotensin II. On phenylephrine preconstricted endothelium intact aortic rings (10(-6) M, WKY = 1.2 +/- 0.04 g; SHR = 1.2 +/- 0.07 g; n = 7), carbachol (10(-5) M) induced a greater relax ation in WKY (84 +/- 2.5 %) than in SHR (63 +/- 8.5 %) rat. This sugge sts the presence of a low NOSc stimulated activity in the hypertensive strain. When the incubation period was limited to 30 min after equili bration period, L-arginine (3.10-4 M) did not induce relaxation. When the incubation period was prolonged (180 min), L-arginine induced a re laxation (WKY = 75 +/- 8 %; SHR = 58 +/- 10 %; n = 7). This relaxation was not observed in a medium containing actinomycin D (10(-6) M) or a fter endothelium removal, indicating the induction of an endothelial N OSi in the two strains.The contractile responses to All (10(-7) M; WKY = 0.49 +/- 0.04 g; SHR = 0.59 +/- 0.08 g), after a 30 min incubation period, were similarly enhanced in presence of Nitro-L-Arginine (NLA; 10(-4) M) or in denuded rings (WKY = + 119 % and + 63 % ; SHR = + 130 % and + 42 %). These results provided evidence that basal NOSc modulat ion was similar in WKY and SHR rats. After a 180 min incubation period , All induced constrictor response was significantly reduced (WKY = 0. 27 +/- 0.07 g; SHR = 0.37 +/- 0.11 g). Addition of cycloheximide (7. 1 .10(-5) M) to the medium or endothelium removal prevented the desensit ization phenomenon. This assessed the participation of the endothelial NOSi in the time dependent reduced contraction of All in rat aorta. M oreover, the prolonged inhibition (180 min) of NO synthesis by NLA pot entiated the contractile response to All in SHR, but not in WKY rings as compared to the 30 mn incubation period. This result is consistent with a specific liberation of vasoconstrictor factors by SHR. These re sults indicate that SHR aorta has a lower stimulated NOSc but a simila r basal NOSc in comparison to WKY. Prolonged incubation of rings allow s the expression of endothelial NOSi in the two strains, but resulted in the release of vasoconstrictor factors in only SHR when the NO synt hesis is inhibited.