SEX-DIFFERENCES IN THE RELATIVE CONTRIBUTIONS OF NITRIC-OXIDE AND EDHF TO AGONIST-STIMULATED ENDOTHELIUM-DEPENDENT RELAXATIONS IN THE RAT ISOLATED MESENTERIC ARTERIAL BED

1 We have used the isolated, buffer-perfused, superior mesenteric arte rial bed of male and female rats to assess the relative contributions of nitric oxide (NO) and the endothelium-derived hyperpolarizing facto r (EDHF) to endothelium-dependent relaxations to carbachol. 2 Carbacho l caused dose-related relaxations of methoxamine-induced tone in mesen teric vascular beds from male rats described by an ED50(M) of 0.43+/-0 .15 nmol and a maximum relaxation (R-max(M)) of 89.6+/-1.2% (n=28) whi ch were not significantly different from those observed in mesenteries from female rats (ED50(F)=0.72+/-0.19 nmol and R-max(F)=90.7+/-0.9%; n=22). 3 In the males, the addition of 100 mu M N-G-nitro-L-arginine m ethyl ester (L-NAME) caused the dose-response curve to carbachol to be significantly (P<0.001) shifted to the right 15 fold (ED50(M)=6.45+/- 3.53 nmol) and significantly (P<0.01) reduced R-max(M) (79.7+/-2.8%, n =13). By contrast, L-NAME had no effect on vasorelaxation to carbachol in mesenteries from female rats (ED50(F)=0.89+/-0.19 nmol, R-max(F)=8 6.9+/-2.3%, n=9). 4 Raising tone with 60 mM KCl significantly reduced the maximum relaxation to carbachol in mesenteries from male rats 2 fo ld (R-max(M)=40.3+/-9.2%, n=4; P<0.001) and female rats by 1.5 fold (R -max(F)=55.3+/-3.3%, n=6; P<0.001), compared with methoxamine-induced tone. The potency of carbachol was also significantly reduced 1.2 fold in preparations from males (ED50(M)=0.87+/-0.26 nmol; P<0.01) but not the females (ED50(F)=4.04+/-1.46 nmol). In the presence of both 60 mM KCl and L-NAME, the vasorelaxation to carbachol was completely abolis hed in mesenteries from both groups. 5 The cannabinoid receptor antago nist SR141716A (1 mu M), which is also a putative EDHF antagonist, had no significant effect on the responses to carbachol in mesenteries fr om males or females (ED50(M)=1.41+/-0.74 nmol, R-max(M)=89.4+/-2.5%, n =7; ED50(F)=2.17+/-0.95 nmol, R-max(F)=89.9+/-1.8%, n=9). In mesenteri es from male rats, in the presence of 100 mu M L-NAME, SR141716A signi ficantly (P<0.05) shifted the dose-response curve to carbachol 8 fold further to the right than that seen in the presence of L-NAME alone (E D50(M)=53.8+/-36.5 nmol) without affecting R-max(M) (72.4+/-4.8%, n=10 ). In mesenteries from female rats, the combined presence of L-NAME an d SR 141716A, significantly (P<0.01) shifted the dose-response curve t o carbachol 7.5 fold, (ED50(F)=6.66+/-2.46 nmol), as compared to L-NAM E alone and significantly (P<0.001) decreased R-max(F) (70.1+/-5.5%, n =8). 6 Vasorelaxations to the nitric oxide donor sodium nitroprusside (SNP), to the endogenous cannabinoid, anandamide (a putative EDHF) and to the ATP-sensitive potassium channel activator, levcromakalim, did not differ significantly between male and female mesenteric vascular b eds. 7 The continuous presence of sodium nitroprusside (SNP; 20-60 nM) had no effect on vasorelaxation to carbachol in mesenteries from eith er males or females. In the presence of L-NAME, SNP significantly (P<0 .05) reduced the potency of carbachol 6 fold, without affecting the ma ximal relaxation in mesenteries from male rats (ED50(M)=40.9+/-19.6 nm ol, R-max(M)=79.4+/-2.5%, n=11). Similarly in mesenteries from female rats, the ED50(F) was also significantly (P<0.01) increased 7 fold (6. 24+/-3.02 nmol), while the R-max(F) was unaffected (81.9+/-11.0%; n=4) . 8 The results of the present investigation demonstrate that the rela tive contributions of agonist-stimulated NO and EDHF to endothelium-de pendent relaxations in the rat isolated mesenteric arterial bed, diffe r between males and females. Specifically, although both NO and EDHF a ppear to contribute towards endothelium-dependent relaxations in males and females, blockade of NO synthesis alone has no effect in the fema le. This suggests that EDHF is functionally more important in females; one possible explanation for this is that in the absence of NO, the r ecently identified ability of EDHF to compensate for the loss of NO, i s functionally more important in females than males.