Influence of oestrous cycle and pregnancy on the reactivity of the rat mesenteric vascular bed

In isolated, perfused mesenteric vascular beds from female rats, it was ass essed whether the constrictor response to cirazoline, an alpha(1)-adrenergi c agonist, or acetylcholine (ACh)-induced relaxation was altered by oestrou s cycle or pregnancy and the ability of nitric oxide (NO), prostanoids and endothelium-derived hyperpolarizing factor (EDHF) to modulate these respons es. Mesenteries, removed from female rats on each oestrous cycle day and ge station day 16, were perfused with physiological salt solution. Tone was in duced with cirazoline (1 mu mol/l), and concentration-response curves to AC h generated. Responsiveness to ACh was tested in the presence of N-omega-ni tro-L-arginine (L-NA), ibuprofen (IBU) and tetrabutylammonium (TBA), to inh ibit nitric oxide synthase (NOS), cyclo-oxygenase and K+ channels respectiv ely. Cirazoline-induced tone was smaller in pro-oestrous and pregnant group s, but the increase in tone to L-NA was larger in pregnant compared with oe strous and dioestrous groups. Control responses to ACh were not different, but L-NA attenuated the response in virgin groups only. IBU did not affect the ACh response, but TEA attenuated it in all groups. When TEA was introdu ced first, ACh-induced dilatation was significantly reduced and not altered by L-NA addition, These results suggest that in the mesenteric vascular be d from cycling and pregnant rats, EDHF is the major mediator of ACh-induced dilatation and NOS may be up-regulated in pregnant and pro-oestrous rats.