Myogenic and flow-mediated responses in isolated mesenteric small arteries from pregnant and nonpregnant rats

Isolated mesenteric arteries from pregnant rats show enhanced flow‐mediated relaxation but normal myogenic tone.

“…**P < 0-02, *P < 0 05 vs. L-NAME. (Learmont et al 1996). In agreement with other studies, myogenic tone was not demonstrable in these larger vessels without prior constriction to a vasoconstrictor agonist (VanBavel & Mulvany, 1994 Meyer et al (1993), there is to our knowledge, only one previous investigation which has Figure 5.…”

“…The vessel was then incubated in PSS and NAl-nitro-L-arginine methyl ester (L-NAME; 100 /M; Sigma) for 20 min and the flow protocol then repeated in the continued presence of L-NAME. We have previously shown in this vascular bed that L-NAME is an effective inhibitor of endothelialdependent relaxation (Learmont et al 1996). Reproducibility of the flow response was ascertained in a separate group of arteries in which, after the initial flow protocol, the vessel was maintained in PSS for 20 min at 80 mmHg pressure in the absence of intraluminal flow and the protocol then repeated.…”

“…A number of contributory mechanisms have been proposed, including a reduction of myogenic tone (Meyer, Brayden & McLaughlin, 1993) and an increase in vasodilatation through enhanced release of nitric oxide (NO). A role for NO has been implied by the increased response to endothelium-dependent vasodilator agonists, including acetylcholine (ACh) in the carotid and uterine arteries of the pregnant guinea-pig (Weiner, Martinez, Liu, Ghodsi & Chestnut, 1989) and bradykinin in small mesenteric arteries of the pregnant rat (Learmont, Cockell, Knock & Poston, 1996). In turn, it is hypothesized that this may arise from oestrogen-induced stimulation of eNOS, the calcium-dependent isoform of NO synthase associated with the vascular endothelium (Weiner, Lizasoian, Bayliss, Knowles, Charles & Moncada, 1994).…”

Vascular adaptations to pregnancy in mice: effects on myogenic tone

“…In contrast, others (8,24) have revealed no change in myogenic tone in mesenteric vessels obtained from pregnant rats. Interestingly, it was demonstrated that the presence of intraluminal flow was necessary to reduce myogenic tone in mesenteric arteries from late pregnant rats (31), suggesting that pregnancy alters the interaction between intraluminal forces (pressure and flow) with the arterial wall, thereby causing an overall reduction of vascular tone in pregnancy (31).…”

“…Some studies have demonstrated reduced myogenic tone in mesenteric arteries (30) and small renal arteries (14) from pregnant rats. However, others (8,24) have reported no change of myogenic tone in mesenteric arteries obtained from late pregnant rats. Results from these latter studies suggested that flow-induced dilation, mediated by nitric oxide (NO), may be involved in vasodilation of pregnancy rather than a reduction of myogenic tone…”