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

Abstract

1. The mechanisms of vascular adaptation to pregnancy remain to be fully elucidated. In this study we L-NAME, n = 8; n.s.). This was also reflected in a greater response to shear stress in the arteries from pregnant animals. 5. We conclude that myogenic tone is not significantly different in pregnant and non-pregnant resistance sized mesenteric arteries of Wistar rats. However, flow-induced dilatation, mediated by nitric oxide release, may play a major role in lowering vascular resistance during pregnancy.In normal human pregnancy the cardiac output rises by 40% and the blood volume by 45-55%, but despite this, the blood pressure falls reaching its nadir in the second trimester (Halligan et al. 1993). The lowering of blood pressure is achieved by a substantial reduction in the peripheral vascular resistance, but the pathways underlying this physiological adaptation to the state of pregnancy remain unknown. 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,

Flow-induced dilatation in isolated resistance arteries from control and streptozotocin-diabetic rats

“…The magnitude of the dilatation elicited by flow in these preconstricted control arteries (13.4 %) was comparable to that observed in non-preconstricted rat cerebral arteries [21] and mesenteric arteries from pregnant rats [14]. The response to flow was biphasic, vessels dilated at low flow rates but at higher flow rates there was a tendency for vessel diameter to decrease.…”

“…The maximum flow-induced dilatation has varied greatly between studies, an observation which may reflect variation among arteries to develop and maintain spontaneous tone. Thus, mesenteric arteries are reported to develop 14 % tone at an intraluminal pressure of 80 mmHg [14] whereas gracilis arteries (which exhibit greater vasodilatation) develop approximately 60 % myogenic tone [23]. The dependence of flow responses on tone has also been reported in studies of rabbit ear arteries [26] and rat skeletal arteries [25], the greater the initial tone, the more pronounced the subsequent flow-induced dilatation.…”

“…Endothelium-dependence of flow-mediated dilatation has been demonstrated in most [14,21,23,24,29] but not all [18] arteries studied. In this study, dilatation was predominantly NO-mediated, as the response was abolished by the NO synthase inhibitor, l-NAME.…”

“…There are fewer studies of this phenomenon in the resistance vasculature, although flow-induced dilatation has been observed in small mesenteric arteries in vivo [22], isolated small mesenteric arteries from pregnant rats [14], resistance sized arteries from rat skeletal muscle [23--25], rabbit ear arteries [26] and small coronary vessels [27]. The maximum flow-induced dilatation has varied greatly between studies, an observation which may reflect variation among arteries to develop and maintain spontaneous tone.…”

Determination of an Optimal Axial‐Length Tension for the Study of Isolated Resistance Arteries on a Pressure Myograph

“…Myogenic tone was determined as the percentage change of the i.d. in the Ca¥-free PSS (Cockell & Poston, 1996).…”