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1 Physiology, University of Alberta, Edmonton, Alberta, Canada
* To whom correspondence should be addressed. E-mail: susan.jacobs{at}ualberta.ca.
In the rat, the spleen is a major site of fluid efflux out of the blood. By contrast, the mesenteric vasculature serves as a blood reservoir. We proposed that the compliance and myogenic responses of these vascular beds would reflect their different functional demands. Mesenteric and splenic arterioles (~150-200µm) and venules (<250µm) from rats anesthetized with Sodium Pentobarbital were mounted in a pressurized myograph. Mesenteric arterial diameter decreased from 146±6 to 133±6µm upon raising intraluminal pressures from 80-120mmHg. This response was enhanced in the presence of L-NAME (139±6 to 112±7µm). There was no such myogenic response in the splenic arterioles, except in the presence of L-NAME (194±4 to 164±4.2µm). We propose that, whereas mesenteric arterioles exhibit myogenic responses, this is normally masked by NO-mediated dilation in the splenic vessels. The mesenteric venules were highly distensible (Active: 184±15 to 320±30.9µm; Passive in Ca2+-free media: 209±31 to 344±27µm; 4-8mmHg) compared with the splenic vessels (Active: 169±11 to 184±16µm; Passive: 187±12 to 207±17µm). We conclude that, in response to an increase in perfusion pressure, mesenteric arterial diameter would decrease to limit the changes in flow and microvascular pressure. In addition, mesenteric venous capacitance would increase. By contrast, splenic arterial diameter would increase, while there would be little change in venous diameter. This would enhance the increase in intrasplenic microvascular pressure and increase fluid extravasation.
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