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Department of Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 2S2
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 (~150-200 µm) and venules (<250 µm) from rats anesthetized with
pentobarbital sodium were mounted in a pressurized myograph. Mesenteric
arterial diameter decreased from 146 ± 6 to 133 ± 6 µm on
raising intraluminal pressures from 80 to 120 mmHg. This response was
enhanced in the presence of
N
-nitro-L-arginine methyl ester
(L-NAME; 139 ± 6 to 112 ± 7 µm). There was no
such myogenic response in the splenic arterioles, except in the
presence of L-NAME (194 ± 4 to 164 ± 4.2 µm).
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 ± 30.9 µm; passive in
Ca2+-free media, 209 ± 31 to 344 ± 27 µm;
4-8 mmHg) compared with the splenic vessels (active, 169 ± 11 to 184 ± 16 µm; passive, 187 ± 12 to 207 ± 17 µm). 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.
arteriole; venule; blood volume
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