Transient regulation of transport by pericytes in post capillary venules

A phenomenon that has defied explanation for nearly two decades is the time scale for transient reabsorption in the classic experiments of Michel and Phillips on individually perfused frog mesentery microvessels. One finds that transient reabsorption lasts < 2 minutes before a new steady-state of low filtration is established when the lumen pressure is abruptly dropped from a high to a low value. In contrast, theoretical models for the filling of the tissue space predict that several hours should be required to achieve the new steady state. In this paper we propose a new hypothesis for this rapid regulation, namely that pericytes covering the interendothelial cleft exits create small trapped tissue regions outside the cleft exits which regulate this transient behavior. Our theoretical model predicts for rat mesenteric microvessels: (1) that water flux and solute concentration at the cleft exit establish their steady-state values within ~1 min after the lumen pressure is dropped to a low value, while the pressure and width of the trapped region establish their steady-state values in ~4 min: (2) that the restoring force of elastic elements in the trapped region is responsible for preventing the region from continuously expanding due to slow filtration at long times.