Pial Microvascular Responses to Transient Bilateral Common Carotid Artery Occlusion: Effects of Hypertonic Glycerol

Abstract

Objective: The aim of the study was to assess the rat pial microvessel alterations due to transient bilateral common carotid artery occlusion (BCCAO) and to investigate the mechanism of 10% hypertonic glycerol neuroprotection. Our suggestion was that 10% glycerol solution infusion could dilate pial arterioles through nitric oxide release and/or stimulation of ATP-sensitive potassium (K_ATP) channels. Therefore, we studied the effects of hypertonic glycerol after inhibition of nitric oxide synthase, with N^G-nitro- L-arginine-methyl ester or N^G-nitro- L-arginine, or K_ATP channels with glibenclamide. Methods: Pial microcirculation of male Wistar rats was visualized by a fluorescent microscopy technique through an open cranial window, using fluorescein isothiocyanate bound to dextran (molecular weight 70 kDa). BCCAO was induced for 30 min and reperfusion lasted 60 min. The arterioles were classified according to the Strahler ordering scheme. Permeability increase was quantified by normalized grey levels (NGL). Leucocytes were stained with rhodamine 6G. Perfused capillary length and capillary red blood cell (RBC) velocity were measured by computer-assisted methods. Results: The arterioles were assigned 5 orders of branchings, from order 1 (diameter 16.0 ± 2.5 µm) to order 5 (62.0 ± 5.0 µm). BCCAO caused inhomogenous changes in diameter of arterioles and leakage of fluorescent dextran, that was further enhanced by reperfusion (0.45 ± 0.05 NGL, p < 0.01). Adhesion of leukocytes to venules was marked and capillary perfusion was reduced by 39.2 ± 6.0% of baseline as well as capillary RBC velocity. 10% glycerol solution caused an increase in diameter of all arterioles within 25 ± 2 min of administration (by 20 ± 5% in order 4, 25 ± 4% in order 3 and 18 ± 3% in order 2; p < 0.01). Leakage (0.19 ± 0.03 NGL, p < 0.01), leukocyte adhesion (2.0 ± 1.0/100 µm of venular length, p < 0.01) and capillary occlusion (reduction by 13.0 ± 5.5% of baseline) were prevented compared with controls. Capillary RBC velocity increased compared with controls. N^G-nitro- L-arginine-methyl ester or N^G-nitro- L-arginine infused prior to glycerol caused vasoconstriction and reduced the protective effects of hypertonic glycerol on permeability increase. The number of adherent leukocytes and perfused capillary length decreased, while capillary RBC velocity was higher than baseline. Glibenclamide prior to 10% glycerol solution blunted glycerol-induced vasodilatation, but did not affect protection by hypertonic glycerol on blood-brain barrier disruption, leukocyte adhesion and capillary perfusion, preserving high capillary RBC velocity. Papaverine (20 mg/kg body weight) induced an increase in arteriolar diameter, enhancing interstitial edema; adhesion of leukocytes was marked as well as capillary occlusion, while capillary RBC velocity increased. Conclusions: 10% glycerol solution was able to prevent microvascular alterations due to BCCAO protecting cerebral tissue. The effects appear to be due to hyperosmolality causing stimulation of K_ATP channels, increase in vessel wall shear stress and release of nitric oxide.