Arteriolar occlusion causes independent cellular responses in endothelium and smooth muscle

Objectives: To test the hypothesis that arteriolar occlusion causes different cellular changes in endothelial and smooth muscle cells. Methods: Cheek pouch arterioles (resting diameter 41 +/- 2 mum) of anesthetized hamsters were occluded briefly (<60 seconds) either upstream or downstream from an observation site. Changes in membrane potential and intracellular calcium concentration ([Ca(2+)](i)) of the endothelial or smooth muscle cells were determined by using fluorescence microscopy (ratiometric analysis). Results: The pressure in the occluded segment decreased by 17.4 +/- 2.6 cm H(2)O during upstream occlusion and increased by 16.8 +/- 6 cm H(2)O during downstream occlusion (n = 5). Upstream occlusion caused vasoconstriction of the occluded segment by 2.4 +/- 0.4 mu m, whereas downstream occlusion produced brief vasodilatation by 1.1 +/- 0.2 mu m. The endothelial cells hyperpolarized during upstream or downstream occlusion (ratio change: 2.26 +/- 0.24% and 2.39 +/- 0.42%, respectively; p < 0.01, n = 5). There were no changes in endothelial [Ca(2+)](i). The sinooth muscle cells depolarized (ratio change: -2.08 +/- 0.14%, n = 5) with an increase in [Ca(2+)](i) (ratio change: 2.92 +/- 0.16%, n = 6) during downstream occlusion. However, there was no detectable change in membrane potential or [Ca(2+)](i) of smooth muscle cells during upstream occlusion. All the changes rapidly recovered when occlusion was released. Responses of an in-situ. isolated segment on a side branch revealed conducted dilatory signals caused by the occlusions. Conclusions: Our results show that the endothelial and smooth muscle cells respond independently to arteriolar occlusion. The endothelial and smooth muscle cells do not effectively communicate in [Ca(2+)](i) or membrane potential during acute arteriolar occlusion. Hyerpolarizing signals in endothelium cause conducted dilation.