Capillaries demonstrate changes in membrane potential in response to pharmacological stimuli.

It has been proposed that capillaries can detect changes in tissue metabolites and generate signals that are communicated upstream to resistance vessels. The mechanism for this communication may involve changes in capillary endothelial cell membrane potentials which are then conducted to upstream arterioles. We have tested the capacity of capillary endothelial cells in vivo to respond to pharmacological stimuli. In a hamster cheek pouch preparation, capillary endothelial cells were labeled with the voltage-sensitive dye di-8-ANEPPS. Fluorescence from capillary segments (75-150 μm long) was excited at 475 nm and recorded at 560 and 620 nm with a dual-wavelength photomultiplier system. KCl was applied using pressure injection, and acetylcholine (ACh) and phenylephrine (PE) were applied iontophoretically to these capillaries. Changes in the ratio of the fluorescence emission at two emission wavelengths were used to estimate changes in the capillary endothelial membrane potential. Application of KCl resulted in depolarization, whereas application of the vehicle did not. Application of ACh and PE resulted in hyperpolarization and depolarization, respectively. The capillary responses could be blocked by including a receptor antagonist (atropine or prazosin, respectively) in the superfusate. We conclude that the capillary membrane potential is capable of responding to pharmacological stimuli. We hypothesize that capillaries can respond to changes in the milieu of surrounding tissue via changes in endothelial membrane potential.