Endothelin-induced changes in the physiology of retinal pericytes

Pericytes are positioned on the abluminal wall of capillaries and are thought to play a role in regulating retinal blood flow. Although endothelin (ET)-1 is a putative endothelium-pericyte signal, the mechanisms by which this molecule regulates pericyte function remain unclear. Because ion channels play a vital role in the response of pericytes to extracellular signals, this study was undertaken to assess the effects of ET-1 on ionic currents.

The perforated-patch configuration of the patch-clamp technique was used to monitor whole-cell currents of pericytes located on microvessels freshly isolated from the rat retina. To assay cell-to-cell coupling within retinal microvessels, a gap junction--permeant tracer was loaded through patch pipettes into pericytes and the spreading of the tracer detected by immunohistochemistry.

ET-1 acting through ET(A) receptors altered pericyte currents and caused depolarization of the membrane potential. The effects on pericyte currents were dynamic over time. Initially, the nonspecific cation (NSC) and calcium-activated chloride (Cl(Ca)) currents were activated and the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) current inhibited. Subsequently, by a mechanism sensitive to a protein kinase C (PKC) inhibitor, the NSC, Cl(Ca), and voltage-dependent potassium currents diminished as gap junction pathways closed within the microvessels.

ET-1 regulates pericyte conductances by multiple mechanisms. One process involves a PKC-dependent closure of gap junction pathways resulting in loss of electrotonic input from neighboring cells. Thus, ET-1 not only affects individual microvascular cells, but also regulates the effective size of the multicellular functional units that may serve to control capillary blood flow. This regulation of intercellular communication within pericyte-containing microvessels may be an important, previously unrecognized, action of ET-1.