NAD+ as a Vasoactive Signal in the Retinal Microvasculature

Abstract

Purpose: : In this study, we assessed the possibility that extracellular NAD⁺ (nicotinamide adenine dinucleotide) regulates the contractility of pericytes. Because these mural cells contract or relax in response to a variety of vasoactive signals, it appears likely that the pericyte–containing microvasculature plays a role in the regulation of retinal blood flow. NAD⁺ is of interest not only because it can be released from damaged cells, but also because it may enter the extracellularspace by effluxing through gap junction hemichannels.

Methods: : Complexes of pericyte–containing microvessels were isolated from papain–treated adult rat retinas. Changes in the contractility of pericytes located on freshly isolated vessels were visualized with the aid of differential interference optics and time–lapse photography.

Results: : Time–lapse movies showed that at concentrations of 20 µM or more, NAD⁺ induced the relaxation of abluminal pericytes. Adjacent to relaxing pericytes, the microvascular lumens dilated. We also observed that the NAD⁺ catabolite, ADP–ribose, caused pericytes to relax. On the other hand, nicotinamide, which is another catabolite of NAD⁺, had no detectable effect on the contractility of these mural cells. In addition, we found that adenosine, a breakdown product of ADP–ribose, caused vasorelaxation. However, although pericyte relaxation induced by adenosine is blocked by glibenclamide, this K_ATP channel inhibitor did not prevent mural cells from relaxing during exposure to ADP–ribose or NAD⁺. Because reactive blue–2 prevented NAD⁺ and ADP–ribose from inducing pericytes to relax, the activation of P2 purinoceptors appears to be a critical step in the contractile response of these mural cells to these putative vasoactive signals.

Conclusions: : Extracellular NAD⁺ and ADP–ribose induce the relaxation of pericytes located on retinal microvessels. Our experiments support the hypothesis that these molecules may play a role in regulating capillary perfusion.

Keywords: retina • vascular cells • blood supply