ABSTRACT
An endothelium-derived hyperpolarizing factor (EDHF) is widely hypothesized to underpin vascular relaxations that are independent of nitric oxide (NO) and prostanoids. Although a diverse variety of agents, ranging from ions (specifically K) to products of arachidonic acid metabolism, have been proposed as the active mediator, bioassay techniques have failed to provide unequivocal evidence for the existence of a freely diffusible EDHF. The present chapter summarizes evidence that the EDHF-type response to acetylcholine is mediated by electrotonic conduction of endothelial hyperpolarization via myoendothelial and homocellular smooth muscle gap junctions whose conductance and permeability are regulated by an associated synthesis of cAMP. In rabbit iliac arteries relaxation is accompanied by transient elevations in cAMP levels of the smooth muscle and mechanical responses, nucleotide accumulation and subintimal hyperpolarization are abolished by the connexin-mimetic peptide Gap 27 and by endothelial denudation. Subintimal hyperpolarization is also abolished by inhibition of adenylyl cyclase, whereas inhibition of cAMP phosphodiesterase enhances transmission of endothelial hyperpolarization through the media and potentiates relaxation. Parallel studies confirm that cAMP increases dye transfer from the endothelium into the media whereas Gap 27 promotes sequestration of dye within the intima. In perfused rabbit ear preparations, phenylephrine stimulates an endothelium-dependent efflux of cAMP that is abolished by blockade of gap junctions and therefore mediated by a signal transmitted from activated smooth muscle to overlying endothelial cells directly. However, cAMP formed via this pathway does not influence the constrictor response of the smooth muscle to phenylephrine, indicating that endothelium-derived cAMP does not affect tone in the absence of co-existent endothelial hyperpolarization. Taken together, the findings suggest that the contribution of cAMP to the EDHF phenomenon is to enhance passive electrotonic spread of endothelial hyperpolarization. Its action may therefore be regarded as permissive.
