Essential role of gap junctions in NO- and prostanoid-independent relaxations evoked by acetylcholine in rabbit intracerebral arteries

Background and Purpose-Direct intercellular communication via gap junctions may play a central role in endothelium-dependent relaxations that are mediated by a conducted hyperpolarization and do not involve the synthesis of NO and prostanoids. In the present study, inhibitory peptides homologous to the Gap27 domain of the second extracellular loop of connexin37/connexin43 and connexin40, designated as (37,43)Gap27 and (40)Gap27, respectively, were used to evaluate the role of this mechanism in intracerebral arteries. Methods-Isolated rings of rabbit middle cerebral artery were constricted by histamine (10 mumol/L) in the presence of N-G-nitro-L-arginine methyl ester (300 mumol/L) and indomethacin (10 mumol/L). Concentration-relaxation curves for acetylcholine were constructed in the presence and absence of (37,43)Gap27 and (40)Gap27. Specific antibodies were used to delineate the distribution of connexin37, connexin40, connexin43, and connexin45 within the arterial wall. Results-Individually, (37,43)Gap27 and (40)Gap27 minimally affected endothelium-dependent relaxations to acetylcholine at concentrations of 300 mumol/L, whereas their combination (at 300 mumol/L each) inhibited the maximal response by approximate to70% and increased the EC50 value for relaxation by approximate to15-fold. In endothelium-denuded rings, this peptide combination did not attenuate responses to sodium nitroprusside, an exogenous source of NO. Gap junction plaques, whose incidence was highest in endothelium, were constructed from connexin40 and connexin43 in the media and connexin37, connexin40, and connexin43 in the endothelium. Conclusions-The findings confirm that direct communication via gap junctions contributes to agonist-induced relaxations of intracerebral arteries. More than one connexin subtype appears to participate in such responses.