Endothelial control of vascular smooth muscle plays a major role in the resulting vasoreactivity implicated in physiological or pathological circulatory processes. However, a comprehensive understanding of endothelial (EC) / smooth muscle cells (SMC) crosstalk is far from complete. Here, we have examined the role of gap junctions in this crosstalk and we demonstrate an active contribution of SMC to endothelial control of vasomotor tone.

In small intrapulmonary arteries, quantitative RT-PCR, Western Blot analyses revealed the presence of 3 connexins (Cx): 37, 40 and 43. Immunofluorescence labelling showed the presence of Cx 37, 40 and 43 in EC and the presence of Cx 37 and 40 in SMC. Functional experiments showed that the peptides homologous to the gap 26 and 27 domains of the extracellular loops of Cx 43 (43Gap 26) or Cx 37 and 43 (37-43Gap 27) respectively, (1) interrupt intercellular communications in a specific manner, (2) attenuate the calcium and contractile responses to serotonin (5-HT) simultaneously recorded in pulmonary arteries and (3) abolish the diffusion in SMC of carboxyfluorescein-AM loaded in EC. Similarly, contractile and calcium responses to 5-HT were decreased by superoxide dismutase and catalase which, catabolize superoxide anion (O2•-) and H2O2, respectively. Both Cx- and reactive oxygen species (ROS)-mediated effects on the responses to 5-HT were reversed by (1) L-NAME, a NO synthase inhibitor, or (2) endothelium removal with CHAPS. Electron paramagnetic resonance directly demonstrated that 5-HTinduced ROS production, and more specifically O2•-, originated from the SMC. Finally, 5-HT decreased basal cyclic GMP content in isolated intact arteries, whereas a cocktail of 5-HT, superoxide dismutase and catalase increased basal cyclic GMP content.

Altogether, the findings suggest that 5-HT produces O2•- in the smooth muscle and NO in the endothelium. O2•- passes through the myoendothelial junctions to decrease endothelial NO production and thus strengthen pulmonary vasoreactivity. This is the first time that we demonstrate a control of the endothelial NO function by the smooth muscle is demonstrated. Since ROS is often overproduced in many cardiovascular diseases, such process could also be of great importance in pathological conditions.