Conducted vasomotor responses in arterioles: characteristics, mechanisms and physiological significance.

Micropipette application of certain vasoconstrictor or -dilator substances onto the surface of arterioles induces both a local vasomotor response and a response which is propagated up- and downstream along the vessel, a so-called conducted vasomotor response. In some vascular beds conducted vasoconstrictor and dilator responses are detectable more than a millimetre from the site of agonist delivery. While agonists such as acetylcholine, noradrenaline, and KCl almost invariably give rise to conducted vasomotor responses others, such as sodium nitroprusside or vasopressin, do not. Conducted vasomotor responses in arterioles appear to rely on passive electrotonic spread of the change in membrane potential induced by the agonist at the tip of the pipette. Presumably the current spreads up- and downstream along the arteriolar wall through endothelial or smooth muscle cell gap junctions. Whether the electrical signal is conducted primarily through the endothelial or the smooth muscle cell layer or both is currently not known, but it may depend on the agonist used. Experiments have suggested that conducted vasodilation in skeletal muscle feed arterioles plays an important role in the development of functional hyperaemia at the onset of exercise. In the kidney, conducted vasoconstriction is believed to be responsible for the upstream contraction of the afferent arteriole and interlobular artery known to occur in response to activation of the macula densa. Therefore conducted vasoconstriction could be important for the tubuloglomerular feedback mechanism. Finally, experimental studies have shown that conduction of vasomotor responses in arterioles may be altered in pathological conditions associated with microvascular dysfunction such as arterial hypertension and sepsis.