Endothelium-dependent vascular smooth muscle relaxation activated by electrical field stimulation

Are voltage-dependent ion channels involved in the endothelial cell control of vasomotor tone?

In the microcirculation, longitudinal conduction of vasomotor responses provides an essential means of coordinating flow distribution among vessels in a complex network. Spread of current along the vessel axis can display a regenerative component, which leads to propagation of vasomotor signals over many millimeters; the ionic basis for the regenerative response is unknown. We examined the responses to 10 s of focal electrical stimulation (30 Hz, 2 ms, 30 V) of mouse cremaster arterioles to test the hypothesis that voltage-dependent Na(+) (Na(v)) and Ca(2+) channels might be activated in long-distance signaling in microvessels. Electrical stimulation evoked a vasoconstriction at the site of stimulation and a spreading, nondecremental conducted dilation. Endothelial damage (air bubble) blocked conduction of the vasodilation, indicating an involvement of the endothelium. The Na(v) channel blocker bupivacaine also blocked conduction, and TTX attenuated it. The Na(v) channel activator veratridine induced an endothelium-dependent dilation. The Na(v) channel isoforms Na(v)1.2, Na(v)1.6, and Na(v)1.9 were detected in the endothelial cells of cremaster arterioles by immunocytochemistry. These findings are consistent with the involvement of Na(v) channels in the conducted response. BAPTA buffering of endothelial cell Ca(2+) delayed and reduced the conducted dilation, which was almost eliminated by Ni(2+), amiloride, or deletion of alpha(1H) T-type Ca(2+) (Ca(v)3.2) channels. Blockade of endothelial nitric oxide synthase or Ca(2+)-activated K(+) channels also inhibited the conducted vasodilation. Our findings indicate that an electrically induced signal can propagate along the vessel axis via the endothelium and can induce sequential activation of Na(v) and Ca(v)3.2 channels. The resultant Ca(2+) influx activates endothelial nitric oxide synthase and Ca(2+)-activated K(+) channels, triggering vasodilation.

Central role of connexin40 in the propagation of electrically activated vasodilation in mouse cremasteric arterioles in vivo

When a short segment of arteriole is stimulated, vasomotor responses spread bidirectionally along the vessel axis purportedly via gap junctions. We used connexin40 knockout (Cx40-/-) mice to study vasomotor responses induced by 10-second trains of electrical stimulation (30 Hz, 1 ms, 30 to 50 V) in 2nd or 3rd order arterioles of the cremaster muscle. Measurements were made at the stimulation site (local) and at conducted sites (500, 1000, and 2000 microm upstream). In wild-type (Cx40+/+) animals, electrical stimulation evoked a local vasoconstriction and a conducted vasodilation that spread very rapidly along the vessel length without detectable decay. In Cx40-/- mice, the conducted dilation was converted into either vasoconstriction or a slowly developing vasodilation that decayed along the vessel length. Tetrodotoxin (TTX, 1 micromol/L) had no effect on the local vasoconstriction in either Cx40+/+ or Cx40-/- mice, but enhanced the conducted vasodilation in Cx40+/+ animals. In Cx40-/- mice, TTX abolished the conducted vasoconstriction when present and revealed a small vasodilation that decayed with distance. In the group of Cx40-/- mice in which electrical stimulation elicited a conducted vasodilation, TTX had no effect. Immunocytochemistry revealed Cx40 only in the endothelial layer of arterioles from Cx40+/+ mice and complete elimination of this connexin in the Cx40-/- animals. These results indicate that focal current stimulation causes vasoconstriction by a combination of perivascular nerve stimulation and smooth muscle activation. Moreover, electrical stimulation activates a nonneuronal, Cx40-dependent vasodilator response that spreads along the vessel length without decay.

Superoxide anion and K+ channels mediate electrical stimulation-induced relaxation in the rat basilar artery

Electrical field stimulation (a single pulse, 0.2 ms) caused a rapid relaxation of rat basilar artery segments precontracted with different agents, but not with 30 mM KCl. This relaxation was not modified by endothelium removal, 10 microM tetrodotoxin, 1 microM propranolol, 1 microM atropine, 30 microM indomethacin, 10 microM methylene blue, 100 microM N(G)-nitro-L-arginine methyl ester or 1 microM cimetidine but it was significantly reduced by 50 and 100 U/ml superoxide dismutase. Charybdotoxin (0.1 and 0.2 microM), a blocker of large-conductance Ca2+-activated K+ channels (BK(Ca)), decreased the relaxation elicited by electrical stimulation, whereas it was unaltered by 10 microM glibenclamide or 1 microM apamin, blockers of ATP-sensitive (K(ATP)) or small-conductance K(Ca) channels, respectively. Thapsigargin (0.01 and 0.1 microM), an inhibitor of sarcoplasmic reticulum Ca2+-ATPase, increased the electrical stimulation-induced relaxation, which was nearly abolished by charybdotoxin. These results show that electrical stimulation induces endothelium-independent and non-neurogenic relaxations in the rat basilar artery. This response appears to involve generation of superoxide anion, increase of cytosolic free Ca2+ concentration and subsequent activation of BK(Ca) channels.