Facilitation of sympathetic neurotransmission by phosphatidylinositol-4,5-bisphosphate-dependent regulation of KCNQ channels in rat mesenteric arteries.

Sympathetic nerves regulate vascular tone by releasing neurotransmitters into the vasculature. We previously demonstrated that bradykinin facilitates sympathetic neurotransmission in rat mesenteric arteries. Although little is known about the intracellular mechanism modulating this neurotransmission, recent cell line experiments have shown that the KCNQ channel, which is inhibited by the depletion of membrane phosphatidylinositol-4,5-bisphosphate (PIP₂), participates in the control of neurotransmission by bradykinin. In the present study, we examined the mechanism regulating neurotransmitter release from rat perivascular sympathetic nerves. Excitatory junction potentials (EJPs) elicited by repetitive nerve stimulation (1 Hz, 11 pulses, 20 μs, 20-50 V), a measure of sympathetic purinergic neurotransmission, were recorded with a conventional microelectrode technique in rat mesenteric arteries. Bradykinin (10⁻⁷  mol l⁻¹) significantly enhanced the amplitude of EJPs (n=22, P<0.05). This enhancing effect was abolished by N-type calcium-channel inhibition with ω-conotoxin GVIA (2 × 10⁻⁹ mol ⁻¹l, n=8). The blockade of phospholipase C with U-73122 (10(-6) mol l⁻¹, n=17) also eliminated the facilitatory effect of bradykinin. In addition, the effects of bradykinin were diminished by the prevention of PIP₂ resynthesis with wortmannin (10⁻⁵ mol l⁻¹ n=7) or KCNQ channel inhibition with XE-991 (10⁻⁵ mol l⁻¹, n=7). On the other hand, depletion of intracellular calcium stores with cyclopiazonic acid (3 × 10⁻⁶ mol l⁻¹, n=6) or the inhibition of protein kinase C with bisindolylmaleimide-I (10⁻⁶ mol l⁻¹, n=9) did not alter the action of bradykinin. These data demonstrate that the hydrolysis of PIP₂ by phospholipase C, which is activated by G(q/11)-coupled receptors, and subsequent KCNQ channel inhibition enhance sympathetic purinergic neurotransmission presumably via the activation of N-type calcium channels in rat mesenteric arteries.