Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C83
The role of smooth muscle K+ channels in conducted hyperpolarization
T. Beleznai1, P. Yarova1,2, K. A. Dora1
1. Pharmacology, University of Oxford, Oxford, United Kingdom. 2. Pharmacy and Pharmacology, University of Bath, Bath, United Kingdom.
Agonists that stimulate smooth muscle cell hyperpolarization have the potential to evoke homo- and heterocellular intercellular electrical coupling and conducted dilatation. We investigated the role of smooth muscle K+ channels in conducted dilatation in phenylephrine pre-contracted, isolated mesenteric arteries. Triple-cannulated arteries (male, Wistar rat) were mounted in a gravity-fed pressure myograph and kept at 37C. Flow into the feed artery allowed luminal perfusion of agonists into Branch1 (local response) without reaching the feed artery (conducted response). Using confocal microscopy we simultaneously measured local and conducted dilatation into the feed artery, and used a fluorescent dye to demark agonist delivery. Intracellular recordings of membrane potential (Em) were made using sharp glass microelectrodes filled with 2M KCl ~2000µm upstream from the bifurcation. The effect of luminally applied acetylcholine (ACh) on endothelial cell [Ca2+]i was examined by luminal loading of Oregon Green 488 BAPTA-1. In some experiments endothelial cells were removed from the feed artery with an air bubble. The NO synthase inhibitor L-NAME (100µM) and the vasoconstrictor phenylephrine were present in all experiments. Values are means±S.E.M., compared by Mann Whitney test. Agonists acting selectively at endothelial (ACh, 1µM) and smooth muscle (isoprenaline: Iso, 1µM) K+ channels each evoked conducted dilatation at 0, 500, 1000, 1500, 2000µm upstream from the bifurcation; increases in diameter (controls): ACh: 125±6, 81±4, 46±4, 25±4, 14±3µm, n=23; Iso: 127±13, 71±8, 47±9, 40±9, 35±10µm, n=9. A combination of either iberiotoxin (BKCa channel blocker) or tetraethylammonium (TEA: BKCa, KV channel blocker) with 4-aminopyridine (4-AP: KV channel blocker), significantly increased the magnitude of conducted dilatation to both ACh and Iso; ACh: 123±25, 120±23, 103±21, 92±20, 85±22µm vs. controls (except at 0µm) p<0.05, n=5; Iso: 121±6, 105±23, 104±25, 92±20, 93±23µm vs. controls (except at 0µm) p<0.05, n=3. In the presence of 1µM nifedipine, the smooth muscle Em was -38.3±6.2mV (n=6). Local perfusion of ACh evoked 3.5±0.6mV conducted hyperpolarization (control, n=3). In the additional presence of TEA and 4-AP, ACh evoked a significantly greater conducted hyperpolarization: 13.8±3.4mV vs. control, p<0.05, n=4. The conducted dilatation to both ACh and Iso was effectively abolished beyond the bifurcation in arteries with damaged endothelial cells (n=3). Changes in endothelial cell [Ca2+]i could not account for the apparently regenerating current. These data show that activated smooth muscle cell K+ channels not only serve to modulate contraction, but also reduce the effectiveness of conducted dilatation. Our data further confirm a key role for the endothelium as a conduit for conducted dilatation responses, and support bidirectional electrical coupling through myoendothelial gap junctions.
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