fk-409 and iberiotoxin

The vascular endothelium plays an important role in regulating retinal blood flow via actions of several vasodilators, including nitric oxide (NO), prostaglandin I₂, and an endothelium-derived hyperpolarizing factor (EDHF). Our previous in vivo studies demonstrated that acetylcholine (ACh) dilates the rat retinal arteriole partly through NO- and prostaglandin-independent pathway, possibly the EDHF-mediated pathway, but the underlying mechanism(s) remains to be elucidated. It has been suggested that activation of Ca²+-activated K+ (K(Ca)) channels contributes to the EDHF-mediated responses; therefore, the roles of K(Ca) channels in ACh-induced vasodilation of retinal arterioles were examined in rats. The retinal vascular responses were assessed by determining changes in diameters of retinal arterioles in ocular fundus images that were captured with an original fundus camera system. Intravitreal injection of charybdotoxin, an inhibitor of intermediate- and large-conductance K(Ca) (I/BK(Ca)) channels, or iberiotoxin, an inhibitor of large-conductance K(Ca) (BK(Ca)) channels, significantly reduced the ACh-induced vasodilation of retinal arterioles, whereas neither apamin, an inhibitor of small-conductance K(Ca) (SK(Ca)) channels, nor TRAM-34, an inhibitor of intermediate-conductance K(Ca) (IK(Ca)) channels, altered the response. The vasodilator response to ACh observed under the combined blockade of NO synthase and cyclooxygenase with N(G)-nitro-L-arginine methyl ester plus indomethacin was also diminished by iberiotoxin. Iberiotoxin did not affect the NO donor NOR3-induced vasodilation of retinal arterioles, whereas it significantly reduced the BK(Ca) channel opener BMS-191011-induced responses. These results suggest that activation of BK(Ca) channels is involved in the EDHF-mediated component of the vasodilator response to ACh in the rat retinal arterioles in vivo.

Topics: Acetylcholine; Animals; Apamin; Arterioles; Blood Pressure; Calcium Channel Agonists; Calcium Channel Blockers; Charybdotoxin; Heart Rate; Hydroxylamines; Indomethacin; Intermediate-Conductance Calcium-Activated Potassium Channels; Large-Conductance Calcium-Activated Potassium Channels; Male; NG-Nitroarginine Methyl Ester; Nitric Oxide Donors; Nitro Compounds; Oxadiazoles; Peptides; Potassium Channels, Calcium-Activated; Pyrazoles; Rats; Rats, Wistar; Retinal Artery; Small-Conductance Calcium-Activated Potassium Channels; Vasodilation

The role of L-type Ca(2+) channels in the relaxation to nitric oxide (NO)-mediated MaxiK(Ca) channel activation was examined in guinea pig aorta. Acetylcholine (ACh) produced an endothelium-dependent relaxation of guinea pig aorta precontracted with noradrenaline (NA), which was abolished by an NO synthase inhibitor, N(G)-nitro-L-arginine (L-NNA). Both endothelium-dependent relaxation by ACh and endothelium-independent relaxation by an NO donor, (+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexeneamide (NOR3), were strongly suppressed by a soluble guanylate cyclase (sGC) inhibitor, 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxalin-1-one (ODQ), suggesting that increased intracellular cGMP plays the key role in both responses. ACh- and NOR3-induced relaxations were significantly suppressed by iberiotoxin (IbTX), a selective blocker of MaxiK(Ca) channels. ACh- and NOR3-induced relaxations were greatly attenuated when arteries were precontracted with high KCl instead of NA, supporting the idea that K(+) channel activation mediates the relaxant responses. (6) NOR3-induced relaxations were not affected by a L-type Ca(2+) channel blocker, diltiazem. Furthermore, endothelium-independent relaxation by a K(ATP) channel opener, (+)-7,8-dihydro-6, 6-dimethyl-7-hydroxy-8-(2-oxo-1-piperidinyl)-6H-pyrano[2,3-f] benz-2,1, 3-oxadiazole (NIP-121) was not affected by diltiazem and nicardipine. These findings suggest that blockade of L-type Ca(2+) channels is not a major mechanism responsible for the vascular relaxation due to NO-mediated MaxiK(Ca) channel activation in guinea pig aorta.

Topics: Acetylcholine; Animals; Aorta, Thoracic; Calcium Channel Blockers; Calcium Channels, L-Type; Drug Interactions; Female; Guinea Pigs; In Vitro Techniques; Large-Conductance Calcium-Activated Potassium Channels; Male; Nitric Oxide; Nitric Oxide Donors; Nitro Compounds; Nitroarginine; Norepinephrine; Oxadiazoles; Peptides; Piperidines; Potassium Channels; Potassium Channels, Calcium-Activated; Potassium Chloride; Quinoxalines; Vasodilation