Kinetic studies of U-37883A (4-morpholinecarboximidine-N-1-adamantyl- N′-cyclohexyl-hydrochloride), a vascular ATP-sensitive K + channel (K ATP channel) blocker, were performed on pig urethral myocytes to investigate inhibitory effects on large-conductance intracellular Ca 2+-sensitive K + channels (i.e., BK Ca channels; 225 pS K + channels) by use of single-channel recordings (outside-out and inside-out configuration). BK Ca channels in pig urethral smooth muscles showed extracellular iberiotoxin (300 nM) sensitivity and voltage dependency. The α subunit of BK Ca channel proteins was detected in the membrane fraction by use of Western blot technique. Application of U-37883A (≥10 μM) reduced the activity of BK Ca channels in a concentration-dependent manner, not only by decreasing mean openlife time but also by prolonging the mean closed time. These results shows that U-37883A affects channels other than the vascular K ATP channel, and demonstrates how it inhibits the activities of BK Ca channels in urethral smooth muscles.
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