A target K⁺ channel for the LP-805-induced hyperpolarization in smooth muscle cells of the rabbit portal vein

The resting membrane potential of smooth muscle cells of the rabbit portal vein was −51.2 mV. LP-805 (8-tert-butyl-6,7-dihydropyrrolo[3,2-e] 5-methylpyrazolo [1,5-a] pyrimidine-3-carbonitrile) hyperpolarized the membrane to −62.3 mV (10 μM) and inhibited the burst spike discharges as measured using the microelectrode method. In dispersed smooth muscle cells, LP-805 (10 μM) generated an outward-current with a maximum amplitude of 68 pA at a holding potential of −40 mV in experiments using the voltage-clamp procedure. The reversal potential of the outward current evoked by LP-805 was −82 mV and this value was close to the equilibrium potential for K⁺ (−80 mV) in the present ionic conditions, suggesting that LP-805 activated the K⁺ channel. Generation of both the hyperpolarization and the outward c urrent by LP-805 was inhibited by glibenclamide (≥ 1 μM). Using the cell-attached and cell-free patch-clamp (in the presence of GDP) procedures, the maxi-K⁺ channel current (150 pS) could be recorded in the absence of LP-805; application of LP-805 additionally opened a small conductance K⁺ channel current (15 pS) without change in the activity of the maxi-K⁺ channel. The maxi-K⁺ channel was sensitive to charybdotoxin (0.1 μM) and to intracellular Ca²⁺ ([Ca²⁺]_i) concentration. The 15 pS channel was insensitive to [Ca²⁺]_i and charybdotoxin, but sensitive to intracellular ATP concentration. Glibenclamide (> 1 μM) inhibited the 15 pS K⁺ channel activated by LP-805. These actions of LP-805 on the maxi-K⁺ and 15 pS K⁺ channels are the same as those previously observed for nicorandil and pinacidil. Thus, LP-805 is a K⁺ channel opener with a chemical structure different from those of the known openers.