Characterization of the large conductance calcium(II)-activated potassium channel of rabbit aorta and its pharmacological modulation

Large conductance Ca$\sp{2+}$-activated K$\sp+$ channels from rabbit aortic microsomes were incorporated into planar lipid bilayers and single channel currents were recorded. The biophysical characterization of the channel demonstrated that this large conductance channel was indistinguishable from the BK channel of various preparations. The channel conductance was approximately 300 pS in 250 mM KCl and was highly selective for the monovalent cations K$\sp+$, Rb$\sp+$, and NH$\sb4\sp+$. The channel showed marked (Ca$\sp{2+}$) $\sb{\rm i}$ sensitivity between 1-100 $\mu$M and P(open) changed e-fold for an 11-13 mV change in membrane potential. Finally, this channel was also inhibited by the scorpion toxin charybdotoxin.The channel was pharmacologically modulated by cromakalim, pinacidil, glibenclamide, and ATP from the internal side of the membrane. Cromakalim and pinacidil (cis and trans) act as channel openers while glibenclamide and ATP (cis) were inhibitors. Both cromakalim and pinacidil increased P(open) of the channel in a dose-dependent manner. This increase in P(open) was caused by a significant decrease in the time constant ($\tau\sb{\rm slow}$) of the component of the closed state. Neither drug affected the mean open time or the fast component ($\tau\sb{\rm fast}$) of the closed state. Cromakalim was approximately 2-3 times more potent than pinacidil in increasing P(open).Glibenclamide inhibited only the cromakalim or pinacidil induced increases in P(open). The mechanism of action apparently is either direct competition for the activator binding site or by binding to an allosteric site that was exposed upon the agonist binding thus altering channel gating. ATP, on the other hand, inhibited the channel without prior activation by a K$\sp+$ channel opener in a dose-dependent manner. Cromakalim could reverse the inhibition of ATP and this effect was found to be Ca$\sp{2+}$ sensitive. The effect of ATP may be on a site that is different from the glibenclamide binding site. Glibenclamide ant ATP did not inhibit voltage- or Ca$\sp{2+}$-induced increases in P(open). Thus the large conductance Ca$\sp{2+}$-activated K$\sp+$ channel may be an important pharmacological target for these novel antihypertensives and may regulate the cell\u27s membrane potential under both physiological and pathophysiological conditions such as anoxia, hypoxia, and ischemia