Depletion of membrane cholesterol eliminates the Ca²⁺ -activated component of outward potassium current and decreases membrane capacitance in rat uterine myocytes

Changes in membrane cholesterol content have potent effects on cell signalling and contractility in rat myometrium and other smooth muscles. We have previously shown that depletion of cholesterol with methyl-β-cyclodextrin (MCD) disrupts caveolar microdomains. The aim of this work was to determine the mechanism underlying the increase in Ca²⁺ signalling and contractility occurring in the myometrium with MCD. Patch clamp data obtained on freshly isolated myocytes from the uterus of day 19-21 rats showed that outward K⁺ current was significantly reduced by MCD. Membrane capacitance was also reduced. Cholesterol-saturated MCD had no effect on the amplitude of outward current suggesting that the reduction in the outward current was due to cholesterol depletion induced by MCD rather than a direct inhibitory action of MCD on the K⁺ channels. Confocal visualization of the membrane bound indicator Calcium Green C18, revealed internalization of the surface membrane with MCD treatment. Large conductance, Ca²⁺-sensitive K⁺ channel proteins have been shown to localize to caveolae. When these channels were blocked by iberiotoxin outward current was significantly reduced in the uterine myocytes; MCD treatment reduced the density of outward current. Following reduction of outward current by MCD pretreatment, iberiotoxin was unable to produce any additional decrease in the current, suggesting a common target. MCD treatment also increased the amplitude and frequency of spontaneous rises in cytosolic Ca²⁺ level ([Ca²⁺]_i transients) in isolated myocytes. In intact rat myometrium, MCD treatment increased Ca²⁺ signalling and contractility, consistent with previous findings, and this effect was also found to be reduced by BK channel inhibition. These data suggest that (1) disruption of cholesterol-rich microdomains and caveolae by MCD leads to a decrease in the BK channel current thus increasing cell excitability, and (2) the changes in membrane excitability produced by MCD underlie the changes found in Ca²⁺ signalling and uterine contractility.