ATTENUATION OF CHANNEL KINETICS AND CONDUCTANCE BY CHOLESTEROL - AN INTERPRETATION USING STRUCTURAL STRESS AS A UNIFYING CONCEPT

The ubiquity of cholesterol in cell membranes and changes in its conce ntration during development, aging and in various diseases suggest tha t it plays an important role in modulating cell function. We examined this possibility by monitoring the effects of cholesterol on the activ ity of the calcium-activated potassium (BK) channel reconstituted into lipid bilayers from rat brain homogenates. Increasing the cholesterol concentration to 11% of total lipid weight resulted in a 70% reductio n in channel mean open time and a reduction of the open probability of the channel by 80%. Channel conductance was reduced by 7%. Cholestero l is known to change the order state and the modulus of compressibilit y of bilayers. These physico-chemical changes may be translated into a n overall increase in the structural stress in the bilayer, and this f orce may be transmitted to proteins residing therein. By examining the characteristics of the BK channel as a function of temperature, in th e presence and absence of cholesterol, we were able to estimate the ac tivation energy based on Arrhenius plots of channel kinetics. Choleste rol reduced the activation energy of the BK channel by 50% for the ope n to closed transition. This result is consistent with an increased st ress energy in the bilayer and favors the channel moving into the clos ed state. Taken together, these data are consistent with a model in wh ich cholesterol induces structural stress which enhances the transitio n from the open to the closed state of the channel. We suggest that th is is an important mechanism for regulating the activity of membrane-i ntegral proteins and therefore membrane function, and that the concept of structural stress may be relevant to understanding the modulation of ion channel activity in cell membranes.