KINETIC-ANALYSIS OF THE INHIBITORY EFFECT OF GLIBENCLAMIDE ON K-ATP CHANNELS OF MAMMALIAN SKELETAL-MUSCLE

We investigated the block of K-ATP channels by glibenclamide in inside -out membrane patches of rat flexor digitorum brevis muscle. (1) We fo und that glibenclamide inhibited K-ATP channels with an apparent K-i o f 63 nM and a Hill coefficient of 0.85. The inhibition of K-ATP channe ls by glibenclamide was unaffected by internal Mg2+. (2) Glibenclamide altered all kinetic parameters measured; mean open time and burst len gth were reduced, whereas mean closed time was increased. (3) By makin g the assumption that binding of glibenclamide to the sulphonylurea re ceptor (SUR) leads to channel closure, we have used the relation betwe en mean open time, glibenclamide concentration and K-D to estimate bin ding and unbinding rate constants. We found an apparent rate constant for glibenclamide binding of 9.9 x 10(7) M(-1) sec(-1) and an unbindin g rate of 6.26 sec(-1). (4) Glibenclamide is a lipophilic molecule and is likely to act on sulfonylurea receptors from within the hydrophobi c phase of the cell membrane. The glibenclamide concentration within t his phase will be greater than that in the aqueous solution and we hav e taken this into account to estimate a true binding rate constant of 1.66 x 10(6) M(-1) sec(-1).