Block of ATP-regulated and Ca2(+)-activated K+ channels in mouse pancreatic beta-cells by external tetraethylammonium and quinine.
Bokvist K., Rorsman P., Smith PA.
1. The whole-cell and outside-out patch configurations of the patch-clamp technique were used to investigate the effects of extracellular tetraethylammonium ions (TEA+) and quinine on both Ca2(+)-activated and ATP-regulated K+ channels in mouse pancreatic beta-cells. 2. The Ca2(+)-activated K+ channel has a single-channel K+ permeability of 4.7 x 10(-13) cm3 s-1 when recorded with physiological ionic gradients. This value decreased to 2.9 x 10(-13) cm3 s-1 after addition of 0.3 mM-TEA+. 3. Two exponentials with time constants of 0.2 and 4.7 ms were required to describe the distribution of the channel openings suggesting that the Ca2(+)-activated K+ channel has at least two open states. The fast and slow components comprised 16 and 84% of the total number of openings respectively. 4. TEA+ caused a concentration-dependent decrease in the single-channel amplitude and open probability of the Ca2(+)-activated K+ channel. A Kd for the reduction in the mean current of 0.14 mM was observed. The stoichiometry was approximately 1:1. 5. Quinine blocked the Ca2(+)-activated K+ channel in a concentration-dependent manner. Half-maximal block was observed at 0.10 mM and binding was 1:1. Inhibition by 20 microM-quinine was not associated with a decrease in channel amplitude but markedly reduced the lifetime of the channel openings. Two exponentials, with time constants of 0.5 and 1.3 ms, were required to describe the channel openings. The rapid component contained 55% of the events. 6. TEA+ reduced the single-channel amplitude of the ATP-regulated K+ channel in a concentration-dependent manner. Kd for the block was 22 mM and the binding approximately 1:1. The block was not associated with changes in the open probability or channel kinetics. Two exponentials were required to describe the distribution of the open times. The time constants for the fast and slow components were approximately 2 and approximately 20 ms respectively. The rapid component accounted for approximately 35% of the events. 7. Quinine (10-20 microM) almost abolished activity of the ATP-regulated K+ channels. Inhibition was characterized by slow onset and reversibility but not associated with a change in the appearance of the single-channel events. Quinine-induced block could not be reversed by diazoxide. 8. We conclude that TEA+ produces rapid block of both Ca2(+)-activated and ATP-regulated K+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)