THE SELECTIVITY OF DIFFERENT EXTERNAL BINDING-SITES FOR QUATERNARY AMMONIUM-IONS IN CLONED POTASSIUM CHANNELS

Tetraethylammonium (TEA) is thought to be the most effective quaternar y ammonium (QA) ion blocker at the external site of K+ channels, and s mall changes to the TEA ion reduce its potency. To examine the propert ies of the external QA receptor, we applied a variety of QA ions to ex cised patches from human embryonic kidney cells or Xenopus oocytes tra nsfected with the delayed rectifying K+ channels Kv 2.1 and Ky 3.1. In outside-out patches of Kv 3.1, the relative potencies were TEA > tetr apropylammonium (TPA) > tetrabutylammonium (TBA). In contrast to Kv 3. 1, the relative potencies in Kv 2.1 were TBA > TEA > TPA. In Kv 3.1 an d Kv 2.1, external tetrapentylammonium (TPeA) blocked K+ currents in a fast, reversible and, in contrast to TEA, time-dependent manner. The external binding of TPeA appeared to be voltage independent, unlike th e effects of TPeA applied to inside-out patches. External n-alkyl-trie thylammonium compounds(C-8, C-10 chain length) had a lower affinity th an TEA in Kv 3.1, but a higher affinity than TEA in Kv 2.1. In Kv 3.1, the decrease in QA affinity was large when one or two methyl groups w ere substituted for ethyl groups in TEA, but minor when propyl groups replaced ethyl groups. Changes in the free energy of binding could be correlated to changes in the free energy of hydration of TEA derivativ es calculated by continuum methodology. These results reveal a substan tial hydrophobic component of external QA ion binding to Kv 2.1, and t o a lesser degree to Kv 3.1, in addition to the generally accepted ele ctrostatic interactions. The chain length of hydrophobic TEA derivativ es affects the affinity for the hydrophobic binding site, whereas the hydropathy of QA ions determines the electrostatic interaction energy.