Voltage-dependent Ba2+ block of K+ channels in apical membrane of frog skin.
The characteristics of the blockage of apical K+ channels in the frog skin (Rana temporaria) by Ba2+ were investigated with current-voltage measurements and current-fluctuation analysis. Inward K+ currents were recorded with a transepithelial K+ concentration gradient oriented from mucosa to serosa. When Ba2+ (5-10,000 microM) was added to the mucosal solution, the inward K+ current was progressively depressed. This blockage was strongly voltage dependent. The apparent macroscopic Michaelis-Menten constant decreased exponentially with increasing transepithelial voltage (Vt) (mucosa positive). According to Eyring rate theory, the binding site for the Ba2+ ion in the K+ channel was found to be located at a relative electrical distance of 0.72 of the apical membrane potential field, measured from the cytosolic side. From current-voltage (I-V) measurements and Michaelis-Menten kinetics, the Ba2+ dissociation constant was calculated: KBa = 57 microM (Vt = 0 mV). The voltage dependence of the on- and off-rate of the Ba2+-receptor interaction was obtained from the analysis of current fluctuations induced by Ba2+. An exponential relationship between the rates and Vt was obtained, as was found for KBa from the I-V measurements. KBa (at Vt = 0 mV) calculated from the noise experiments was 66 microM.
D000818:Animals; D001464:Barium / Q000378:metabolism / Q000494:pharmacology*; D002462:Cell Membrane / Q000502:physiology; D004553:Electric Conductivity; D004848:Epithelium / Q000502:physiology; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D011896:Rana temporaria; D012879:Skin Physiological Phenomena
D007473:Ion Channels; D001464:Barium; D011188:Potassium