K+ current stimulation by Cl- in the midgut epithelium of tobacco hornworm (Manduca sexta)

Summary Chloride-stimulated K⁺ secretion by Manduca sexta midgut (5th-instar larvae) was measured as K⁺-carried short-circuit current of the tissue mounted in an Ussing chamber. Microscopic parameters, such as single-channel current and channel density for the rate-determining passive transport step across the basolateral goblet cell membrane (i.e. K⁺ channels), were estimated by means of current-fluctuation analysis of the K⁺ channel blockade by haemolymph-side Ba²⁺ ions. Ba²⁺ was equally effective with Cl^- or gluconate (Glu^-) as the principal ambient anion. The Ba²⁺-induced K⁺ channel conduction noise is reflected by a Lorentzian, or relaxation, noise component in the power spectrum of the K⁺ current fluctuations. A reduced Lorentzian plateau value, but an unchanged corner frequency, were observed when Cl^- was replaced by Glu^-. The results from the analysis of a two-state model of K⁺ channel block by Ba²⁺, with respect to the anion-replacement effects, suggest that the observed changes in K⁺ current and Lorentzian plateau value mirror a complex change of the underlying parameters: Cl^- omission reduces single channel current but increases channel density so that the product of single channel current and channel density is smaller in Glu^- than in Cl^-. It seems likely that basolateral K⁺ channels (1) are subject to anionic gating ligands, and (2) depend on anions with respect to the rate of K⁺ transfer through and open K⁺ channel.Abbreviations a.c. alternating current - single-channel conductance - E_K K⁺ Nernst potential - f frequency contained in current noise - f_c corner frequency - Glu^- gluconate - G_t transepithelial conductance - I_sc short-circuit current - I_K K⁺ current - I_K(max) maximal K⁺ current - i single-channel current - K_Ba barium inhibition constant - K_m Michaelis constant of saturating K⁺ current - k₀₁ and k₁₀ barium association and dissociation rate constant, respectively - M K⁺ channel density - S_f power density - S_o Lorentzian plateau value - P_o channel-open probability - P_K K⁺ permeability - V_sc cellular potential at short-circuitThese results have already been presented in part, at the 1989 joint meeting of the German and Israel Physiological Societies in Jerusalem (Zeiske et al. 1990).