MEMBRANE POTENTIAL AND ENDOGENOUS ION CURRENT OF PHYCOMYCES SPORANGIOPHORES

Abstract

Glass microelectrodes were inserted into the growing zone of sporangiophores of Phycomyces blakesleeanus that had been submersed in artificial pond water. The membrane potential (inside negative) increased with increasing pH of the bathing solution from an average of −98 mV at pH 5 up to −131 mV at pH 7. Removal of Ca²⁺ from the medium hyperpolarized the membrane potential in the wild type, but caused a significant depolarization in the blue-light-insensitive madC mutant. KCN, diethylstilbestrol, and N,N′-dicyclohexylcarbodiimide depolarized the membrane potential in both the wild type and the madC mutant, while fusicoccin had no effect. Endogenous ion current of up to 2 μA cm⁻² was measured in the growing zone of sporangiophores with an extracellular vibrating electrode. The current density and current pattern varied with the pH of the medium. At pH 5 most sporangiophores had weak inward current along the growing zone, whereas at pH 7 most sporangiophores had strong outward current. The response of the membrane potential to specific inhibitors and the presence of an endogenous ion current indicate an electrogenic H⁺-ATPase in the plasma membrane. The results show a negative correlation between growth rate of sporangiophores growing in buffered aqueous medium and magnitude of membrane potential, as well as density of outward current. They also indicate an important role of protons in controlling the growth of Phycomyces sporangiophores.