The primary process of reception in the Type 3 (‘water’) receptor cell of the fly, Phormia terranovae

Abstract

In so-called water receptors, the amplitude of the depolarizing receptor potential and frequency of action potential generation (APGF) increases as the solute concentration of the stimulus solution decreases. These electro-physiological experiments test the hypothesis that electrokinetic streaming potentials are responsible for the receptor potential. This requires that there be pores in the receptor membrane which have a lining carrying fixed negative charges, and which thus have greater cationic permeability than anionic. All electrical recordings were made extracellularly. The receptor showed a mean unstimulated discharge rate of 0.07 s -1 . The APGF was found to be linearly proportional to the receptor potential, with no voltage or frequency intercepts. Stimulation with electrolytes showed that the response was almost independent of the concentration, species and valency of the anion in solution, but strongly dependent upon cationic valency and concentration; (full inhibition by LaCl 3 at 1.5 mM, by CaCl 2 at 10 mM and by NaCI at 70 mM). The unimportance of anions rules out diffusion potentials as a possible source of the receptor potential. It is suggested that cations can govern the streaming potential by modifying the pore-surface ζ potential and the specific conductance of the pore contents. The effect of pH was to inhibit the response below pH = 5.0 and above pH = 11.0. Negatively charged groups lining the transmembrane pores with an isoelectric point at about pH = 3.7 could be responsible for these effects. Approximately isosmotic concentrations (mean 5.5 Osmol kg -1 ) of non-electrolytes (urea, glucose, glycerol, sucrose) much greater than those of electrolytes were needed to inhibit the response fully. A dendritic osmotic pressure of about 4.5 Osmol kg -1 was deduced. Nonelectrolytes probably modify the response by reducing the osmotic pressure difference across the receptor membrane, and thus the streaming potential. The response to water was not affected by the presence of dinitrophenol or ATP (0.1 to 10 mM) or ouabain (up to 1 g/1). No cholinergic effects were observed, although acetylcholine chloride causes inhibition in a similar manner to other 1-1 (see notation, p. 473) electrolytes. The effects of sodium cyanide and sodium chloride were similar.