Transport systems of Ventricaria ventricosa: I/V analysis of both membranes in series as a function of [K⁺](o)

The current-voltage (I/V) profiles of Ventricaria (formerly Valonia) membranes were measured at a range of external potassium concentrations, [K⁺](o), from 0.1 to 100 mm. The conductance-voltage (G/V) characteristics were computed to facilitate better resolution of the profile change with time after exposure to different [K⁺](o). The resistance-voltage (R/V) characteristics were computed to attempt resolution of plasmalemma and tonoplast. Four basic electrophysiological stages emerged: (1) Uniform low resistance between -60 and +60 mV after the cell impalement. (2) High resistance between +50 and +150 for [K⁺](o) from 0.1 to 1.0 mm and hypotonic media. (3) High resistance between -150 and -20 mV for [K⁺](o) of 10 mm (close to natural seawater) and hypertonic media. (4) High resistance between -150 and +170 mV at [K⁺](o) of 100 mm. The changes between these states were slow, requiring minutes to hours and sometimes exhibiting spontaneous oscillations of the membrane p.d. (potential difference). Our analysis of the I/V data supports a previous hypothesis, that Ventricaria tonoplast is the more resistive membrane containing a pump, which transports K⁺ into the vacuole to regulate turgor. We associate state (1) with the plasmalemma conductance being dominant and the K⁺ pump at the tonoplast short-circuited probably by a K⁺ channel, state (2) with the K⁺ pump 'off' or short- circuited at p.d.s more negative than +50 mV, state (3) with the K⁺ pump 'on,' and state (4) with the pump dominant, but affected by high K⁺. A model for the Ventricaria membrane system is proposed.