Modeling the Current-Voltage Characteristics of Charophyte Membranes. II. The Effect of Salinity on Membranes of Lamprothamnium papulosum

Lamprothamnium is a salt-tolerant charophyte that inhabits a broad range of saline environments. The electrical characteristics of Lamprothamnium cell membranes were modeled in environments of different salinity: full seawater (SW), 0.5 SW, 0.4 SW, and 0.2 SW. The cells were voltage-clamped to obtain the I/V (current-voltage) and G/V (conductance-voltage) profiles of the cell membranes. Cells growing at the different salinities exhibited one of three types of I/V profiles (states): pump-, background- and K(+)-states. This study concentrates on the pump- and background-states. Curved (pump-dominated) I/V characteristics were found in cells with resting membrane PDs (potential differences) of -219 +/- 12 mV (in 0.2 SW: 6 cells, 16 profiles), -161 +/- 12 mV (in 0.4 SW: 6 cells, 7 profiles), -151 +/- 12 mV (in 0.5 SW: 6 cells, 12 profiles) and -137 +/- 12 mV (in full SW: 8 cells, 13 profiles). The linear I/V characteristics of the background-state were found in cells with resting PDs of -107 +/- 12 mV (in 0.4 SW: 7 cells, 12 profiles), -108 +/- 12 mV (in 0.5 SW: 7 cells, 10 profiles) and -104 +/- 12 mV (in full SW: 3 cells, 5 profiles). The resting conductance (G) of the cells progressively increased with salinity, from 0.5 S x m(-2) (in 0.2 SW) to 22.0 S x m(-2) (in full SW). The pump peak conductance only rose from 2 S x m(-2) (0.2 SW) to 5 S x m(-2) (full SW), accounting for the increasingly depolarized resting PD observed in cells in more saline media. Upon exposure to hypertonic medium, both the pump and an inward K+ rectifier were stimulated. The modeling of the I/V profiles identified the inward K+ rectifier as an early electrical response to hypertonic challenge.