Turgor regulation in Lamprothamnium papulosum .1. I/V analysis and pharmacological dissection of the hypotonic effect

Current-voltage (I/V) analysis and pharmacological dissection were applied to membranes of Lamprothamnium at the time of hypotonic stress. At least three types of process were found to be involved in the response to this stress. (1) The first 10 min of exposure to hypotonic medium resulted in a depolarization of about 50 mV accompanied by a decrease or no change in conductance. This depolarization occurred with either K+ or Ca2+ (and consequently Cl-) channels inactivated. (2) The Cl- channels opened mainly in the first 15 min of the hypotonic stress, increasing the membrane conductance by about an order of magnitude. (3) The K+ conductance rose as the Cl- conductance started to diminish and reached a maximum after about 40 min. Both types of channel were strongly potential-dependent with a conductance peak between -150 and 0 mV. An inactivation of K+ or Cl- channels resulted in moving the membrane potential away from the conductance maximum toward either E(K) or E(Cl), diminishing the ion efflux (and turgor regulation). The time courses of the conductance increases remained the same, suggesting that the conductance changes are not driven by feedback to some preset turgor level. The electrophysiology of the Lamprothamnium transporters is compared to that of salt-sensitive charophytes.