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 app lied to membranes of Lamprothamnium at the time of hypotonic stress. A t least three types of process were found to be involved in the respon se to this stress. (1) The first 10 min of exposure to hypotonic mediu m resulted in a depolarization of about 50 mV accompanied by a decreas e or no change in conductance. This depolarization occurred with eithe r K+ or Ca2+ (and consequently Cl-) channels inactivated. (2) The Cl- channels opened mainly in the first 15 min of the hypotonic stress, in creasing 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 stro ngly potential-dependent with a conductance peak between -150 and 0 mV . An inactivation of K+ or Cl- channels resulted in moving the membran e potential away from the conductance maximum toward either E(K) or E( Cl), diminishing the ion efflux (and turgor regulation). The time cour ses of the conductance increases remained the same, suggesting that th e conductance changes are not driven by feedback to some preset turgor level. The electrophysiology of the Lamprothamnium transporters is co mpared to that of salt-sensitive charophytes.