ION CHANNELS IN THE PLASMA-MEMBRANE OF PROTOPLASTS FROM THE HALOPHYTIC ANGIOSPERM ZOSTERA-MUELLERI

Patch clamp studies show that there may be as many as seven different channel types in the plasma membrane of protoplasts derived from young leaves of the halophytic angiosperm Zostera muelleri. In whole-cell p reparations, both outward and inward rectifying currents that activate in a time- and voltage-dependent manner are observed as the membrane is either depolarized or hyperpolarized. Current voltage plots of the tail currents indicate that both currents are carried by K+ The channe ls responsible for the outward currents have a unit conductance of app roximately 70 pS and are five times more permeable to K+ than to Na+. In outside-out patches we have identified a stretch-activated channel with a conductance of 100 pS and a channel that inwardly rectifies wit h a conductance of 6 pS. The reversal potentials of these channels ind icate a significant permeability to K+. In addition, the plasma membra ne contains a much larger K+ channel with a conductance of 300 pS. Sin gle channel recordings also indicate the existence of two Cl- channels , with conductances of 20 and 80 pS with distinct substates. The membr ane potential difference of perfused protoplasts showed rapid action p otentials of up to 50 mV from the resting level. The frequency of thes e action potentials increased as the external osmolarity was decreased . The action potentials disappeared with the addition of Gd3+, an effe ct that is reversible upon washout.