Raising the intracellular level of inositol 1,4,5-trisphosphate changes plasma membrane ion transport in characean algae.

Abstract

Inositol 1,4,5-trisphosphate (InsP 3 ) was introduced into the cytoplasm of characean algae in two different ways: (i) by iontophoretic injection into cytoplasm-enriched fragments from Chara and (ii) by adding InsP 3 to the permeabilization medium of locally permeabilized cells of Nitella. In both systems this operation induced a depolarization of the membrane potential, ranging from a few mV to sequences of action potentials. The effect of InsP 3 on locally permeabilized Nitella cells was abolished when InsP 3 was added together with 30 mM EGTA. When inositol 1,4-bisphosphate or myo-inositol were substituted for InsP 3 in this system, there was no change in the membrane potential. On the other hand, increasing the free Ca 2+ concentration in the permeabilization medium induced, in a similar fashion to InsP 3 , action potentials. Similarities between InsP 3 and Ca 2+ action were also observed upon injection into Chara fragments. Both injections increased an inward current. In the first few seconds after injection the current/voltage characteristics of the InsP 3 -induced current resembled those of the Ca 2+ -sensitive current. Subsequently, differences between the InsP 3 - and Ca 2+ -induced phenomena became apparent in that the InsP 3 -induced current continued to increase while the Ca 2+ -induced current declined, returning to the resting level. Our results suggest that these plant cells contain an InsP 3 sensitive system that, under experimental conditions, is able to affect membrane transport via an increase in cytoplasmic free Ca 2+ .