Changes in extrinsic fluorescence intensity of the electroplax membrane during electrical excitation

The fluorescent dye 1-anilinonapthalene-8-sulfonic acid (ANS) has been used as a probe of changes in membrane conformation accompanying excitation of the electroplax ofElectrophorus electricus. ANS binds reversibly to the excitable membrane at rest. During generation of an action potential, an increase in ANS-fluorescence intensity is observed which resembles but does not strictly follow the membrane potential. Experiments using the current-clamp technique have demonstrated a linear relationship between the change in membrane potential and the change in ANS fluorescence intensity. The change in fluorescence intensity is not a consequence of binding to membrane sites of increased affinity nor of an electrophoretic concentration of ANS molecules at the membrane surface.It is not known whether the change in fluorescence intensity is due to a change in quantum yield of bound ANS or to an increase in the amount of bound ANS. In either case, the change in fluorescence intensity may be interpreted as a change in membrane conformation.

In Vitro excitation of purified membrane fragments by cholinergic agonists : II. The permeability change caused by cholinergic agonists

The effect of carbamylcholine (Carb) on the release of(22)Na(+) by excitable microsacs is reversible: Carb does not promote an irreversible lysis of the microsacs. The microsacs form closed vesicles which are more permeable to water than to solutes present in physiological media; their apparent volume is controlled by the osmotic pressure of the medium in which they are equilibrated; they behave like micro-osmometers. In the presence of Carb, the apparent volume of the microsacs does not change: Carb has no significant effect on water permeability.The time course of(22)Na(+) release does not follow a simple exponential law and is fitted by a minimum of three exponentials. The complex kinetics are not due to electrical effects but presumably are caused by an heterogeneity both in size and in nature of the microsacs population.Microsacs at rest are permeable to(45)Ca(+),(42)K(+) and(22)Na(+), and to(36)Cl(-) but slightly or not at all to(32)S-SO -2 (4) . In the presence of Carb, the permeability to(22)Na(+),(42)K(+) and(45)Ca(++) increases, whereas the permeability to(14)C-tetraethylammonium and(14)C-choline does not change. Carb has no effect on the permeability to negatively charged or uncharged permeants.The kinetics of(22)Na(+) efflux is independent of the total concentration of Na(+) either inside or outside the microsacs. The outward transport of Na(+) thus varies only with the concentration of ion facing theinside of the membrane. Efflux of(42)K(+) follows the same laws as the efflux of(22)Na(+) except that it is blocked by(+)Na ions present in the outside medium.

Use of a snake venom toxin to characterize the cholinergic receptor protein

alpha-Bungarotoxin, a polypeptide of mol wt 8000 purified from the venom of Bungarus multicinctus, blocks irreversibly and specifically the excitation by cholinergic agonists on the isolated electroplax and on purified membrane fragments in vitro. The toxin also blocks the in vitro binding of decamethonium to a protein recently isolated from electric tissue. This observation strengthens our earlier conclusion that this protein is the cholinergic receptor macromolecule.