Coupling between action potential and bioluminescence inNoctiluca: Effects of inorganic ions and pH in vacuolar sap

Abstract

1. The ionic dependency of the flash-triggering action potential (FTP) evoked across the vacuolar membrane inNoctiluca was studied by injecting various salt and pH buffer solutions into the vacuole. 2. Electron microscopic observations showed that the peripheral cytoplasmic compartment sandwiched between vacuole and pellicle is 0.11±0.07 μm in thickness, so that it has a very small volume (Figs. 2 and 15). 3. The resting membrane resistance and capacitance measured across the cell surface were about 10 KΩ·cm2 and 0.8 μF/cm2, respectively. Under normal conditions the FTP, which is 50∼60 mV in amplitude, arises from a potential of −150 to −160 mV. 4. More than a tenfold increase in the concentration of K+, Ca2+, Mg2+, NH 4 + , or SO 4 2− in the vacuole produced no significant change in the amplitude of FTP (Figs. 4 and 5). 5. Injections of HCl-glycine buffer solutions of various pH were used to alter the intrinsic vacuolar pH (about 3.5) within the range of 2.5 to 3.7 without damaging the cells. The FTP amplitude rose about 58 mV per unit drop in pH (Figs. 6–9, Table 1). 6. Crude aqueous extracts from the cells emitted light most effectively when the pH was lowered from 8.2 to 5.5. None of the other major cations found in the vacuole showed any effect on the light emission of the extract (Figs. 12 and 13). 7. From these findings, we propose a proton hypothesis explaining the coupling between the bioluminescence and the action potential.