An electrogenic sodium pump in Limulus ventral photoreceptor cells

Area of Science:

• Neuroscience
• Cellular Physiology

Background:

• Intracellular recordings reveal hyperpolarization after light or sodium injection.
• This hyperpolarization is sensitive to strophanthidin and extracellular potassium levels.

Purpose of the Study:

• To investigate the role of the electrogenic sodium pump in cellular membrane potential and light responses.
• To elucidate the mechanisms underlying the observed after-hyperpolarizations and light response modulation.

Main Methods:

• Intracellular electrophysiological recordings in dark-adapted cells.
• Manipulations of extracellular ion concentrations (K+, Ca++).
• Application of strophanthidin to inhibit the sodium pump.

Main Results:

• Electrogenic sodium pump inhibition abolishes after-hyperpolarization and causes rapid depolarization.
• Prolonged inhibition leads to a progressive decrease in light response amplitude, especially with repetitive stimuli.
• The rate of response diminution is dependent on extracellular calcium concentration.

Conclusions:

• The electrogenic sodium pump directly contributes to membrane voltage and generates after-hyperpolarizations.
• Pump inhibition causes intracellular sodium accumulation, leading to increased intracellular calcium.
• Elevated intracellular calcium likely mediates the progressive reduction in receptor potential size.