Internal calcium concentration and potassium permeability in Paramecium

Area of Science:

• Cellular electrophysiology
• Ion transport mechanisms
• Protozoan biology

Background:

• Intracellular calcium ([Ca]i) is a critical second messenger regulating numerous cellular processes.
• Understanding ion channel function in excitable cells like Paramecium is essential for deciphering cell signaling.
• Potassium (K) permeability plays a key role in determining the resting membrane potential and excitability of Paramecium.

Purpose of the Study:

• To investigate the role of intracellular calcium ([Ca]i) in controlling steady-state potassium (K) permeability in Paramecium tetraurelia.
• To elucidate the effects of altered [Ca]i on membrane potential, resistance, and ion currents.
• To characterize the contribution of calcium currents to depolarization under conditions of suppressed K permeability.

Main Methods:

• Ionophoretic injection of calcium (Ca) and ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA) into Paramecium tetraurelia.
• Measurement of resting membrane potential and resistance.
• Voltage-clamp analysis of current-voltage (I-V) relationships under varying ionic conditions (K- and TEA-solutions).
• Recording of prolonged calcium spikes.

Main Results:

• Calcium injection decreased membrane resistance and hyperpolarized the membrane, while EGTA had opposite effects.
• EGTA injection, particularly after TEA (a K channel blocker), mimicked TEA's effects on resistance and resting potential, indicating [Ca]i controls steady-state K permeability.
• In TEA-solution, altered [Ca]i influenced the I-V relationship, with inward currents resembling those seen after TEA injection.
• Prolonged depolarization in TEA-solution with low [Ca]i was attributed to a calcium current, consistent with the Nernst slope for Ca2+.

Conclusions:

• Intracellular calcium ([Ca]i) is a significant regulator of steady-state potassium (K) permeability in Paramecium.
• Calcium currents contribute to depolarization in Paramecium, especially when K permeability is suppressed.
• Further research is needed to address quantitative challenges in measuring internal calcium and fully differentiating EGTA and TEA effects.