Ionomycin activates electrogenic Ca2+ influx in rat thymic lymphocytes

Area of Science:

• Cellular Biology
• Immunology
• Biochemistry

Background:

• Depolarization inhibits ionomycin-induced intracellular calcium ([Ca2+]i) increases in lymphocytes, previously attributed to plasma membrane Ca2+ pump effects.
• The role of intracellular calcium pools and electrogenic uptake in this phenomenon remains unclear.

Purpose of the Study:

• To test the hypothesis that electrogenic Ca2+ uptake, triggered by intracellular calcium release, underlies the membrane potential (Em)-sensitivity of ionomycin-mediated Ca2+ uptake in T-lymphocytes.
• To compare ionomycin-induced Ca2+ influx with store-regulated Ca2+ uptake (SRCU).

Main Methods:

• Compared Ca2+ influx characteristics induced by ionomycin and by thapsigargin (an endosomal Ca2+-ATPase inhibitor) to induce SRCU.
• Investigated the effect of compounds known to inhibit SRCU on ionomycin-mediated Ca2+ and Mn2+ uptake.
• Assessed the Em-sensitivity of ionomycin-mediated Ca2+ uptake in the absence of SRCU.

Main Results:

• Ionomycin releases Ca2+ from a thapsigargin-sensitive endosomal pool, which regulates electrogenic SRCU.
• Low doses of ionomycin-mediated Ca2+ and Mn2+ uptake are inhibited by SRCU inhibitors.
• Ionomycin-mediated Ca2+ uptake shows no Em-sensitivity when SRCU is absent.

Conclusions:

• Electrogenic SRCU activation adequately explains the Em-sensitivity of Ca2+ flux previously attributed to plasma membrane Ca2+-pump alterations.
• This mechanism may also explain ionomycin-mediated electrogenic Ca2+ transport observed in other tissues.