Discriminating between capacitative and arachidonate-activated Ca(2+) entry pathways in HEK293 cells

Area of Science:

• Cellular Biology
• Physiology
• Biochemistry

Background:

• Receptor-activated calcium (Ca2+) entry is crucial for cellular signaling.
• Existing models, like the capacitative or store-operated model, may not fully explain Ca2+ influx at low agonist concentrations, which often results in intracellular Ca2+ ([Ca2+]i) oscillations.
• An arachidonic acid-regulated, noncapacitative Ca2+ entry mechanism has been identified for these conditions.

Purpose of the Study:

• To determine if the arachidonic acid-regulated Ca2+ entry pathway is distinct from the capacitative Ca2+ entry pathway.
• To differentiate between these two Ca2+ influx mechanisms using the selectivity of Ca2+-stimulated type VIII adenylyl cyclase.

Main Methods:

• Utilized HEK293 cells to investigate Ca2+ entry mechanisms.
• Employed thapsigargin to induce capacitative Ca2+ entry.
• Measured adenylyl cyclase activity as an indicator of Ca2+ entry via the capacitative pathway.
• Administered arachidonic acid to induce noncapacitative Ca2+ entry.
• Monitored intracellular Ca2+ ([Ca2+]i) levels and Ca2+ release from internal stores.

Main Results:

• Thapsigargin induced a significant increase in adenylyl cyclase activity, dependent on capacitative Ca2+ entry and independent of global [Ca2+]i or internal Ca2+ stores.
• Arachidonic acid-induced Ca2+ entry, despite being faster than thapsigargin-induced entry, did not affect adenylyl cyclase activity.
• This differential effect on adenylyl cyclase activity indicates distinct pathways for Ca2+ influx.

Conclusions:

• The arachidonic acid-activated Ca2+ entry pathway is separate and distinct from the capacitative Ca2+ entry pathway.
• These findings clarify the mechanisms of receptor-activated Ca2+ entry, particularly at low agonist concentrations.
• The study provides a molecular basis for understanding different modes of cellular calcium influx.