Highly cooperative Ca2+ elevations in response to Ins(1,4,5)P3 microperfusion through a patch-clamp pipette

Area of Science:

• Cellular Biology
• Biochemistry
• Calcium Signaling

Background:

• Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a key second messenger regulating intracellular calcium (Ca2+) release.
• Understanding the kinetics of Ins(1,4,5)P3-induced Ca2+ signaling is crucial for deciphering cellular responses.
• Cell-to-cell heterogeneity can complicate the study of initial signaling events.

Purpose of the Study:

• To investigate the initial kinetics of Ins(1,4,5)P3-induced Ca2+ elevations with high temporal resolution.
• To address cell-to-cell variability using a combined experimental approach.
• To elucidate the mechanisms underlying the cooperativity of Ca2+ release.

Main Methods:

• Utilized a combined patch-clamp and microfluorimetry technique for high-resolution kinetic studies.
• Employed mathematical modeling, including monoexponential decay and Hill equations, to describe Ins(1,4,5)P3 perfusion and Ca2+ release.
• Analyzed Ca2+ release kinetics with and without considering intracellular Ca2+ ([Ca2+]i) feedback.

Main Results:

• Observed high cooperativity in Ins(1,4,5)P3-induced Ca2+ release, with Hill coefficients ranging from 4 to 12 when [Ca2+]i dependence was excluded.
• Confirmed high cooperativity using a poorly metabolized analog (Ins(2,4,5)P3) and found it independent of extracellular Ca2+.
• Mathematical analysis incorporating positive [Ca2+]i feedback suggested cooperativity in channel opening (n=2) and Ca2+-dependent enhancement of Ins(1,4,5)P3-induced Ca2+ release.

Conclusions:

• The initial kinetics of Ins(1,4,5)P3-induced Ca2+ signaling in single HL-60 granulocytes display significant cooperativity.
• This high cooperativity is likely due to cooperative channel gating and positive Ca2+ feedback, not Ca2+ influx or Ins(1,4,5)P3 metabolism.
• The interplay of high cooperativity and negative feedback mechanisms may be vital for fine-tuning cellular Ca2+ signals.