Pumping Ca2+ up H+ gradients: a Ca2(+)-H+ exchanger without a membrane

Area of Science:

• Cellular Physiology
• Ion Transport
• Biochemistry

Background:

• Cellular processes are sensitive to intracellular calcium (Ca2+) and hydrogen (H+) ion concentrations.
• Membrane-bound transporters traditionally regulate these ions by coupling their movement across compartments.
• Spatial and temporal control of ions is crucial for cellular functions, particularly in excitable cells like cardiac myocytes.

Purpose of the Study:

• To investigate a novel mechanism of Ca2+-H+ coupling within the cytoplasm.
• To demonstrate how cytoplasmic Ca2+ can be compartmentalized without a partitioning membrane.
• To compare this novel mechanism with canonical membrane transport.

Main Methods:

• Demonstration in mammalian myocytes.
• Utilized diffusible cytoplasmic buffers (carnosine, homocarnosine, ATP) that bind Ca2+ and H+ ions.
• Investigated the competitive binding of Ca2+ and H+ to cytoplasmic buffers.

Main Results:

• Identified a novel Ca2+-H+ coupling mechanism operating within the cytoplasm.
• Showcased uphill Ca2+ transport energized by spatial H+ gradients.
• Demonstrated Ca2+ microdomain formation through buffer-mediated exchange, independent of membrane partitioning.
• This spatial cytoplasmic Ca2+-H+ exchange (cCHX) functions analogously to a pump but without a membrane.

Conclusions:

• Cytoplasmic Ca2+-H+ exchange (cCHX) provides a novel pathway for regulating intracellular Ca2+ distribution.
• This mechanism allows for the creation of localized Ca2+ microdomains, potentially regulating function locally.
• cCHX may be a widespread mechanism operational across various cell types, offering a new perspective on ion homeostasis.