In squid axons, ATP modulates Na+-Ca2+ exchange by a Ca2+i-dependent phosphorylation

Area of Science:

• Neuroscience
• Cellular Physiology
• Ion Transport

Background:

• ATP modulates Na+-Ca2+ exchange in squid axons, affecting ion affinity.
• The precise mechanism of ATP activation (allosteric vs. hydrolysis) remains unclear.

Purpose of the Study:

• To investigate whether ATP activates Na+-Ca2+ exchange through phosphorylation.
• To differentiate between ATP hydrolysis and allosteric modulation in Na+-Ca2+ exchange activation.

Main Methods:

• Utilized [gamma-S]ATP, an ATP analog, to test kinase-dependent phosphorylation.
• Measured Ca2+ efflux and 22Na+ efflux in squid axons under various ionic conditions.
• Employed steady-state kinetics and radioactive tracer experiments.

Main Results:

• [gamma-S]ATP significantly increased Na+-dependent Ca2+ efflux, requiring Mg2+ and Ca2+.
• [gamma-S]ATP also stimulated both Ca2+-dependent Na+ efflux (reverse exchange) and Na+-dependent Na+ efflux.
• Activation by [gamma-S]ATP was greater than ATP and did not affect other ion transport systems.

Conclusions:

• The findings strongly indicate a Ca2+-dependent phosphorylation event during ATP-mediated activation of Na+-Ca2+ exchange.
• This supports a model where phosphorylation, rather than ATP hydrolysis, is the primary mechanism for ATP's modulation of the Na+-Ca2+ exchanger.