Proton-sensing Ca2+ binding domains regulate the cardiac Na+/Ca2+ exchanger.

Area of Science:

• Cardiovascular Physiology
• Molecular Cardiology
• Ion Transport Mechanisms

Background:

• The cardiac sodium-calcium exchanger (NCX) is crucial for regulating intracellular calcium ([Ca2+]i) and maintaining cardiac function.
• Dysregulation of NCX contributes to various heart diseases.
• The precise molecular mechanisms governing NCX activity, particularly the role of intracellular pH, remain incompletely understood.

Purpose of the Study:

• To investigate the impact of intracellular protons (H+) on the electrogenic cardiac sodium-calcium exchanger (NCX).
• To elucidate how pH modulates the Ca2+ binding domains (CBD1 and CBD2) of the NCX.
• To determine if physiological intracellular acidification can alter NCX function.

Main Methods:

• Measured NCX transport rate as membrane current (INCX) in intact cardiac ventricular myocytes.
• Manipulated intracellular pH (pHi) using an ammonium chloride rebound method.
• Assessed the effects of varying pHi on Ca2+ activation kinetics and Ca2+ binding affinities of NCX regulatory domains.

Main Results:

• Intracellular acidification significantly shifted the threshold for Ca2+-dependent NCX activation to higher [Ca2+]i.
• Low pHi reduced the Ca2+ affinities of both CBD1 and CBD2 regulatory domains.
• The maximum transport rate (Vmax) of NCX was not significantly affected by low pHi, but its activation was impaired.

Conclusions:

• Physiologically relevant intracellular acidification can effectively inhibit cardiac NCX activity.
• Proton binding to CBD1 and CBD2 competitively interferes with Ca2+ binding, thereby modulating NCX function.
• This study reveals a novel mechanism of NCX regulation by pH, with potential implications for cardiac health and disease.