Ca²⁺ ions can affect intracellular pH in mammalian cardiac muscle

Abstract

Although intracellular pH (pH_i) has important effects on both the mechanical and electrical properties of cardiac muscle^1–3, the control of pH_i in the heart is still poorly understood. One important determinant of pH_i appears to be the transmembrane Na⁺ gradient^4,5. It has therefore been suggested that Na⁺–H⁺ exchange assists in the control of pH_i in heart as has been proposed for other excitable cells^6–8. However, pH_i and the intracellular Ca²⁺ concentration ([Ca²⁺]_i) are interdependent in a variety of tissues^9–11 and it has been shown recently that pH_i can affect [Ca²⁺]_i in cardiac muscle^5,12. As [Ca²⁺]_i in cardiac muscle is also strongly influenced by the transmembrane Na⁺ gradient⁵ it is possible that the apparent Na⁺ -dependence of pH_i is secondary to changes in [Ca²⁺]_i. Previous work in cardiac muscle has not been able to separate the effects of Na⁺–H⁺ exchange and [Ca²⁺]_i on pH_i (refs 4, 5). Here we demonstrate in cardiac muscle that an increase in [Ca²⁺]_i produces an intracellular acidification which cannot be ascribed to Na⁺–H⁺ exchange.