The Work of Ion Movements

Abstract

During each cardiac cycle, Na⁺, Ca²⁺, H⁺ and K⁺ move across the sarcolemma. To initiate contraction, Na⁺ and Ca²⁺ move from the interstitium to the intracellular space, and H⁺ and K⁺ move out of the cell. During relaxation, the ions move in the opposite directions. Whether directly through the Na⁺ pump (also known as the Na⁺, K⁺-ATPase) or indirectly through the energy dependence of exchangers on the transmembrane Na⁺ gradient, ATP is the primary energy source maintaining normal ion gradients. Here the energetics of maintaining ion gradients will be illustrated by focusing on one cation, Na⁺. The topics of this chapter are:

• Maintaining the Na₊ Gradient in Normal Mammalian Myocardium

• Characteristics of the Na₊ pump and Why Ouabain is an Inotrope

• The Thermodynamics of the Na₊, K₊ ATPase Reaction

• On the Relationship Between ΔG_∼ATP and Na₊ Efflux by the Na₊ pump

• Definitions

• Facilitated transport is defined by the stoichiometry of the transport process. Antiport is the simultaneous transport of two different molecules in opposite directions. The Na₊/H₊ exchanger or antiporter is an example. Symport is the simultaneous transport of two different molecules in the same direction. Uniport is the movement of a single molecule.

• Electrical character of the ion transport is designated as electroneutral (i.e., no net change in charge) or electrogenic (i.e., transport results in a charge separation in space).