Coupling of the Na⁺/Ca²⁺exchanger, Na⁺/K⁺ pump and sarcoplasmic reticulum in smooth muscle

Abstract

THE Na ⁺ /Ca^{2 +} exchanger, driven by a transmembrane Na⁺ gradient, plays a key role in regulating Ca²⁺concentration in many cells1'2. Although the exchanger influences Ca²⁺concentration, its activity in smooth muscle appears to be closely coupled to Ca²⁺ availability from intracellular stores³. This linkage might result if the exchanger were positioned close to Ca²⁺storage sites within the sarcoplasmic reticulum. To test this hypothesis we have developed methods to assess the relative three-dimensional distribution of proteins involved in Na⁺/K⁺ pumping, Na⁺/Ca²⁺exchange, Ca²⁺ storage within the sarcoplasmic reticulum, and attachment of contractile filaments to the membrane in smooth muscle. Here we report that the Na⁺ /Ca^{2 +} exchanger is largely co-distributed with the Na⁺/K⁺ pump on unique regions of the plasma membrane in register with, and close to, calsequestrin-containing regions of the sarcoplasmic reticulum in sites distinct from the sites where contractile filaments attach to the membrane. This molecular organization suggests that the plasma membrane is divided into at least two functional domains, and appear to provide a mechanism for the strong linkage seen in smooth muscle between Na⁺/K⁺ pumping and Na⁺/Ca²⁺exchange, and between Na⁺/Ca²⁺ exchange and Ca^{2 +} release from the sarcoplasmic reticulum^4–7.