COUPLING OF THE NA+ CA2+ EXCHANGER, NA+/K+ PUMP AND SARCOPLASMIC-RETICULUM IN SMOOTH-MUSCLE/

THE Na+/Ca2+ 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 m uscle appears to be closely coupled to Ca2+ availability from intracel lular stores3. This linkage might result if the exchanger were positio ned 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, an d attachment of contractile filaments to the membrane in smooth muscle . Here we report that the Na+/Ca2+ exchanger is largely co-distributed with the Na+/K+ pump on unique regions, of the plasma membrane in reg ister with, and close to, calsequestrin-containing regions of the sarc oplasmic 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 doma ins, and appear to provide a mechanism for the strong linkage seen in smooth muscle between Na+/K+ pumping and Na+/Ca2+ exchange, and betwee n Na+/Ca2+ exchange and Ca2+ release from the sarcoplasmic reticulum4- 7.