Na/Ca Exchange and the Sarcolemmal Ca-Pump

Abstract

The two known mechanisms responsible for extrusion of Ca from cardiac myocytes are the sarcolemmal Ca-ATPase pump and Na/Ca exchange. A plasma membrane Ca-pump was first reported in erythrocytes (Schatzmann, 1966), but is ubiquitous (Schatzmann, 1982, 1989; Carafoli & Stauffer, 1993; Guerini et al., 1998). The red cell plasma membrane Ca-pump has been most extensively characterized and appears closely related to that in other tissues. Two key features of the Ca-pump are its stimulation by Ca-calmodulin and by PKA-dependent phosphorylation. The plasma membrane Ca-pump is a P-type ATPase (like the Na/K-ATPase, H/K- ATPase and the SR Ca-ATPase). That is, it transfers the energy of ATP to a high energy phosphorylated intermediate (aspartyl residue), energy which is then used in the ion transport step (e.g. Figs 44 & 83). The purified protein (138 kDa) is no more similar to the SR Ca-pump protein than it is to the Na/K-ATPase or H/K-ATPase (Niggli et al., 1981a; Verma et al., 1988). The plasma membrane Ca-ATPase was first cloned by Shull & Greeb (1988) and there are four human isogenes (PMCA1–4) and also numerous splice variants (Carafoli, 1994). A central stretch of ~80 kDa is all that is required for Ca transport (James et al., 1988). The overall structure is similar to the related, and better characterized SR/ER Ca-ATPase (SERCA; see Chapter 7, Fig 81–82). There are probably 10 transmembrane domains (TM1–10) which make up e;~20% of the protein, while the cytosolic domains make up ~80%. The data are much more compelling for the SR Ca-ATPase, but TM4, 5 and 6 may be involved in Ca translocation. The TM2–3 loop is analogous to the hinge domain of SERCA which is thought to couple ATP hydrolysis to Ca transport. The even larger TM4–5 loop contains the site which binds ATP (and fluorescein isothiocyanate or FITC) and also the aspartate which is phosphorylated at the active site of the enzyme. A third important cytosolic domain is the carboxy terminal tail where a 30 amino acid stretch contains the regulatory calmodulin binding domain and also has regulatory sites which are phosphorylated by PKA and PKC (James et al., 1988, 1989; Wang et al., 1991). This carboxy terminal is thought to interact with the other 2 cytosolic domains in an autoinhibitory manner, such that Ca-calmodulin binding and phosphorylation relieve this inhibition (Carafoli, 1994). One Ca ion seems to be transported per ATP hydrolyzed (Rega & Garrahan, 1986) and Ca extrusion by this pump appears coupled to proton influx (1Ca:1H, Kuwayama, 1988). The turnover rate of plasma membrane Ca-pumps may approach ~20/sec with K_m(Ca) ~I μM (Schatzmann, 1989).