MgATP and phosphoinositides activate Na + /Ca 2+ exchange in bovine brain vesicles. Comparison with other Na + /Ca 2+ exchangers

Abstract

We investigated the metabolic modulation of the Na(+)/Ca(2+) exchanger in membrane vesicles obtained from bovine brain. The Na(+)/Ca(2+) exchanger was activated by MgATP with a K(0.5) of 336 micro M. Unlike the squid nerve Na(+)/Ca(2+) exchanger, this effect required no cytosolic component. Also, stimulation is the same in vesicles prepared and/or assayed at the ionic strength found in mammals (160 mM) or marine animals (300 mM). Other differences between squid and bovine nerve are that the bovine brain Na(+)/Ca(2+) exchanger is not stimulated by phosphagens, either phosphoarginine (molluscan source) or phosphocreatine (mammalian source); and that stimulation by MgATP in bovine brain is related to the production of polyphosphatidylinositides. In this regard bovine heart and brain Na(+)/Ca(2+) exchangers behave similarly. These results indicate that the mechanisms of metabolic regulation of the squid and mammalian nerve Na(+)/Ca(2+) exchangers are not alike and represent differences between species. Some differences found between bovine heart and brain exchangers, such as MgATP stimulation even at saturating [Ca(2+)] and the smaller degree of activation by adenosine 5'- O-(3-thiotriphosphate) (ATP-gamma-S) in the brain, may be related to the unequal isoform population in both tissues.

Sodium/Calcium Exchanger: Influence of Metabolic Regulation on Ion Carrier Interactions

“…In dialyzed squid axons, where the four exchange modes are all stimulated by MgATP, stimulation is increased by intracellular vanadate (K 1/2 ϳ10 M) and millimolar concentrations of inorganic phosphate and p-nitrophenylphosphate (75). The mammalian heart exchanger in native cells, or even expressed in heterologous cells, under patch-clamp conditions is stimulated by MgATP; mammalian heart sarcolemmal or brain vesicles require the addition of 100 -500 M vanadate (26,27). Upon removal of ATP, in the presence of Mg 2ϩ , upregulation of the exchangers is lost (93).…”

“…Incidentally, in bovine brain membrane vesicles, MgATP stimulation is related to PtdIns(4,5)P 2 , but ATP␥S is no more than one-third as effective. In this case, however, a mixture of exchanger isoforms cannot be ruled out as a possible explanation (26).…”

“…The hydrolyzable ATP analog 2-deoxy-ATP and AMP-CPP can mimic ATP, while the nonhydrolyzable analogs AMP-PCP and AMP-PNP are ineffective (71,88). Nonhydrolyzable ATP analogs are also ineffective in mammalian heart under giant patch (118) and mammalian heart and nerve vesicles (26,27). This rules out an allosteric, nonphosphorylating, role, similar to that reported for the stimulation of the cation transporting ATPases like the Na ϩ -K ϩ -dependent enzyme (20,206,240).…”

Some Biochemical Properties of the Upregulation of the Squid Nerve Na+/Ca2+ Exchanger by MgATP and Phosphoarginine

“…In a way, this result marks another difference with the mammalian cardiac and nerve exchangers. In the membrane vesicles from the mammalian heart 6 and brain, 8 MgATP upregulation requires the simultaneous presence of vanadate, whereas in dialyzed squid axons 14 and squid nerve vesicles (not shown here) it does not.…”

Effect of Ca2+ on Protein Kinase A‐Mediated Phosphorylation of a Specific Serine Residue in an Expressed Peptide Containing the Ca2+‐Regulatory Domain of Scallop Muscle Na+/Ca2+ Exchanger

“…Many studies have shown that ATP increases the affinity of the transport sites for cytoplasmic Ca 2+ and extracellular Na + on squid axonal, optic lobe, and stellate ganglion Na + /Ca 2+ exchangers, and there is strong evidence that protein kinase activity may be involved in mediating these effects 10–12 . In previous studies, 13 the Na + /Ca 2+ exchanger from the adductor muscle of the bivalve mollusk, Placopecten magellanicus was found to be a substrate for PK‐A.…”