Are Ba2+ and Sr2+ ions transported by the Na+-Ca2+ exchanger in frog atrial cells?
Ba2+ and Sr2+ ions are widely used to replace Ca2+ ions for the study of Ca2+ channel currents in electrophysiological experiments. Using the double sucrose gap technique, we investigated the effects of Sr2+ and Ba2+ ions on the Na+ Ca2+ exchange activity in frog atrial fibres where it is the major relaxation mechanism. With either Sr2+ or Ba2+ ions instead of Ca2+ in the extracellular bath, Na-free contractures reversibly developed but with different kinetics. Voltage clamp experiments showed that the tonic tension recorded in the presence of Sr2+ or Ba2+ was markedly increased following the addition of monensin, a Na+ ionophore known to increase the intracellular Na+ activity. In Na-free solutions (Li-substituted), it was possible to induce contractures by substituting Sr2+ or Ba2+ ions for extracellular Ca2+. These contractures could be relaxed by reintroducing Na+ or Ca2+ ions in the extracellular medium. Taken together, these results suggest that Sr2+ and Ba2+ ions can interact with the Na+-Ca2+ exchange mechanism and potentially participate not only in Na+-cation but also in Ca2+-cation exchanges on either side of the sarcolemmal membrane.
Richard
S
S
Laboratoire d'Electrophysiologie et Pharmacologie Cellulaires, C.R.B.M., C.N.R.S. UPR.41, INSERM U. 249, Montpellier, France.
Potreau
D
D
Charnet
P
P
Raymond
G
G
Nargeot
J
J
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001464:Barium / Q000378:metabolism* / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D002413:Cations, Divalent / Q000378:metabolism; D006325:Heart Atria / Q000378:metabolism; D008985:Monensin / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D011893:Rana esculenta; D012964:Sodium / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D013324:Strontium / Q000378:metabolism* / Q000494:pharmacology
D002352:Carrier Proteins; D002413:Cations, Divalent; D019831:Sodium-Calcium Exchanger; D001464:Barium; D008985:Monensin; D012964:Sodium; D002118:Calcium; D013324:Strontium