Significance of Na/Ca exchange for Ca2+ buffering and electrical activity in mouse pancreatic beta-cells.
We have combined the patch-clamp technique with microfluorimetry of the cytoplasmic Ca2+ concentration ([Ca2+]i) to characterize Na/Ca exchange in mouse beta-cells and to determine its importance for [Ca2+]i buffering and shaping of glucose-induced electrical activity. The exchanger contributes to Ca2+ removal at [Ca2+]i above 1 microM, where it accounts for >35% of the total removal rate. At lower [Ca2+]i, thapsigargin-sensitive Ca2+-ATPases constitute a major (70% at 0.8 microM [Ca2+]i) mechanism for Ca2+ removal. The beta-cell Na/Ca exchanger is electrogenic and has a stoichiometry of three Na+ for one Ca2+. The current arising from its operation reverses at approximately -20 mV (current inward at more negative voltages), has a conductance of 53 pS/pF (14 microM [Ca2+]i), and is abolished by removal of external Na+ or by intracellularly applied XIP (exchange inhibitory peptide). Inhibition of the exchanger results in shortening (50%) of the bursts of action potentials of glucose-stimulated beta-cells in intact islets and a slight (5 mV) hyperpolarization. Mathematical simulations suggest that the stimulatory action of glucose on beta-cell electrical activity may be accounted for in part by glucose-induced reduction of the cytoplasmic Na+ concentration with resultant activation of the exchanger.
Gall
D
D
Laboratoire de Pharmacodynamie et de Thérapeutique, Faculté de Médecine, Université Libre de Bruxelles, B-1070 Bruxelles, Belgium.
Gromada
J
J
Susa
I
I
Rorsman
P
P
Herchuelz
A
A
Bokvist
K
K
D000200:Action Potentials; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005470:Fluorometry; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials; D051379:Mice; D008954:Models, Biological; D018408:Patch-Clamp Techniques; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology
D002021:Buffers; D004791:Enzyme Inhibitors; D019831:Sodium-Calcium Exchanger; D019284:Thapsigargin; D000255:Adenosine Triphosphate; D012964:Sodium; D000252:Calcium-Transporting ATPases; D002118:Calcium