Na(+)-dependent Ca(2+) transport modulates the secretory response to the Fcepsilon receptor stimulus of mast cells.
Immunological stimulation of rat mucosal-type mast cells (RBL-2H3 line) by clustering of their Fcepsilon receptors (FcepsilonRI) causes a rapid and transient increase in free cytoplasmic Ca(2+) ion concentration ([Ca(2+)](i)) because of its release from intracellular stores. This is followed by a sustained elevated [Ca(2+)](i), which is attained by Ca(2+) influx. Because an FcepsilonRI-induced increase in the membrane permeability for Na(+) ions has also been observed, and secretion is at least partially inhibited by lowering of extracellular sodium ion concentrations ([Na(+)](o)), the operation of a Na(+)/Ca(2+) exchanger has been considered. We found significant coupling between the Ca(2+) and Na(+) ion gradients across plasma membranes of RBL-2H3 cells, which we investigated employing (23)Na-NMR, (45)Ca(2+), (85)Sr(2+), and the Ca(2+)-sensitive fluorescent probe indo-1. The reduction in extracellular Ca(2+) concentrations ([Ca(2+)](o)) provoked a [Na(+)](i) increase, and a decrease in [Na(+)](o) results in a Ca(2+) influx as well as an increase in [Ca(2+)](i). Mediator secretion assays, monitoring the released beta-hexosaminidase activity, showed in the presence of extracellular sodium a sigmoidal dependence on [Ca(2+)](o). However, the secretion was not affected by varying [Ca(2+)](o) as [Na(+)](o) was lowered to 0.4 mM, while it was almost completely inhibited at [Na(+)](o) = 136 mM and [Ca(2+)](o) < 0.05 mM. Increasing [Na(+)](o) caused the secretion to reach a minimum at [Na(+)](o) = 20 mM, followed by a steady increase to its maximum value at 136 mM. A parallel [Na(+)](o) dependence of the Ca(2+) fluxes was observed: Antigen stimulation at [Na(+)](o) = 136 mM caused a pronounced Ca(2+) influx. At [Na(+)](o) = 17 mM only a slight Ca(2+) efflux was detected, whereas at [Na(+)](o) = 0.4 mM no Ca(2+) transport across the cell membrane could be observed. Our results clearly indicate that the [Na(+)](o) dependence of the secretory response to FcepsilonRI stimulation is due to its influence on the [Ca(2+)](i), which is mediated by a Na(+)-dependent Ca(2+) transport.
Rumpel
E
E
Department of Physics, University of Bremen, Bremen, Germany.
Pilatus
U
U
Mayer
A
A
Pecht
I
I
D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D005456:Fluorescent Dyes; D007211:Indoles; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008407:Mast Cells / Q000187:drug effects / Q000502:physiology*; D008956:Models, Chemical; D051381:Rats; D017455:Receptors, IgE / Q000502:physiology*; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D013324:Strontium / Q000493:pharmacokinetics / Q000494:pharmacology; D001619:beta-N-Acetylhexosaminidases / Q000378:metabolism
D005456:Fluorescent Dyes; D007211:Indoles; D017455:Receptors, IgE; D000584:Amiloride; C064515:2',4'-dichlorobenzamil amiloride; D012964:Sodium; D001619:beta-N-Acetylhexosaminidases; C048960:indo-1; D002118:Calcium; D013324:Strontium