FUNCTIONAL COUPLING OF THE NA+ CA2+ EXCHANGER WITH CA2+ RELEASE FROM INTRACELLULAR STORES IN CULTURED SMOOTH-MUSCLE CELLS OF GUINEA-PIG ILEUM/

The mechanism of increase in intracellular Ca2+ concentration ([Ca2+]( i)) by removal of extracellular Nat, which phenomena were reported pre viously (Japan. J. Pharmacol. 63 83-91 1993), was investigated in cult ured guinea pig ileum longitudinal muscle cells loaded with a fluoresc ent Ca2+ indicator, fura-2, by digital ratio imaging microscopy. Isoto nic substitution of choline chloride for NaCl induced a transient incr ease in [Ca2+](i). The pretreatment of thapsigargin (0.5 mu M), but no t nicardipine (10 mu M), suppressed the transient increase completely. In solutions containing micromolar concentrations of free Ca2+ (nomin ally Ca2+-free solution), the Na+-free induced transient increase was observed, but neither the second cell exposure to the Na+-free solutio n nor the following application of histamine increased [Ca2+](i), indi cating that removal of extracellular Na+ releases Ca2+ from intracellu lar stores including inositol 1,4,5-trisphosphate (IP3)-releasable poo ls. The Na+-free-induced transient increase required the presence of m ore than micromolar concentrations of extracellular free Ca2+ and rele asable Ca2+ within the stores, but ryanodine did not affect the transi ent increase. These results suggest that undetectable influx of Ca2+ b y the reverse-mode action of the Na+/Ca2+ exchanger can release Ca2+ f rom the thapsigargin-sensitive intracellular stores including IP3-rele asable pools.