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SG Oakes, PA Iaizzo, E Richelson and G Powis
Department of Pharmacology, Mayo Clinic and Foundation, Rochester, Minnesota.
Apparent intracellular free Ca++ concentration [(Ca++]i) was measured in differentiated N1E-115 neuroblastoma by microinjecting cells with aequorin (estimated intracellular concentration, 4 microM) and measuring light emission. Histamine produced a transient, dose- dependent increase in [Ca++]i. Pyrilamine blocked completely the response to histamine whereas cimetidine had no effect. Omitting Ca++ from the external medium reversibly blocked the response. As well as a rise in [Ca++]i, histamine caused a concomitant cell hyperpolarization that was not blocked by ouabain, low Cl-, tetraethylammonium chloride/tetradotoxin or metiamide but was blocked by apamin and pyrilamine. A secondary small depolarization caused by histamine was also blocked by apamin but not by ouabain, low Cl- or tetraethylammonium chloride/tetrodotoxin. Direct iontophoretic injection of Ca++ into cells caused only hyperpolarization. Injection of inositol 1,4,5-trisphosphate [IP3(1,4,5)] caused an increase in [Ca++]i and rapid hyperpolarization. Inositol 1,3,4-trisphosphate [IP3(1,3,4)] caused an increase in [Ca++]i, rapid hyperpolarization and a slower depolarization. Repeated injections of IP3(1,3,4) led to a diminished [Ca++]i response and decreased hyperpolarization but had no effect on depolarization. Inositol 1,3,4,5-tetrakisphosphate was without effect on [Ca++]i or on cellular membrane potential. The results suggest that histamine causes an H1 receptor-dependent increase in [Ca++]i, probably by the increased entry of extracellular Ca++, although there may be a contribution from intracellular Ca++ released by IP3(1,4,5). The increase in [Ca++]i activates K+ channels leading to cell hyperpolarization. IP3(1,3,4) formed from inositol 1,3,4,5- tetrakisphosphate, which is itself a product of IP3(1,4,5), causes a slower depolarization by a mechanism that does not involve Na+ channels or an increase in [Ca++]i.
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