Ammonium ions mobilize calcium from an internal pool which is insensitive to TRH and ionomycin in bovine anterior pituitary cells.
The effects of NH4Cl on cytoplasmic free calcium concentration ([Ca2+]i) and pH (pHi) in single bovine anterior pituitary cells were determined using fluorescence imaging microscopy. Addition of NH4Cl (10-40 mM) in the presence of 1 mM extracellular calcium ([Ca2+]e) increased [Ca2+]i to a peak which then fell to a sustained plateau, returning to resting levels upon removal of NH4Cl. In medium containing 0.1 microM [Ca2+]e, or in 1 mM [Ca2+]e medium containing 0.1 microM nitrendipine, the plateau was absent leaving only a transient [Ca2+]i spike. NH4Cl also increased pHi and this, like the [Ca2+]i plateau, remained elevated during the continued presence of NH4Cl. In medium containing only 0.1 microM [Ca2+]e, to preclude refilling of internal stores by entry of external calcium, repeated exposures to NH4Cl induced repeated [Ca2+]i transients. In contrast, only the initial exposure to thyrotropin releasing hormone (TRH; 20-500 nM) caused a [Ca2+]i rise but, after an additional exposure to NH4CI, TRH responses re-emerged in some cells. Pre-treatment with the calcium ionophore ionomycin abolished the rise caused by TRH, but neither TRH nor ionomycin pretreatment affected the response to NH4Cl. Neither acetate removal nor methylamine increased [Ca2+]i in medium containing 0.1 microM [Ca2+]e, although in both cases pHi increased. We conclude that in bovine anterior pituitary cells NH4Cl raises [Ca2+]i by two independent pathways, increasing net calcium entry and mobilizing Ca2+ from a TRH-insensitive calcium store.
Shorte
S L
SL
Department of Biochemistry, Medical School, University of Bristol, University Walk, UK.
Collingridge
G L
GL
Randall
A D
AD
Chappell
J B
JB
Schofield
J G
JG
D000643:Ammonium Chloride / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000032:analysis / Q000378:metabolism*; D002417:Cattle; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D016257:Fura-2; D006863:Hydrogen-Ion Concentration; D015759:Ionomycin / Q000494:pharmacology*; D008856:Microscopy, Fluorescence; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D012710:Serum Albumin, Bovine; D013973:Thyrotropin-Releasing Hormone / Q000032:analysis / Q000378:metabolism / Q000494:pharmacology*
D000643:Ammonium Chloride; D012710:Serum Albumin, Bovine; D015759:Ionomycin; D013973:Thyrotropin-Releasing Hormone; D002118:Calcium; D016257:Fura-2