Intracellular Ca regulation during secretagogue stimulation of the parietal cell

Intracellular [Ca] ([Ca]i) was measured following secretagogue stimulation of rabbit gastric glands loaded with the Ca-sensitive dye fura-2. Glands were mounted on cover slips and placed either in a perfused cuvette (for spectrofluorimetric measurements on whole glands) or in a chamber on the stage of a microscope (for microspectrofluorimetric measurements on single parietal cells within a gland). In parietal cells resting [Ca]i = 91 nM. Either histamine or carbachol caused [Ca]i to increase (spike) rapidly (within 5 s) to greater than 425 nM by releasing Ca from an intracellular store. The two hormones acted on the same store, with carbachol being the more potent releaser. The spike occurred equally well in solutions containing the Ca channel blockers verapamil, nifedipine, Co, or La. After the spike, [Ca]i decreased to a plateau level (130-200 nM) that was dependent on the presence of secretagogue but was independent from the release of the intracellular store. This plateau persisted (up to 40 min) until the addition of antagonist or until the removal of either extracellular Ca or the agonist. Histamine and carbachol were specifically blocked by the H2-antagonist cimetidine and the muscarinic antagonist atropine, respectively. Histamine often induced repeated increases in [Ca]i. A combination of dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) and the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) had no effect on [Ca]i, but if cells were pretreated with histamine, DBcAMP + IBMX would cause [Ca]i to increase. Because exposure to DBcAMP + IBMX stimulates acid secretion yet does not affect [Ca]i, two independent pathways to acid secretion may exist.