Intracellular Ca requirements for stimulus-secretion coupling in parietal cell

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Abstract
The role of cytosolic free Ca (Cai) in stimulating acid production by the parietal cell in response to the secretagogues carbachol (carb), histamine (hist), and dibutyryl adenosine 3',5'-cyclic monophosphate plus 3-isobutyl-1-methylxanthine (DBcAMP + IBMX) was evaluated. Microfluorimetry with fura-2 was used to measure Cai in single parietal cells within intact rabbit gastric glands. Acid production was determined in parallel experiments by monitoring the accumulation of [14C]aminopyrine (AP) in suspensions of glands. Carb increased peak Cai levels to 1 microM in a dose-dependent manner [concentration for half-maximal response (K0.5) = 8 microM] that correlated well with the dose dependence of carb-stimulated AP accumulation (K0.5 = 18 microM). The Ca chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was used to attenuate secretagogue-induced Cai increases. Incubating glands for 10 min with 1 and 10 microM BAPTA/AM caused resting Cai to decrease from 119 to 93 and 80 nM, respectively. BAPTA/AM, 1 and 10 microM, blocked carb-stimulated increases in Cai by 60 and 90% and AP accumulation by 50 and 90%. Hist, which increases cytosolic cAMP, caused small and relatively infrequent increases in Cai. Even so, hist-stimulated AP accumulation was inhibited 8 and 40% by 1 and 10 microM BAPTA. DBcAMP had no effect on Cai, and AP accumulation caused by DBcAMP was unaffected by the concentrations of BAPTA tested. These data suggest that carb requires an increase in Cai as a secretory signal. Hist also exhibited some Cai dependence, which may be attributable to either a small increase in Cai or the necessity of having a specific basal level of Cai. A Cai requirement for DBcAMP-stimulated acid secretion was not detected. Thus the parietal cell possesses both Ca-dependent and Ca-independent stimulatory pathways, and at least one secretagogue (hist) may utilize both pathways.