Role of Ca2+ in H+ transport by rabbit gastric glands studied with A23187 and BAPTA, an incorporated Ca2+ chelator.
The role of Ca2+ in stimulation of H+ gastric secretion by cAMP-dependent and -independent secretagogues was studied in isolated rabbit glands using Ca2+ ionophore, A23187, and an intracellular Ca2+ chelator (BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) incorporated as its acetoxymethyl ester (BAPTA-AM). Acetylcholine (ACh), tetragastrin (TG), histamine and forskolin induced a transitory increase of intracellular Ca2+ concentration, [Ca2+]i, measured in gastric glands loaded with Ca2+-sensitive dye fura-2, and provoked an acid secretory response evaluated with aminopyrine accumulation ratio (AP ratio). The Ca2+-ionophore A23187 also induced an increase in [Ca2+]i and in AP ratio. cAMP-dependent secretagogues were more potent stimulants of acid secretion than cAMP-independent secretagogues. cAMP analogue, 8-bromo-adenosine 3',5'-cyclic monophosphate (8-BR-cAMP) induced an increase in AP ratio without modifying [Ca2+]i. BAPTA-AM (5-25 microM) induced a transient decrease of resting [Ca2+]i which returned to basal level due to extracellular Ca2+ entry. Increases in [Ca2+]i produced by ACh and TG were abolished by BAPTA and those produced by Ca2+ ionophore A23187 were partially buffered. BAPTA inhibited in a dose-dependent manner H+ secretion induced by cholinergic and gastrinergic stimulants in the presence of cimetidine. A23187 increased the AP ratio to values similar to those obtained with ACh or TG and was not inhibited by BAPTA. BAPTA partially inhibited (40%) the increase in AP ratio induced by forskolin and histamine inspite of the complete inhibition of the Ca2+ response. BAPTA did not inhibit the response to 8-BR-cAMP. BAPTA inhibition of forskolin stimulation was reversed by A23187 and the response was potentiated. These results indicate that ACh and TG response are completely dependent on an increase of [Ca2+]i. The response to cAMP-dependent agonists histamine and forskolin depend both on Ca2+ and cAMP. For forskolin stimulation the response may be the result of a potentiation between Ca2+ and cAMP.
Michelangeli
F
F
Centro de Biofisica y Bioquimica, Instituto Venezolano de Investigaciones Cientificas, Caracas, Venezuela.
Ruiz
M C
MC
Fernández
E
E
Ciarrocchi
A
A
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000109:Acetylcholine / Q000494:pharmacology; D000632:Aminopyrine / Q000378:metabolism; D001572:Benzofurans; D000001:Calcimycin / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002614:Chelating Agents / Q000494:pharmacology*; D005576:Colforsin / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology* / Q000031:analogs & derivatives; D016257:Fura-2; D005744:Gastric Acid / Q000378:metabolism*; D005753:Gastric Mucosa / Q000187:drug effects / Q000378:metabolism*; D006632:Histamine / Q000494:pharmacology; D007700:Kinetics
D001572:Benzofurans; D002614:Chelating Agents; D000632:Aminopyrine; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; D000001:Calcimycin; D004533:Egtazic Acid; D006632:Histamine; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D000109:Acetylcholine; D002118:Calcium; D016257:Fura-2