Regulation of basal gastric acid secretion by the glycogen synthase kinase GSK3

According to previous observations, basal gastric acid secretion is downregulated by phosphoinositol-3-(PI3)-kinase, phosphoinositide-dependent kinase (PDK1), and protein kinase B (PKB beta/Akt2) signaling. PKB/Akt phosphorylates glycogen synthase kinase GSK3. The present study explored whether PKB/Akt-dependent GSK3-phosphorylation modifies gastric acid secretion. Utilizing 2',7'-bis-(carboxyethyl)-5(6')-carboxyfluorescein (BCECF)-fluorescence, basal gastric acid secretion was determined from Na+-independent pH recovery (a dagger pH/min) following an ammonium pulse, which reflects H+/K+-ATPase activity. Experiments were performed in gastric glands from gene-targeted mice (gsk3 (KI) ) with PKB/serum and glucocorticoid-inducible kinase (SGK)-insensitive GSK alpha,beta, in which the serines within the PKB/SGK phosphorylation site were replaced by alanine (GSK3 alpha(21A/21A), GSK3 beta(9A/9A)). The cytosolic pH in isolated gastric glands was similar in gsk3 (KI) and their wild-type littermates (gsk3 (WT) ). However, a dagger pH/min was significantly larger in gsk3 (KI) than in gsk3 (WT) mice and a dagger pH/min was virtually abolished by the H+/K+-ATPase inhibitor omeprazole (100 mu M) in gastric glands from both gsk3 (KI) and gsk3 (WT) . Plasma gastrin levels were lower in gsk3 (KI) than in gsk3 (WT) . Both, an increase of extracellular K+ concentration to 35 mM [replacing Na+/N-methyl-d-glucamine (NMDG)] and treatment with forskolin (5 mu M), significantly increased a dagger pH/min to virtually the same value in both genotypes. The protein kinase A (PKA) inhibitor H89 (150 nM) and the H-2-receptor antagonist ranitidine (100 mu M) decreased a dagger pH/min in gsk3 (KI) but not gsk3 (WT) and again abrogated the differences between the genotypes. The protein abundance of phosphorylated but not of total PKA was significantly larger in gsk3 (KI) than in gsk3 (WT) . Basal gastric acid secretion is enhanced by the disruption of PKB/SGK-dependent phosphorylation and the inhibition of GSK3. Thus, the inhibition of GSK3 participates in the signaling of PI3-kinase-dependent downregulation of basal gastric acid secretion.