Bile acid accumulation in gastric mucosal cells.

Bile acids are one of the components of the gastric contents capable of disrupting the mucosal barrier to diffusion. The mechanism by which bile acids can damage the gastric epithelium is not completely understood. Several studies have emphasized mucosal lipid solubilization by bile acids in the pathogenesis of mucosal injury. Bile acid entry into gastric mucosal cells may be a critical and early step in the genesis of mucosal injury, but this possibility has not yet been investigated. The present study was designed to explore the interaction of bile acids with dispersed gastric mucosal cells isolated from the rabbit and guinea pig stomach. Results showed that both glycocholic and deoxycholic acid rapidly associated with the gastric cells and reached a steady state concentration by 30 min. Glycocholic acid accumulated in the cells to a concentration approximately eight times greater than that in the surrounding medium. The amount of bile acid associated with the cells was greater at an acidic than at a neutral pH, and was a function of the concentration of both the cells and the bile acid. The process did not require cellular energy, was nonsaturable, and was not species specific. Experiments with 86Rb, a cytoplasmic marker, revealed that approximately one half of the cellular glycocholic acid was associated with the cytoplasmic compartment and the rest with the membranes. These findings are consistent with a combination of intracellular entrapment of the bile acids due to intracellular ionization and bile acid binding to cellular membrane components being the mechanisms by which bile acids accumulate in cells. Acid-driven bile acid accumulation may explain how relatively low luminal concentrations of bile acid can be damaging to the gastrointestinal mucosa.

Distortion of H2-antagonist equilibrium constants by uptake in rabbit gastric mucosal cells.

In rabbit gastric cells the new H2-antagonists, BL-6341A, SK&F 93479 and L-643,441 were highly potent inhibitors of the H2-receptor mediated action of histamine as monitored by 14C-aminopyrine uptake and cyclic AMP formation. BL-6341A and SK&F 93479 acted as competitive antagonists to histamine and dimaprit but they were less potent against dimaprit (Ki, 8.9 nM) than against histamine (Ki, 3.5-4.4 nM). Furthermore, the Schild slope for L-643,441 against histamine was significantly higher than unity (1.69-1.79), which is inconsistent with competitive antagonism, whereas against dimaprit it was close to unity. In contrast to these antagonists, cimetidine was an equally potent competitive antagonist of both histamine and dimaprit. 3H-histamine was taken up by gastric cells as evidenced by the loss of 60-70% of the cell-associated radioactivity upon hypotonic lysis. These results suggest that uptake and possibly metabolism of histamine by rabbit gastric cells is partially responsible for the distortion of the estimated equilibrium constants for these H2-antagonists.

Comparison of cimetidine with new H2-antagonists in rabbit and guinea pig gastric cells.

We compared the effect of three relatively new H2-antagonists (compounds L-643,411, BL-6341A and SK&F 93479) to cimetidine in two preparations of mucosal cells isolated from rabbit and guinea pig stomachs. The indices for the histamine-stimulated acid secretory response were the changes in [14C]aminopyrine uptake in the rabbit and in cellular cyclic AMP, in the guinea pig. Both functions were mediated by the histamine H2-receptors and hence, can be used to examine antagonist-receptor interaction in vitro. In both rabbit and guinea pig, the new antagonists were highly potent competitive inhibitors of histamine on the H2-receptor, 30- to 200-fold more potent than cimetidine. The Ki values for cimetidine (500-800 nM) and L-643,411 (6-12 nM) were the same in the two animal species, but those for SK&F 93479 and BL-6431A were significantly lower in rabbit than in guinea pig cells. In inhibiting the changes in [14C]aminopyrine uptake in rabbit cells the Ki values for SK&F 93479 and BL-6341A were 2.4-3.5 nM whereas on cyclic AMP in guinea pig cells they were 10-fold higher (25-30 nM). These differences may reflect the structural requirements of the H2-receptors in that in rabbit these antagonists possess higher affinity for the H2-receptors than in guinea pig, or alternatively, uptake or metabolism of histamine by rabbit gastric cells may be responsible for these differences. Furthermore, these preparations appeared to be satisfactory for in vitro assay of gastric acid secretion to test for competitiveness of new H2-receptor antagonists.

Secretion of intrinsic factor from dispersed mucosal cells isolated from guinea pig and rabbit stomach.

In dispersed mucosal cells prepared from rabbit and guinea pig stomach, the secretion of intrinsic factor was constant (0.3-0.4%/min) for at least 30 min incubation at 37 degrees C. Histamine or isobutyl methylxanthine increased cyclic AMP and intrinsic factor secretion in both cells preparations. Isobutyl methylxanthine potentiated and cimetidine competitively inhibited (ki = 5.10-7 M) both effects of histamine. Dibutyryl cyclic AMP (1.0 mM), also caused a 3-fold increase in intrinsic factor secretion. These results suggest that in rabbit and guinea pig histamine interacts with H2-receptors to increase cyclic AMP which mediates the rise in the rate of intrinsic factor secretion.