Role of bradykinin B2 receptors in the modulation of the peristaltic reflex of the guinea pig isolated ileum

Neurophysiological mechanisms of bradykinin-evoked mucosal chloride secretion in guinea pig small intestine

AIM: To investigate the mechanism for bradykinin (BK) to stimulate intestinal secretomotor neurons and intestinal chloride secretion.

METHODS: Muscle-stripped guinea pig ileal preparations were mounted in Ussing flux chambers for the recording of short-circuit current (Isc). Basal Isc and Isc stimulated by BK when preincubated with the BK receptors antagonist and other chemicals were recorded using the Ussing chamber system. Prostaglandin E2 (PGE₂) production in the intestine was determined by enzyme immunologic assay (EIA).

RESULTS: Application of BK or B2 receptor (B2R) agonist significantly increased the baseline Isc compared to the control. B2R antagonist, tetrodotoxin and scopolamine (blockade of muscarinic receptors) significantly suppressed the increase in Isc evoked by BK. The BK-evoked Isc was suppressed by cyclooxygenase (COX)-1 or COX-2 specific inhibitor as well as nonselective COX inhibitors. Preincubation of submucosa/mucosa preparations with BK for 10 min significantly increased PGE₂ production and this was abolished by the COX-1 and COX-2 inhibitors. The BK-evoked Isc was suppressed by nonselective EP receptors and EP4 receptor antagonists, but selective EP1 receptor antagonist did not have a significant effect on the BK-evoked Isc. Inhibitors of PLC, PKC, calmodulin or CaMKII failed to suppress BK-induced PGE₂ production.

CONCLUSION: The results suggest that BK stimulates neurogenic chloride secretion in the guinea pig ileum by activating B2R, through COX increasing PGE₂ production. The post-receptor transduction cascade includes activation of PLC, PKC, CaMK, IP₃ and MAPK.

Action of GLP-1 (7-36) amide and exendin-4 on Suncus murinus (house musk shrew) isolated ileum

Glucagon-like peptide-1 (GLP-1) receptor agonists have been reported to modulate gastrointestinal motility but the mechanism is essentially unknown. In the present studies, we investigated the potency and mechanism of action of GLP-1 receptor ligands on the isolated ileum of Suncus murinus, an insectivore used in anti-emetic research. Ileal segments were mounted in organ baths containing Kreb's solution. Cumulative concentration-response curves to GLP-1 (7-36) amide (0.1-300 nM) and exendin-4 (0.1-100 nM) were constructed in the absence and presence of exendin (9-39) amide (0.3-3 nM). GLP-1 (7-36) amide and exendin-4 induced concentration-dependent contractions yielding pEC50 values of 8.4+/-0.2 and 8.4+/-0.4, respectively. Exendin (9-39) antagonized the action of both agonists in a non-competitive reversible manner, with apparent pKB values of 9.5 and 9.7, respectively. Tetrodotoxin (1 microM), atropine (1 microM) and hexamethonium (500 microM) were used to determine the contractile mechanism of action of exendin-4. Tetrodotoxin and atropine significantly antagonized (P<0.01) the contractile action of exendin-4 (10 nM); hexamethonium (500 microM) had no action. These studies suggest that GLP-1 receptor agonists contract the ileum indirectly via postganglionic enteric neurones and an involvement of muscarinic receptors. These studies provide information relevant to the use of this species to estimate the therapeutic indexes of GLP-1 receptor agonists.