Naloxone-insensitive modulation of gastric acid output by [D-Met2,Pro5]enkephalinamide in rats

Studies from our laboratory have previously shown that two opioid agonists (morphine and [D-Met2,Pro5]enkephalinamide) aggravate, whereas naloxone inhibits cold-restraint stress-induced ulceration in rats. In the present study the effects of these substances were examined using the Shay-model. Contrary to expectations, both naloxone and the opioid agonists decreased gastric acid output. Naloxone in combination with either opioid agonist failed to reverse their inhibitory action. Thus the secretory activity of the stomach may be modulated by opioids in both naloxone-reversible and irreversible ways.

Effect of [D-Met2,Pro5]enkephalinamide on gastric ulceration and transmucosal potential difference

The effects of [D-Met2,Pro5]enkephalinamide, morphine and naloxone have been examined in two different models of experimentally elicited gastric mucosal lesions. One of them was the classic cold-restraint stress-induced ulceration. The other was a less frequently applied procedure, involving the measurement of decreases in the transmucosal potential difference, which is also a sensitive indicator of mucosal damage. The opioid agonists studied, [D-Met2,Pro5]enkephalinamide and morphine, aggravated, whereas naloxone, the pure opiate antagonist, mitigated the lesions in both models. The protective action of naloxone points to an eventual role of endogenous opioids in the generation of these types of mucosal lesions. Morphine is selective ligand of mu-opiate receptors. The enkephalin analogue, however, binds to both mu- and delta-receptors. Therefore, the potent pro-ulcerogenic action of the enkephalin analogue indicates that both the mu- and delta-receptors were involved in these models of experimental gastric lesions. The clarification of the eventual role of kappa-receptors requires further experimental work with a selective ligand of this subtype of opiate receptors.

The changed activity of the serum kallikrein-kinin system after ligation of the superior mesenteric artery in rats

The aim of the experiments was to study the effect of ligation of the superior mesenteric artery on the activity of the kinin-kallikrein system in the plasma and on arterial blood pressure. Ligation of 60 minutes did not change the free kallikrein activity of the plasma, while prekallikrein activity significantly decreased. Blood pressure showed a tendency to increase but not to a significant degree. Five minutes after the release of the ligation (during the period when the functional parameters of the intestinal mucosa returned to normal) free kallikrein activity increased and the arterial blood pressure decreased significantly. A rinse of the intestine with 5 p.c. glucose solution prior to the release of ligation prevents the increase in free kallikrein activity and the fall of blood pressure. The substances absorbed from the bowel lumen and activating the kinin system are probably responsible for the circulation failure following the ligation of the superior mesenteric artery.

Effect of vibration on the activity of the plasma kallikrein-kinin system in the rat

Rats were exposed to whole body vibration horizontally for four hours at 5 Hz frequency and 2 cm amplitude. Of the components of the plasma kallikrein-kinin system the free (spontaneous) and kaolin-activated kallikrein (prekallikrein) activities, the concentration of bradykinin, the bradykinin splitting total kininase activity, total kallikrein-inhibitor activity and the concentration of alpha 2-macroglobulin, a major plasma kallikrein inhibitor, were estimated. Results showed that in response to acute vibration plasma free kallikrein activity was increased significantly in association with a significant reduction of prekallikrein concentration. The concentrations of bradykinin and total kininase activity were significantly elevated, too. Neither total kallikrein-inhibitor activity nor the concentration of alpha 2-macroglobulin were changed indicating that the plasma kallikrein inhibitors did not play a role in the alterations of plasma free kallikrein and prekallikrein activities. During acute vibration the plasma kallikrein-kinin system was activated probably by the enhanced catecholamine secretion. We suggest that the biological importance of this phenomenon is in the defense against the impaired microcirculation caused by catecholamines.