| Abstract: | When rats were exposed to immobilization stress for 1-12 h, gastric lesions did not occur at 1-6 h but did at 12 h of immobilization. Exogenous adenosine increased stress-induced gastric lesions, and dipyridamole, a blocker of adenosine uptake, potentiated the action of adenosine. The selective adenosine A1-receptor stimulants N6-cyclohexyl adenosine (CHA) and N6-(L-phenylisopropyl) adenosine (L-PIA) produced gastric lesions even in non-stressed state and markedly potentiated in dose- and time-dependent manner in stressed state. The stimulatory effect of N6-(D-phenylisopropyl) adenosine (D-PIA) on ulceration was weaker than that of CHA or L-PIA. Furthermore, intracerebral ventricular (i.c.v.) injection of adenosine or adenosine analogues produced the most rapid and most potent exacerbation of stress-induced gastric lesions relative to those induced with subcutaneous (s.c.) injection. The stress lesions enhanced by CHA were not affected by phentolamine, yohimbine, prazosin, naloxone and cholecystokinin (CCK8) but were inhibited by caffeine, clonidine, morphine and beta-endorphin. The inhibitory effect of clonidine was not antagonized by yohimbine or prazosin. The inhibition by morphine was selectively antagonized by exogenous CCK8 as well as naloxone. These results suggest that endogenous adenosine is tonically active in stress lesion formation which is modulated by opiate systems. Clonidine as well as caffeine may function as a purinoceptor antagonist, and it seems unlikely that the inhibitory effect of clonidine on stress ulcer is due to activation of alpha-adrenoceptors. |
