Effects of caffeine and 8-phenyltheophylline on the actions of purines and opiates in the guinea-pig ileum.

The antagonist effects of 8-phenyltheophylline (8-PT) and caffeine against the actions of adenosine, (-)-N6-(R-phenyl-isopropyl)-adenosine (PIA), morphine and nalorphine on the guinea pig ileum preparation were examined. Antagonism of both adenosine and PIA by caffeine and 8-PT was concentration dependent. The slopes of Schild plots for both alkylxanthines vs. adenosine were significantly less than -1 unless the adenosine reuptake blocker dipyridamole (0.1 microM) was include in the tissue bath. Under these conditions, the 95% confidence intervals of the Schild plot slopes embraced theoretical unity, suggesting competitive antagonism. The antagonism of PIA by the alkylxanthine was also competitive. Dipyridamole had no effect on the potency of PIA. The pA2 value for caffeine-adenosine was not different from that for caffeine-PIA, and the pA2 values of 8-PT-adenosine and 8-PT-PIA were similar, suggesting that these two agonists interacted with similar receptors. The pA2 values using 8-PT were approximately 1.5-fold higher than those employing caffeine, suggesting higher affinity for 8-PT at these receptors. Caffeine significantly antagonized morphine at all concentrations used (0.5-1.0 mM), but only antagonized nalorphine at the two highest concentrations. After treating ilea with the mu-specific irreversible antagonist beta-FNA (beta-funaltrexamine) (resulting in a preparation with relatively pure kappa receptor population), the antagonist effect of caffeine against morphine was reduced such that only a concentration of 1 mM resulted in significant antagonism, while the effects of caffeine against nalorphine were unchanged. 8-PT did not antagonize morphine or nalorphine.(ABSTRACT TRUNCATED AT 250 WORDS)