Contribution of cAMP to the inhibitory effect of non-steroidal anti-inflammatory drugs in rat uterine smooth muscle

1 The effect of the non-steroidal anti-inflammatory drugs naproxen, mefenamic acid, phenylbutazone, piroxicam and tolmetin on the vanadate (0.3 mM)-induced tonic contraction, as well as the modifications of these effects by the G-protein inhibitor pertussis toxin, and the inhibitors of protein kinase A, Rp-cAMPS (Rp-Adenosine 3',5'-cyclic monophosphothioate triethylamine salt) and protein kinase C, H-7 [1(5-isoquinolynilsulionyl)-2-methyl-piperazine], have been assayed to study the possible nature of intracellular mediators contributing to the inhibitory effects of NSAIDs in rat uterine smooth muscle incubated in medium lacking calcium plus EDTA. The effect of phorbol 12,13-dibutyrate on vanadate contraction and its modification with H-7 has also been examined. 2 Naproxen (6-600 mu M), mefenamic acid (6-300 mu M), phenylbutazone (6-300 mu M), piroxicam (6-600 mu M) and tolmetin (6-600 mu M) produced concentration-dependent relaxation of vanadate-induced tonic contraction. The potency order, in acordance with their respective IC50 values was: phenylbutazone greater than or equal to mefenamic acid greater than or equal to naproxen > tolmetin greater than or equal to piroxicam. 3 The relaxant effects of naproxen, phenylbutazone, piroxicam and tolmetin were significantly antagonized with pertussis toxin (50 ng ml(-1)), Rp-cAMPS (100 mu M) and H-7 (1 mu M). However, the effect of mefenamic acid was unmodified by the three drugs. This suggests that the effect of mefenamic acid and other NSAIDs occur by different mechanisms. 4 Phorbol 12,13-dibutyrate relaxed the vanadate contraction but the maximal relaxation achieved (54.8 +/- 8.3%, n = 4) was lower than those induced with the NSAIDs. On the other hand, H-7 (1 mu M) did not modify the relaxant effect of phorbol 12,13-dibutyrate. This suggests that H-7 behaves as a PKA, but not a PKC inhibitor, under the present experimental conditions. 5 The relaxation by naproxen, phenylbutazone, piroxicam and tolmetin is presumably produced by increasing cAMP because the effects of these are antagonized with Rp-cAMPS and H-7, and by pertussis-toxin-sensitive mechanisms.