A comparison of the augmentation of H1 and H2 receptor agonist stimulation of rabbit atrial chronotropic response by two phosphodiesterase inhibitors, papaverine and theophylline.

Papaverine and theophylline both increase the chronotropic response of isolated rabbit atria, however, the maximum stimulation following papaverine is much smaller than that obtained with theophylline. When these phosphodiesterase inhibitors are used in conjunction with histamine (H1 and H2 receptor agonist), 2-(2-aminoethyl) pyrazole (H2 agonist) and 3-(2-aminoethyl) triazole (H1 agonist) the responses are all considerably more than additive. This suggests that both H1 and H2 histamine receptors can activate adenyl cyclase and therefore increase the formation of cyclic AMP in spite of reports that only H2 receptors stimulate adenyl cyclase.