Effects of cyclic AMP and analogues on neurogenic transmission in the rat tail artery.

1 The effects of two 8-substituted analogues of adenosine 3':5'-cyclic monophosphate (cyclic AMP) were compared with those of forskolin and isoprenaline on [3H]-noradrenaline release and vasoconstriction induced by electrical field stimulation (24 pulses at 0.4 Hz, 200 mA, 0.3 ms duration) in the rat tail artery, in the absence and in the presence of protein kinase inhibitors. 2 8-Bromo-adenosine 3':5'-cyclic monophosphate (8-bromo-cyclic AMP, 10-300 microM), 8-(4-chlorophenyl-thio)-adenosine 3':5' cyclic monophosphate (8-pCPT-cyclic AMP, 3-300 microM), forskolin (0.3-10 microM) and isoprenaline (1 nM-1 microM) all concentration-dependently enhanced stimulation-induced [3H]-noradrenaline release. The effect of cyclic AMP analogues was larger (2.5 fold at 300 microM) than those of cyclic AMP elevating drugs (1.6 fold at 10 microM for forskolin and 1.5 fold at 30 nM for isoprenaline). 3 At concentrations active at the prejunctional level, the four drugs had differential effects on stimulation-induced vasoconstriction, which was enhanced by the two cyclic AMP analogues, decreased by forskolin and not significantly altered by isoprenaline. 4 The [3H]-noradrenaline release-enhancing effects of 8-bromo-cyclic AMP, forskolin and isoprenaline were significantly decreased by the cyclic AMP-dependent protein kinase (PKA) inhibitor (N-[2-((3-(4-bromophenyl)-2-propenyl)-amino)-ethyl]-5- isoquinolinesulphonamide, di-hydrochloride) (H-89; 100 nM). By contrast they were unaffected by the cyclic GMP-dependent protein kinase (PKG) inhibitor, 8-bromo-guanosine 3':5'-cyclic monophosphorothioate, Rp-isomer (Rp-8-bromo-cyclic GMPS; 10 microM). By contrast they were unaffected by the cyclic GMP-dependent protein kinase (PKG) inhibitor,8-bromo-guanosine 3':5'-cyclic monophosphorothioate, Rp-isomer (Rp-8-bromo-cyclic GMPS; 10 MicroM).At the same concentrations the PKA inhibitor attenuated only the nerve-induced vasoconstrictor responses obtained in the presence of 8-bromo-cyclic AMP, whereas the PKG inhibitor did not modify that obtained in the presence of 8-bromo-cycic AMP or forskolin.5. Exposure to the protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (1 MicroM) enhanced nerve-evoked [3H]-noradrenaline release, and this effect was decreased by the PKC inhibitor, 2-[1-(3-dimethylaminopropyl)-indol-3-yl]-3-(-indol-3-yl)-maleimide (GF 109203X; 100 nM). However, the latter drug did not modify the enhancing effect of 8-bromo-cyclic AMP on [3H]-noradrenaline release.6. It is concluded that activation of cyclic AMP-dependent protein kinase is involved in the enhancing effect of cyclic AMP-elevating compounds on prejunctional release of noradrenaline. In addition the results provide no clear-cut evidence for a vasodilator role of PKA.