Examination of the role of inhibition of cyclic AMP in α2‐adrenoceptor mediated contractions of the porcine isolated palmar lateral vein

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Abstract
1 We have examined the effect of elevation of cellular adenosine 3′:5′-cyclic monophosphate (cyclic AMP) on α1- and α2-adrenoceptor-mediated contraction of the isolated palmar lateral vein of the pig. Cellular cyclic AMP was increased by either inhibition of phosphodiesterase by rolipram, or direct activation of adenylyl cyclase by forskolin. 2 Noradrenaline (1 nm–10 μm) caused concentration-dependent contractions of the porcine isolated palmar lateral vein (pD2 7.32 ± 0.07, n = 10). The selective α1-adrenoceptor antagonist, prazosin (0.1 μm) and the selective α2-adrenoceptor antagonist, rauwolscine (1 μm) caused a 10 fold rightward displacement of the concentration-response curve and a combination of the two antagonists caused a 200 fold rightward displacement of the concentration-response curve. The selective α2-adrenoceptor agonist, UK-14304, also produced concentration-dependent contractions of the palmar lateral vein (pD2 7.70 ± 0.15, n = 5), but the maximum response was 55.5 ± 7.6% (n = 5) of that produced by noradrenaline. Prazosin (0.1 μm) failed to affect responses to UK-14304 but rauwolscine, 1 μm, caused a 200 fold rightward displacement. The estimated pKB value for rauwolscine (8.28 ± 0.19, n = 10) is consistent with inhibition of α2-adrenoceptors. Thus, the porcine isolated palmar lateral vein has a population of α1- and α2-adrenoceptors capable of producing a contraction. 3 Rolipram, 10 μm, and forskolin, 1 μm, caused a 2–3 fold rightward displacement of the noradrenaline concentration-response curve (CRC), but 1,9-dideoxyforskolin, 1 μm, a forskolin analogue which does not activate adenylyl cyclase, failed to produce a significant inhibition of noradrenaline-induced contractions. The combination of forskolin (1 μm) and rolipram (10 μm) were additive, producing a 20 fold rightward displacement of the noradrenaline CRC. 4 Responses to noradrenaline were similarly affected by a combination of rolipram (10 μm) and prazosin (0.1 μm) (isolation of α2-adrenoceptors) and the combination of rolipram (10 μm) and rauwolscine (1 μm) (isolation of α1-adrenoceptors), resulting in a 100 fold rightward displacement of the noradrenaline CRC. Although forskolin inhibited both α1- and α2-adrenoceptor-mediated contractions, the effects produced were not similar. In particular, noradrenaline, 0.3–3 μm, produced a significant contraction in the presence of forskolin (1 μm) and prazosin (0.1 μm) (an α2-adrenoceptor-mediated response) but not in the presence of forskolin (1 μm) and rauwolscine (1 μm) (an α1-adrenoceptor-mediated response). 5 Five minute exposure to either rolipram (10 μm) or forskolin (1 μm) elevated [3H]-cyclic AMP of the porcine isolated palmar lateral vein by approximately 70% and 150–200%, respectively. Neither noradrenaline (1 nm − 100 μm) nor UK-14304 (1 nm−100 μm) affected basal levels of [3H]-cyclic AMP, but both produced a concentration-dependent inhibition of forskolin-stimulated [3H]-cyclic AMP accumulation with a pKi of 7.43 ± 0.1 (n = 3) and 7.97 ± 0.18 (n = 3), respectively. The effect of noradrenaline against forskolin-stimulated [3H]-cyclic AMP accumulation was reversed by rauwolscine (1 μm) but not by prazosin (0.1 μm). In contrast, α2-adrenoceptor activation did not affect rolipram-induced elevation of [3H]-cyclic AMP. 6 These findings indicate that α2-adrenoceptor contractions of the porcine isolated palmar lateral vein are not produced by reduction in cellular cyclic AMP per se. It is proposed that this response involves a novel signal transduction mechanism. However, when cellular cyclic AMP has been elevated by agents that stimulate adenylyl cyclase, rather than through inhibition of phosphodiesterase, the ability of α2-adrenoceptors to inhibit cyclic AMP formation may be of functional importance in vascular smooth muscle.