Potentiation of cyclic AMP‐mediated vasorelaxation by phenylephrine in pulmonary arteries of the rat

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Abstract
α1‐adrenoceptor agonists may potentiate relaxation to β‐adrenoceptor agonists, although the mechanisms are unclear. We compared relaxations induced by β‐adrenoceptor agonists and cyclic AMP‐dependent vasodilators in rat pulmonary arteries constricted with prostaglandin F2α (PGF2α) or the α1‐adrenoceptor agonist phenylephrine (PE). In addition, we examined whether differences were related to cyclic AMP‐ or nitric oxide (NO) and cyclic GMP‐dependent pathways. Isoprenaline‐induced relaxation was substantially potentiated in arteries constricted with PE compared with PGF2α. Methoxamine was similar to PE, whereas there was no difference between PGF2α and 30 mM KCl. The potentiation was primarily due to a marked increase in the NO‐independent component of relaxation, from 9.1±1.7% for PGF2α to 55.1±4.4% for PE. NO‐dependent relaxation was also enhanced, but to a lesser extent (∼50%). Relaxation to salbutamol was almost entirely NO‐dependent in both groups, and was potentiated ∼50% by PE. Relaxation to forskolin (activator of adenylate cyclase) was also enhanced in PE constricted arteries. Part of this relaxation was NO‐dependent, but the major effect of PE was to increase the NO‐independent component. Propranolol diminished but did not abolish the potentiation. There was no difference in response to CPT cyclic AMP (membrane permeant analogue) between PE and PGF2α, suggesting that mechanisms distal to the production of cyclic AMP were unchanged. Relaxation to sodium nitroprusside (SNP) was the same for PE and PGF2α, although relaxation to acetylcholine (ACh) was slightly depressed. This implies that potentiation by PE does not involve the cyclic GMP pathway directly. Mesenteric arteries constricted with PE did not show potentiation of isoprenaline‐induced relaxation compared to those constricted with PGF2α, suggesting that this effect may be specific to the pulmonary circulation. These results clearly show that PE potentiates both the NO‐independent and ‐dependent components of cyclic AMP‐mediated relaxation in pulmonary arteries of the rat, although the effect on the former is more profound. We suggest that potentiation of both components is largely due to direct activation of adenylate cyclase via α1‐adrenoceptors, within the smooth muscle and endothelial cells respectively. British Journal of Pharmacology (1999) 127, 291–299; doi:10.1038/sj.bjp.0702525

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