The role of cyclic nucleotides and calcium in the relaxation produced by amrinone in rat aorta

(1) The vasorelaxation produced by the phosphodiesterase 3 (PDE3) inhibitor, amrinone was investigated in isolated rat aorta denuded of endothelium. In the presence of extracellular Ca²⁺, amrinone, milrinone and 3-isobutyl-1-methylxanthine (IBMX), relaxed endothelium-denuded rat aortic rings constricted with phenylephrine. While the actions of milrinone and IBMX were inhibited by the protein kinase G (PKG) inhibitor, Rp-8-Bromo guanosine-3′,5′ monophosphothioate (Rp-8-Br-cGMPS; 0.5 mM), that of amrinone was only slightly affected; whereas the protein kinase A (PKA) inhibitor, Rp-adenosine-3′,5′ cyclic monophosphothioate (Rp-cAMPS; 0.5 mM) had no effect on any agent. (2) Amrinone (100 μM) inhibited ⁴⁵Ca²⁺ influx through receptor- or store-operated Ca²⁺ channels following stimulation with phenylephrine (1 μM) or thapsigargin (1 μM). In contrast, amrinone had no effect on KCl (120 mM)-stimulated Ca²⁺ influx. (3) In the absence of extracellular Ca²⁺, amrinone (30 μM) inhibited the constriction produced by phenylephrine, 5-hydroxytryptamine (5HT) and U46619, and this effect was not affected by Rp-cAMPS or Rp-8-Br-cGMPS. (4) The intracellular mechanism of action of amrinone may involve the phospholipase C (PLC)-inositol 1,4,5 trisphosphate (IP₃)-intracellular Ca²⁺ signal transduction pathway. However, amrinone (100 μM) had no effect on either basal- or noradrenaline (100 μM)-stimulated PLC activity. Similarly, IP₃ stimulated a concentration-dependent release of Ca²⁺ from rat brain microsomes that was not affected by amrinone (30 and 100 μM). (5) In conclusion, the vasorelaxant action of amrinone does not involve adenosine 3′,5′ cyclic monophosphate (cAMP) or involve guanosine 3′,5′ cyclic monophosphate (cGMP) but may include an inhibition of Ca²⁺ influx through receptor- or store-operated Ca²⁺ channels, although it does not directly affect intracellular Ca²⁺ release.