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. I n the presence of extracellular Ca2+, amrinone, milrinone and 3-isobutyl-1- methylxanthine (IBMX), relaxed endothelium-denuded rat aortic rings constri cted with phenylephrine. While the actions of milrinone and IBMX were inhib ited by the protein kinase G (PKG) inhibitor, Rp-8-Bromo guanosine-3',5' mo nophosphothioate (Rp-8-Br-cGMPS: 0.5 mM), that of amrinone was only slightl y 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 muM) inhibited Ca-45(2+) influx through receptor- or stor e-operated Ca2+ channels following stimulation with phenylephrine (1 muM) o r thapsigargin (1 muM). In contrast, amrinone had no effect on KCL (120 mM) -stimulated Ca2+ influx. (3) In the absence of extracellular Ca2+, amrinone (30 muM) inhibited the constriction produced by phenylephrine, 5-hydroxytr yptamine (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 in volve the phospholipase C (PLC)-inositol 1,4.5 trisphosphate (IP3)-intracel lular Ca2+ signal transduction pathway. However, amrinone (100 muM) had no effect on either basal- or noradrenaline (100 muM)-stimulated PLC activity. Similarly, IP3 stimulated a concentration-dependent release of Ca2+ from r at brain microsomes that was not affected by amrinone (30 and 100 muM) (5) In conclusion, the vasorelaxant action of amrinone does not involve adenosi ne 3',5' cyclic monophosphate (cAMP) or involve guanosine 3',5' cyclic mono phosphate (cGMP) but may include an inhibition of Ca2+ influx through recep tor- or store-operated Ca2+ channels, although it does not directly affect intracellular Ca2+ release. (C) 2001 Elsevier Science Inc. All rights reser ved.