h-89 and pimobendan

The effects of pimobendan, a Ca(2+) sensitizer with inhibitory action against cyclic-GMP-inhibited phosphodiesterase (PDE-III), on catecholamine (CA) secretion were studied in bovine adrenal chromaffin cells. In intact cells, pimobendan (10 - 100 microM) inhibited CA secretion stimulated by acetylcholine (10 and 30 microM) and 1,1-dimethyl-4-phenyl-piperazinium (DMPP) (3 and 10 microM), but facilitated CA secretion stimulated by high K(+) (30 mM), histamine (3 microM), and angiotensin-II (3 microM). Histamine and angiotensin-II had no effect on CA secretion in Ca(2+)-free medium. The inhibition or facilitation by pimobendan of the stimulation-evoked CA secretion was not affected by H-89 (1 microM) and H-8 (30 microM), inhibitors of cyclic-AMP-dependent protein kinase. Milrinone (10 and 30 microM) and amrinone (100 and 300 microM), inhibitors of PDE-III, did not affect the stimulation-evoked CA secretion. In beta-escin-permeabilized cells, pimobendan (10 - 100 microM) did not affect CA secretion stimulated by Ca(2+) (0.1 - 10 microM) in the presence and absence of MgATP (2 mM). These results indicate that pimobendan has dual effects, inhibition and facilitation, on CA secretion. The inhibition may be due to an inhibitory action on nicotinic receptors and the facilitation may be due to a facilitatory action on stimulation-induced Ca(2+) influx. Neither Ca(2+) sensitizing nor PDE-III inhibiting actions seem to be related to these effects.

Topics: 3',5'-Cyclic-AMP Phosphodiesterases; Acetylcholine; Adrenal Glands; Amrinone; Angiotensin II; Animals; Calcium; Catecholamines; Cattle; Cells, Cultured; Chromaffin Cells; Cyclic AMP-Dependent Protein Kinases; Cyclic Nucleotide Phosphodiesterases, Type 3; Dimethylphenylpiperazinium Iodide; Histamine; Isoquinolines; Milrinone; Potassium; Pyridazines; Sulfonamides

The aim of this study was to clarify the mechanisms of activation of Ca2+-dependent K+ (K(Ca)) channels by pimobendan, a new cardiotonic and vasodilator agent with phosphodiesterase inhibition properties, and its main metabolite, UD-CG 212 Cl, in vascular smooth-muscle cells from porcine coronary arteries. Both pimobendan and UD-CG 212 Cl induced relaxation of porcine coronary artery strips. However, in the presence of 100 microM Rp-cAMPS (Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate), a membrane-permeable antagonist of cAMP, the effects of pimobendan decreased significantly. Application of 1 microM pimobendan activated K(Ca) channels in cell-attached patches, and this increase in activity was suppressed by 100 nM H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoline-sulfonamide), a cAMP-dependent protein-kinase inhibitor. Pimobendan was ineffective in altering the activity of K(Ca) channels in inside-out patches. In contrast, UD-CG 212 Cl, at a concentration of 1 microM, activated K(Ca) channels not only in cell-attached patches but also in inside-out patches. Application of 100 nM H-89 also inhibited UD-CG 212 Cl-induced K(Ca) channels activity but to a lesser degree than that induced by pimobendan in cell-attached patches. In conclusion, there are two mechanisms of activation of K(Ca) channels by pimobendan and UD-CG 212 Cl. Activation by pimobendan occurs primary through the cAMP pathway, whereas UD-CG 212 Cl activates K(Ca) channels directly as well as through the cyclic AMP pathway.

Topics: Animals; Calcium; Cardiotonic Agents; Coronary Vessels; Drug Interactions; Enzyme Inhibitors; Heart; Isoquinolines; Muscle, Smooth, Vascular; Potassium Channels; Pyridazines; Sulfonamides; Swine; Vasodilation