Selective blockade of nicotinic acetylcholine receptors by pimobendan, a drug for the treatment of heart failure: reduction of catecholamine secretion and synthesis in adrenal medullary cells

Q4 · MEDICINE

Article

Author: Miyabi Watanabe ; Susumu Ueno ; Masato Tsutsui ; Futoshi Izumi ; Yasuhito Uezono ; Tatsuhiko Kubo ; Yumiko Toyohira ; Nobuyuki Yanagihara ; Koji Shinkai

Pimobendan, a Ca(2+) sensitizer, is used clinically in the treatment of chronic heart failure. Although chronic heart failure is associated with activation of the sympathetic nervous system, it remains unknown whether pimobendan affects the function of sympathetic neurons and the adrenal medulla. Here, we report the inhibitory effects of pimobendan on catecholamine secretion and synthesis in cultured bovine adrenal medullary cells. Pimobendan decreased the catecholamine secretion (IC(50)=29.5 microM) elicited by carbachol, an agonist at nicotinic acetylcholine receptors, but not that elicited by veratridine, an activator of voltage-dependent Na(+) channels, or by high K(+), an activator of voltage-dependent Ca(2+) channels. Pimobendan also inhibited carbachol-induced influx of (22)Na(+) (IC(50)=25.9 microM) and (45)Ca(2+) (IC(50)=26.0 microM), but not veratridine-induced (22)Na(+) influx or high K(+)-induced (45)Ca(2+) influx. The reduction of catecholamine secretion caused by pimobendan was not overcome by increasing the concentration of carbachol. UD-CG 212, an active metabolite of pimobendan, lowered carbachol-induced catecholamine secretion with a concentration/inhibition curve similar to that of pimobendan. In experiments in situ, pimobendan suppressed both basal and carbachol-stimulated (14)C-catecholamine synthesis (IC(50)=5.3 and 4.9 microM) from [(14)C] tyrosine [but not from L: -3, 4-dihydroxyphenyl [3-(14)C] alanine ([(14)C]DOPA)], as well as tyrosine hydroxylase activity (IC(50)=3.8 and 4.3 microM). These findings suggest that pimobendan inhibits carbachol-induced catecholamines secretion and synthesis through suppression of nicotinic acetylcholine receptors.

01 Sep 2002·Journal of Cardiovascular PharmacologyQ4 · MEDICINE

Mechanisms of Action of Novel Cardiotonic Agents

Q4 · MEDICINE

Review

Author: Endoh, Masao

Regulation of myocardial contractility by cardiotonic agents is achieved by an increase in intracellular Ca2+ mobilization (upstream mechanism), an increase in Ca2+ binding affinity to troponin C (central mechanism), or facilitation of the process subsequent to Ca2+ binding to troponin C (downstream mechanism). cAMP mediates the regulation induced by Ca2+ mobilizers such as beta-adrenoceptor agonists and selective phosphodiesterase III inhibitors acting through the upstream mechanism. These agents act likewise on the central mechanism to decrease Ca2+ sensitivity of troponin C in association with the cAMP-mediated phosphorylation of troponin I. In addition to such a well-known action of cAMP, recent experimental findings have revealed that Ca2+ sensitizers, such as levosimendan, OR-1896, and UD-CG 212 Cl, require the cAMP-mediated signaling for induction of Ca2+ sensitizing effect. These agents shift the [Ca2+] -force relationship to the left, but their positive inotropic effect (PIE) is inhibited by carbachol, which suppresses selectively the cAMP-mediated PIE. These findings imply that cAMP may play a crucial role in increasing the myofilament Ca2+ sensitivity by cross-talk with the action of individual cardiotonic agents. No clinically available cardiotonic agents act primarily via Ca2+ sensitization, but the PIE of pimobendan and levosimendan is partly mediated by an increase in myofilament Ca2+ sensitivity. Evidence is accumulating that cardiotonic agents with Ca2+ sensitizing action are more effective than agents that act purely via the upstream mechanism in clinical settings. Further clinical trials are required to establish the effectiveness of Ca2+ sensitizers in long-term therapy for congestive heart failure patients.

01 Sep 2001·The Journal of pharmacology and experimental therapeutics

Decrease in Ca(2+)-sensitizing effect of UD-CG 212 Cl, a metabolite of pimobendan, under acidotic condition in canine ventricular myocardium.

Article

Author: Shimazaki, Y ; Takahashi, R ; Endoh, M

We studied the influence of acidosis on the positive inotropic effect of UD-CG 212 Cl (4,5-dihydro-6-[2-(4-hydroxyphenyl)-1H-benzimidazole-5-yl]-5-methyl-3(2H)-pyridazinone), an active metabolite of pimobendan, in canine ventricular trabeculae loaded with aequorin. The positive inotropic effect of UD-CG 212 Cl was markedly suppressed under acidotic conditions. The maximal contractile response to UD-CG 212 Cl was attained at 10(-5) M in the control condition at pH 7.4, but was not achieved even at 10(-4) M during acidosis. The maximal inotropic effect of UD-CG 212 Cl was 18% of the maximal response to isoproterenol (ISO(max)) in association with an increase in Ca(2+) transients of 7% of ISO(max) in the control, while they are 8 and 6% of ISO(max) under acidosis, respectively. Acidosis abolished the increase in myofilament Ca(2+) sensitivity induced by UD-CG 212 Cl, whereas the increase in Ca(2+) transients induced by the compound was not affected by acidosis. In conclusion, UD-CG 212 Cl elicited a positive inotropic effect even under acidosis, however, UD-CG 212 Cl was much less effective as a cardiotonic agent under acidosis mainly due to a decrease in the Ca(2+)-sensitizing effect under acidotic condition.

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Indication	Highest Phase	Country/Location	Organization	Date
Heart Failure	Preclinical	-	C.H. Boehringer Sohn AG & Co. KG	-

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