Possible mechanism of the negative inotropic effect of α₁‐adrenoceptor agonists in rat isolated left atria after exposure to free radicals

Abstract

1 This study was designed to investigate the mechanism(s) of the negative inotropic eects of a 1 -adrenoceptor agonists observed in rat isolated left atria after exposure to free radicals. 2 Ouabain and calphostin C were used in contraction experiments to block the sodium pump and protein kinase C. Methoxamine-induced phospholipase C and Na + /K + ATPase activities were measured. 3 Methoxamine (300 mM) increased contractile force by 1.6+0.2 mN in control atria but decreased contractile force in electrolysis-treated atria by 2.0+0.1 mN (P50.05), as determined 10 min after methoxamine addition. In contrast, the positive inotropic eects of endothelin-1 (30 nM) and isoprenaline (10 mM) were reduced from 2.6+0.3 to 1.3+0.1 mN and from 2.6+0.3 to 1.7+0.2 mN, respectively, by electrolysis treatment (P50.05), but not converted into a negative inotropic action. 4 In an inositol phosphate assay we observed that the stimulation of phospholipase C by methoxamine was attenuated by electrolysis when the (electrolyzed) medium from the organ bath was used, but the phospholipase C responses were restored by the use of fresh medium. However, fresh medium did not counteract the negative inotropic eect of methoxamine. Accordingly, the negative inotropic eect of methoxamine is not directly related to the impaired phospholipase C responses seen in atria subjected to electrolysis. 5 Ouabain (10 mM) and the protein kinase C inhibitor calphostin C (50 nM), completely prevented the negative inotropic eect of 300 mM methoxamine in electrolysis-treated atria. 6 Measurement of the Na + /K + ATPase activity, revealed that in control atria, a 1 -adrenoceptor stimulation with 300 mM methoxamine, decreased the Na + /K + ATPase activity by 14.4+7.7%. In contrast, methoxamine increased the Na + /K + ATPase activity by 48.8+8.9% (P50.05) in electrolysistreated atria. Interestingly, this increase in Na + /K + ATPase activity was completely counteracted by calphostin C (1.4+0.1% over basal). 7 These results indicate that the negative inotropic eects of a 1 -adrenoceptor agonists, observed in rat isolated left atria exposed to free radicals, are likely to be caused by protein kinase C-mediated phosphorylation and subsequent activation of the Na + /K + ATPase.

Reactive oxygen species potentiate the negative inotropic effect of cardiac M 2 -muscarinic receptor stimulation

“…This sensitivity was reflected by a reversal of the positive inotropic-to a negative inotropic response to α 1 -adrenoceptor stimulation. This phenomenon is most likely the result of a protein kinase C-mediated activation of the Na + /K + -ATPase in rat left atria exposed to ROS (Peters et al 1998). The functional responses to β-adrenoceptor and endothelin-1 receptor stimulation are significantly blunted in atria exposed to ROS (Peters et al 1997(Peters et al , 1998.…”

Role of Protein Kinase C in Mediating Alpha-1-Adrenoceptor-Induced Negative Inotropic Response in Rat Ventricles

“…Therefore, mechanisms other than stimulation of Na + /K + ATPase may by responsible for NIE of PE in rat ventricles. Since the Na + /K + ATPase activity in healthy rat atria was inhibited by · 1 -agonist [30], species-and/or tissuespecific differences in · 1 -AR-stimulated signal transduction for the inotropic responses may exist.…”

“…· 1 -Adrenergic activation has been reported to produce negative inotropic responses in rat ventricular myocytes [3,12,18] or rat atria 'damaged' by free radicals [30] through the diacylglycerol-PKC signal transduction pathway. The PKC-mediated phosphorylation and subsequent activation of the Na + /K + ATPase may contribute to the NIE of PE.…”

“…The possible mechanisms of the aforementioned NIE by · 1 -AR agonists including the following: increased activity of Na pump [30], stimulation of the forward mode of Na/Ca exchange [35], cytoplasmic acidification [12] or mobilization of cytosolic stored Ca 2+ by inositol-1,4,5-trisphosphate (IP 3 ) [29] have been proposed. However, the modifiers of the ryanodine receptors can produce transient effects on systolic Ca 2+ due to the induced changes in sarcoplasmic reticulum (SR) Ca 2+ stores [7].…”

“…The diacylglycerol-protein kinase C (PKC) signal transduction pathway mediates the negative inotropic effect (NIE) of · 1 -AR stimulation in rat ventricular myocytes [3,12,18] or the left atria [30]. The possible mechanisms of the aforementioned NIE by · 1 -AR agonists including the following: increased activity of Na pump [30], stimulation of the forward mode of Na/Ca exchange [35], cytoplasmic acidification [12] or mobilization of cytosolic stored Ca 2+ by inositol-1,4,5-trisphosphate (IP 3 ) [29] have been proposed.…”