1 This study was designed to investigate the mechanism(s) of the negative inotropic effects of alpha(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 mu M) 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 (P<0.05), as determined 10 min after methoxamine addition. In contrast, the positive inotropic effects of endothelin-1 (30 nM) and isoprenaline (10 mu M) 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 (P<0.05), 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 effect of methoxamine. Accordingly, the negative inotropic effect of methoxamine is not directly related to the impaired phospholipase C responses seen in atria subjected to electrolysis. 5 Ouabain (10 mu M) and the protein kinase C inhibitor calphostin C (50 nM), completely prevented the negative inotropic effect of 300 mu M methoxamine in electrolysis-treated atria. 6 Measurement of the Na+/K+ ATPase activity, revealed that in control atria, alpha(1)-adrenoceptor stimulation with 300 mu M 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% (P<0.05) in electrolysis-treated 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 effects of alpha(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.
