| Abstract: | Modulations of the inotropic and chronotropic effects of ouabain and protein kinase C (PKC) stimulation with phorbol esters in rat right atria were examined. Cumulative administration of ouabain (3–30 μm ) caused a positive inotropic effect in a concentration-dependent manner, but did not produce a chronotropic effect. A single administration of ouabain (30 μm ) also had similar effects: +744 ± 84% (n = 23, P < 0.01) in the contractile force and ?0.7 ± 1.3% (n = 23, P > 0.05) in the sinus rate. Addition of phorbol esters reinforced the ouabain-evoked positive inotropic effect: 26.5 ± 8.9% (n = 6, P < 0.05) with 100 μm 4-β-phorbol-12,13-dibutyrate (PDB), and 6.4 ± 3.3% (n = 6, P > 0.05) with 100 μm 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Simultaneously, the mixture of ouabain and phorbol ester raised the resting tension. Phorbol esters alone caused a positive inotropic effect (by about 21–27%). Non-PKC activating phorbol ester, 4-α-phorbol-12,13-didecanoate (PDD, 100 μm ), did not have any effect. Pretreatment with the PKC inhibitor (staurosporine 100 μm ) significantly decreased the ouabain-induced positive inotropic effect and caused a negative chronotropic effect, but H-7 (1-(5-isoquinolinylsulphonyl)-2-methylpiperazine dihydrochloride) (5 μm ) had no effect. These results suggest that PKC stimulation may be involved in the ouabain-evoked responses in the right atria of rat as seen by increased cellular Ca2+ concentration (and Ca2+-sensitivity); thus the positive inotropic effect may not be due only to modulation of Na+/K+ pump activity. |
