In spontaneously beating guinea pig right atria, levosimendan (LS, or R-[[-(1,4,5,6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)- phenyl]-hydrazono]propanedinitrile) exerted a positive chronotropic effect starting at 0.1 microM. In electrically driven guinea pig left atria, LS (0.1-10 microM) increased force of contraction without changing time parameters of contraction. In electrically driven right papillary muscles, LS (0.1-10 microM) enhanced force of contraction without affecting time parameters of contraction. The maximal effect on force of contraction at 10 microM amounted to 130 +/- 8.6% of predrug value. The positive inotropic effect of LS in papillary muscles was greatly diminished by additionally applied carbachol. In [32P]-labeled guinea pig ventricular cardiomyocytes, LS increased the phosphorylation state of phospholamban, the inhibitory subunit of troponin and C-protein. The maximal effect at 1 microM amounted to 134 +/- 8.6%, 124 +/- 4.2% and 121 +/- 8% of control for phospholamben, the inhibitory subunit of troponin and C-protein, respectively. LS (1 microM) increased cAMP content from 6.3 +/- 0.3 to 8.1 +/- 0.3 pmol/mg protein in guinea pig ventricular cardiomyocytes. Furthermore, whole-cell patch-clamp studies were performed in guinea pig ventricular cardiomyocytes. In this setup, 10 microM LS increased the amplitude of L-type Ca++ current to 402 +/- 86% of predrug value.
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