The acute effects of β-adrenergic stimulation on myocardial stiffness were evaluated. New-Ze aland white rabbits were treated with saline (control group) or do xorubicin to induce heart failure (HF) (DOXO-HF group). Effects of isoprenaline (10-10-10-5 M), a non-selective β-adrenergic agonist, were tested in papillary muscles from both groups. In the control group, the effects of isoprenaline were also evaluate d in the presence of a damaged endocardial endothelium, atenolol (β1-adrenoceptor antagonist), ICI-118551 (β2-adrenoceptor antagonist), KT-5720 (PKA inhibitor), L-NNA (NO-synthase inhibitor), or indomethacin (cyclooxygenase inhibitor). Passive length-tension relations were constructed before and after adding isoprenaline (10-5 M). In the control group, isoprenaline increased resting muscle length up to 1.017±0.006 L/Lmax. Correction of resting muscle length to its initial value resulted in a 28.5±3.1 % decrease of resting tension, indicating decreased muscle stiffness, as confirmed by the isoprenaline-induced right-downwa rd shift of the passive length- tension relation. These effects were modulated by β1- and β2-adrenoceptors and PKA. In DOXO-HF group, the effect on myocardial stiffness was significantly decreased. We conclude that β -adrenergic stimulation is a relevant mechanism of acute neurohumoral modulation of the diastolic function. Furthermore, this study clarifies the mechanisms by which myocardial stiffness is decreased., I. Falcão-Pires ... [et al.]., and Obsahuje bibliografii a bibliografické odkazy