Effects of α1- or β-adrenoceptor stimulation on work-loop and isometric contractions of isolated rat cardiac trabeculae

摘要

class="enumerated" style="list-style-type:decimal">We studied the effects of α1-or β-adrenoceptor stimulation on the contractility of isolated rat ventricular trabeculae at 24 °C using the work-loop technique, which simulates the cyclical changes in length and force that occur during the cardiac cycle. Some muscles were injected with fura-2 to monitor the intracellular Ca²⁺ transient.Comparison of twitch records revealed that peak force was greater and was reached earlier in work-loop contractions than in corresponding isometric contractions. This was attributed to the changes in muscle length and velocity during work-loop contractions, since the Ca²⁺ transients were largely unaffected by the length changes.Stimulation of α1-adrenoceptors (with 100 μm phenylephrine) increased net work, power production, the frequency for maximum work, and the frequency for maximum power production (fopt). The increase in net work was due to the positive inotropic effect of phenylephrine, which was similar at all frequencies investigated (0.33–4.5 Hz). The increase in fopt was attributed to an abbreviation of twitch duration induced by α1-stimulation at higher frequencies (> 1 Hz), even though the twitch became longer at 0.33 Hz.β-Adrenoceptor stimulation (with 5 μm isoprenaline) produced marked increases in net work, power output, the frequency for net work, and fopt. These effects were attributed both to the positive inotropic effect of β-stimulation, which was greater at higher frequencies, and to the reduction in twitch duration. β-Stimulation also abolished the frequency-dependent acceleration of twitch duration.The increase in power output and fopt with α1- as well as β-adrenoceptor stimulation suggested that both receptor types may contribute to the effects of catecholamines, released during stress or exercise, although the greater effects of β-stimulation are likely to predominate.