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

Abstract

1. 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.

2. 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.

3. 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 (f_opt). 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 f_opt 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.

4. β-Adrenoceptor stimulation (with 5 μm isoprenaline) produced marked increases in net work, power output, the frequency for net work, and f_opt. 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.

5. The increase in power output and f_opt 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.