Characteristics of the fast and slow components of the response to noradrenaline in rat aorta.

The fast and slow components of the response to noradrenaline in rat aorta were studied to assess their relative contribution to the total mechanical response. The fast component, the magnitude of which varied with the concentration of noradrenaline in a dose-dependent manner, was shown to be dependent on release of calcium from intracellular stores. In calcium-free medium with ethylene glycol bis-(beta-aminoethylether) N,N,N',N-tetra-acetic acid (EGTA) a noradrenaline challenge (1 microM) produced a transient increase in force which faded in about 10 mins. Addition of calcium at this point produces a slow component of similar amplitude to the total response to noradrenaline. Recycling of calcium through the intracellular store does not appear to play an important role under these experimental conditions, since the presence of noradrenaline in the bath prevented its refilling. It is concluded that extracellular calcium by itself (ie, the slow component) can account for the total contractile response to noradrenaline in rat aorta.