[2-component contraction of mammalian myocardium due to noradrenaline].

The effect of noradrenaline (2 mg/l) on contractile response, its first derivative, and transmembrane action potential of isolated papillary muscles of guinea pigs, was studied. The muscles were continuously superfused with normal Tyrode's solution at 20-22 degrees C and stimulated at 0.3 Hz. Under the effect of noradrenaline the contraction consisted of two components which became more obvious at low-frequency stimulation or in the first contraction after a long resting period, the second component's amplitude being increased, at that. The increase corresponded to prolongation of the AP. In presence of noradrenaline the Bowditch's staircase became positive.