Negative inotropic responses of the isolated heart of the rat to isoprenaline.

The isolated heart of the rat perfused at 38 degrees C responded to isoprenaline with an initial positive inotropism followed by a bi-phasic negative inotropic effect. The latter was associated with the simultaneous biphasic rate increase. On reduction of the perfusion temperature to 20 degrees C, isoprenaline no longer increased the rate but a negative inotropic response remained. Blockade of this by practolol suggested that it was mediated via beta(1)-adrenoceptors.