Time- and rate-dependence of the inotropic action of noradrenaline in the isolated guinea-pig's atrium.

UNLABELLED: The inotropic action of noradrenaline (10-6 mol/l)) was studied on a left guinea-pig's atrium which was isolated functionally by ligations and perfused during artificial stimulation (basal rhythm: 120/min) over a period of 5 to 10 hrs. Each half hour the rate was varied within a range from 60 to 300 imp/min.

RESULTS: When modernaline is applied continuously throughout the experienment, contractility in terms of amplitude and rate of pressure development is almost maintained on the control level at the stimulation rates over at least 4 hrs of perfusion. When noradrenaline is applied at various stages of perfusion it compensates for the foregoing decay of contractility even after 8 hrs. Since contractility during normal perfusion decreases much more at a lower than at a higher rate the inotropic action of noradrenaline is minimal at a high rate in the fresh preparation (this means: under physiological conditions) and is fully developed only during later stages of perfusion at a low rate (which is far below the physiological range.