Homeometric autoregulation in the heart

THE PROPER understanding of the inte-grated function of an organism is predi-cated, at least in part, on understanding the function of its component organs. A deter-mination of the extent to which an organ makes intrinsic readjustments to varying con-ditions, i.e., autoregulates, is a necessary, pre-cursor to any precise analysis of the effects of extrinsic influences. The term autoregula-tion will be used to describe phenomena oc-curring in an organ not subject to nervous or chemical influences originating outside of it and which phenomena can reasonably be con-strued as being of value to the performance of the total organism. One type of intrinsic response exhibited by the isolated heart is the well-known Prank-Starling mechanism which endows the ven-tricles with performance eharacte ' istics such that the heart ejects whatever volume is put into it. If inflow is augmented and end-dias-tolic pressure and fiber length are thus in-creased, the ventricle contracts more force-fully and expels an augmented stroke volume. This occurs on a beat-to-beat basis and may be designated as heterometric autoregulation. A second type of autoregulation in the iso-lated heart, one which apparently does not utilize the Frank-Starling mechanism, will be the subject of this communication. It requires at least a few beats to develop fully after an increase in activity. The ventricle then exhib-its performance characteristics such that its end-diastolic pressure and fiber length tend to be maintained because of an increase in myocardial contractility. It will, therefore, be referred to as homeometric autoregulation. Methods Experiments were performed on mongrel dogs of both sexes weighing from 16.0 to 25.5 Kg1., anesthetized with morphine sulfate (2 mg./Kg.