PERFUSION-INDUCED CHANGES IN CARDIAC O-2 CONSUMPTION AND CONTRACTILITY ARE BASED ON DIFFERENT MECHANISMS

Increased cardiac perfusion results in increased oxygen consumption (V O2) and increased contractility (Gregg phenomenon) in the isolated hea rt. We investigated whether these two aspects of the Gregg phenomenon are related to coronary flow or arterial pressure. Coronary flow and, thus, arterial pressure were changed in the reference state and during vasoconstriction (3 nM vasopressin) in the Langendorff-perfused rat h eart contracting isovolumically (ventricular balloon) at 27 degrees C (n = 5). All hearts showed an increase in developed isovolumic left ve ntricular pressure (measure of contractility) and in VO2 with increase d perfusion. Developed left ventricular pressure depended primarily on arterial pressure, so its relationship with coronary flow was shifted by vasoconstriction. Conversely, VO2 primarily depended on coronary f low, so its relationship with arterial pressure was shifted with vasoc onstriction. By use of vasoconstriction (decreased vascular radii), th e effects of arterial pressure and wall shear stress (proportional to arterial pressure X radius) should be separable, but the results did n ot reach significance. Thus contractility is related to arterial press ure or shear stress, whereas VO2 is related to coronary flow. We concl ude that the two aspects of the Gregg phenomenon are based on differen t mechanisms.