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This study examines the transient response of coronary blood flow to acute changes in O2 content at normal and high arterial PO2 (Pa, O2) in the blood-perfused, working isolated rat heart. The perfusion system used in this study presents the following advantages: it eliminates the gas/blood interface, includes a peripheral circulation for control of pre-load and after-load, and allows for rapid change of perfusates and continuous recording of aortic and coronary blood flow. With this system the isolated rat heart is capable of stable haemodynamic performance for periods in excess of 4 h. A sudden decrease in O2 content from 0.147 to 0.067 11(-11) at constant Pa,O2 (133 mmHg; n = 15) was associated with a marked increase in coronary blood flow (QCOR). This increase showed two phases: a rapid phase which reached 200% of the control value in 20 s, followed by a slow phase (235% in 90 s). When the same decrease in O2 content (0.135 to 0.057 11(-1] was associated with an increase in Pa, O2 (n = 22; 143 to 412 mmHg), the response of QCOR was limited both in amplitude (175% rather than 235%) and in rate of onset (response time of 15.6 instead of 9.2 s). These results are consistent with the majority of currently popular hypotheses regarding control of QCOR including the adenosine hypothesis and that of vessel wall PO2 being a direct mediator. The time course of changes in coronary vascular resistance, with a Pa, O2-dependent rapid phase, suggests the simultaneous function of the two mechanisms.

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