CORONARY FLOW-PRESSURE RELATIONSHIP IN THE WORKING ISOLATED FISH HEART - TROUT (ONCORHYNCHUS-MYKISS) TORPEDO (TORPEDO-MARMORATA)

Isolated hearts of rainbow trout and torpedo were perfused via the atrium and coronary artery under conditions of low and high work and with two different levels of oxygen to determine the effects on coronary flow-pressure relationships and to estimate coronary resistance. In all cases, an increase in input pressure to the coronary artery resulted in an increase in coronary flow through a reduction in coronary resistance. The relationship between flow and pressure was linear but the resistance became less pressure dependent at higher input pressures. When the trout heart was perfused with oxygenated saline an increase in the atrial filling pressure (volume loading, high work condition) reduced coronary resistance and increased flow for a given coronary artery input pressure. The opposite effects were seen when atrial filling pressure was reduced (volume loading, low work condition). Increasing ventricular output pressure (pressure loading) resulted in an increased coronary resistance. In torpedo, changes in preload and afterload did not affect coronary perfusion. In both preparations, reduced levels of oxygen in the coronary perfusion fluid reduced coronary resistance. It is concluded that in trout the coronary resistance is intrinsically sensitive to input pressure and oxygen demand of the ventricle; these susceptibilities assist in the maintenance of the oxygen supply to the compact layer of ventricular muscle. In contrast, the coronary resistance of the torpedo heart appears to be insensitive to load conditions when steady-state perfusion pressure is used. The difference between the two species is discussed in terms of morphofunctional differences in the coronary system.