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 atr ium and coronary artery under conditions of low and high work and with two different levels of oxygen to determine the effects on coronary f low-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 resist ance. The relationship between flow and pressure was linear but the re sistance became less pressure dependent at higher input pressures. Whe n the trout heart was perfused with oxygenated saline an increase in t he atrial filling pressure (volume loading, high work condition) reduc ed coronary resistance and increased flow for a given coronary artery input pressure. The opposite effects were seen when atrial filling pre ssure was reduced (volume loading, low work condition). Increasing ven tricular output pressure (pressure loading) resulted in an increased c oronary resistance. In torpedo, changes in preload and afterload did n ot 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 s ensitive to input pressure and oxygen demand of the ventricle; these s usceptibilities assist in the maintenance of the oxygen supply to the compact layer of ventricular muscle. In contrast, the coronary resista nce of the torpedo heart appears to be insensitive to load conditions when steady-state perfusion pressure is used. The difference between t he two species is discussed in terms of morphofunctional differences i n the coronary system.