TRANSMURAL BIOENERGETIC RESPONSES OF NORMAL MYOCARDIUM TO HIGH WORKSTATES

The response of myocardial high energy and inorganic phosphates (HEP a nd P-i respectively) and associated changes in myocardial blood flow, lactate uptake, and O-2 consumption (MVO(2)) rates were examined in an open-chest canine model during progressively increasing workloads ach ieved by catecholamine infusion. HEP and P-i levels (measured with tra nsmurally localized P-31-nuclear magnetic resonance spectroscopy) were unaffected by moderate increases in the level of energy expenditure b ut were significantly altered by high workloads, especially in the sub epicardium. The MVO(2) and HEP data from three different protocols tha t utilized pharmacological augmentation of blood flow demonstrated tha t the maximal rate of myocardial energy production during inotropic st imulation was dictated by perfusion limitation. This limitation was mo re severe in the subepicardial layer at the high workloads despite equ ivalent or even higher increases in blood flow to this layer, reflecti ng a preferential enhancement of demand in the outer layer by catechol amines. In contrast, under basal conditions, existence of a marginal p erfusion limitation was evident in the inner but not in the outer laye r.