REGIONAL MYOCARDIAL FLOW AND CAPILLARY PERMEABILITY-SURFACE AREA PRODUCTS ARE NEARLY PROPORTIONAL

Analyses of data on the transcapillary exchange and cellular uptake in the normal heart have generally been based on the assumption that loc al membrane conductances and volumes of distribution are everywhere th e same. The question is whether such an assumption is justified in vie w of the marked (sixfold) heterogeneity of local blood flows per gram tissue. The method was to estimate both flow and capillary membrane pe rmeability-surface area products (PS) locally in the heart. For each o f five dogs running on a sloped treadmill, the deposition of tracer mi crospheres and of [I-131]iodophenylpentadecanoic acid (IPPA), after le ft atrial injection, was determined in 256 pieces of left ventricular myocardium by killing the animals at similar to 100 s after radiotrace r injection. A hydraulic occluder stopped the flow to a portion of the myocardium supplied by the left circumflex coronary artery 30 s befor e tracer injection. Regional flows ranged from 0.1 to 7.0 ml.g(-1).min (-1). IPPA extractions ranged from 20 to 49%. Using the known flows, w e assumed the applicability of an axially distributed blood-tissue exc hange model to estimate the PS for the capillary (PS,) and the parench ymal cell. It was impossible to explain the data if the PS, values for membrane transport were uniform throughout the organ. Rather, the onl y reasonable descriptors of the data required that local PS, values in crease with local flow, almost in proportion. Current methods of analy sis using data based on deposition methods need to be revised to take into account the near proportionality of PS to flow for at least some substrates.