Heterogeneity and sampling volume dependence of epicardial adenosine concentrations

Rapid steady-state estimates of interstitial fluid (ISF) adenosine concentrations (ADOi) in the left ventricular epicardium of anesthetized dogs were obtained by the epicardial porous disc (EPD) method described herein. Because of the high temporal and spatial resolution of this method, it was ideally suited to test the hypothesis that ADOi may vary in these domains. Variance in steady-state EPD solute concentrations was quantified statistically by the coefficient of variation (CV = standard deviation/mean), which we used as an index of heterogeneity. A significant temporal variation in steady-state EPD adenosine concentrations was observed when samples were sequentially collected from one epicardial location (CV = 42.9 +/- 3.5%). When steady-state sample pairs (n = 45) were collected simultaneously from two distinct epicardial locations, a 2.6 +/- 0.3-fold mean difference in their respective adenosine concentrations was measured. About 25% of this variation was inherent in procedural methodology, based on the variability of steady-state EPD concentrations of extracellularly-equilibrated 14C sucrose (CV = 12.7 +/- 1.2%) and the variability of steady-state concentrations of both solutes measured using in vitro preparations (mean CV = 9.7 +/- 1.2%). Thus, we contend that endogenous myocardial ISF adenosine is temporally and perhaps spatially heterogeneous. Our estimates of steady-state ADOi obtained with the EPD method ranged from 0.47 to 0.99 microM. Using modifications of the EPD technique and the epicardial chamber, we also demonstrated that the adenosine concentration in 'steady-state' epicardial samples is reduced when the volume/surface area ratio of the sample buffer is increased. We hypothesize that sampling-induced decreases in steady-state ADOi underlie these observations, because losses of ISF adenosine to high volumes of sample buffer can be greater than the myocardial cells are capable of replacing. However, with the very low volume/surface area ratio of a single EPD (7.5 microliters/cm2), steady-state ADOi may remain constant during sampling, allowing for accurate determinations of ADOi with this method.