Interstitial fluid of isolated perfused rat hearts: glucose and lactate concentration.

Interstitial fluid (IF) emerging at the ventricular surface of isolated perfused rat hearts was collected and assayed for rate of production, protein content, glucose and lactate concentration. The influence of four perfusion media was investigated during an experimental period of 150 min: Krebs-Henseleit solution (KH) containing glucose (5.5 mM); KH containing glucose and pyruvate (2 mM); Hypoxic KH (20% O2), substrates as 2; KH containing isoprenaline (8 X 10(-9) M), substrates as 2. Interstitial fluid was produced at a rate of 20 to 100 microliters/min/gww and contained proteins (0.5 to 3 g/l). Interstitial glucose concentration was lower than venous concentration by up to 50%. Interstitial lactate concentration was higher by up to 600%. Permeability X surface area products of glucose calculated from transcapillary concentration differences and transfer rates were different depending on experimental conditions, but were within the rather large range of P X S values for molecules of similar size obtained by other authors. Those of lactate were higher by a factor of 3 to 9 and can be interpreted to be influenced by the metabolic activity of the endothelium. The results demonstrate that interstitial substrate concentration can differ very markedly from intravascular concentration and cannot be estimated reliably without assay of the interstitial fluid. Capillary permeability seems to be variable under the experimental conditions of this study.