Interstitial adenosine concentration during norepinephrine infusion in isolated guinea pig hearts

Abstract

This study determined the effect of norepinephrine (NE) on cardiac interstitial fluid adenosine concentration [( ADO]isf). Isolated guinea pig hearts were perfused with a Krebs-Henseleit buffer solution. Radiolabeled albumin, sucrose, and adenosine were injected under control conditions and after 3 and 20 min of NE infusion to obtain multiple indicator dilution curves that were used to determine capillary transport parameters for adenosine. These parameters together with venous adenosine concentrations were used in a mathematical model to a calculate [ADO]isf. Capillary transport parameters were not changed significantly by NE infusion. Because of uncertainty regarding two model parameters, two sets of [ADO]isf values were calculated. One set used best-fit values obtained from indicator dilution curves, and a second set used parameters chosen to provide the highest [ADO]isf values consistent with indicator dilution curves. Venous adenosine concentrations were 1.9 +/- 0.4 nM under control conditions and 243 +/- 110 and 45 +/- 25 nM after 3 and 20 min of NE infusion, respectively. Calculated [ADO]isf was 2.6-9.4, 591-1,288, and 166-324 nM, respectively, under these same conditions. We conclude that NE infusion greatly increases [ADO]isf, and adenosine is responsible for most of the vasodilation at 3 min. The subsequent fall in venous concentration is due to a fall in [ADO]isf rather than to decreased capillary permeability. Vascular resistance remained low while [ADO]isf fell, which suggests that additional vasodilators are important during maintained NE infusion.

FIG. 1

Diagram of capillary-tissue unit showing parameters used to characterize adenonsine transport and metabolism. F, mean flow of solute-containing fluid (ml · min⁻¹ · g⁻¹); G, clearance rate constant for intracellular sequestration and/or consumption (ml· min⁻¹ ·g⁻¹); PS, permeability-surface area product (ml· min⁻ · g⁻¹); V, volume; V′, volume of tracer distribution (ml/g). Subscripts: ec, endothelial cell; eta, endothelial cell abluminal surface; ccl, endothelial cell luminal surface; g, interendothelial gaps; isf, interstitial fluid; p, perfusate; pc, parenchymal cell.

FIG. 2

Representative indicator dilution curves before (A), early (B; 3 min), and late (C; min) during norepinephrine (NE) infusion. During both NE curves, tracer appearance is more rapid, and extractions of sucrose and adenosine are lower due to higher flow. PS products calculated from these curves appear in Table 1. ADO, adenosine.

FIG. 3

Relative specific activity of venous adenosine after selective endothelial labeling with [³H]adenosine (n = 3). Time zero represents resting conditions. Specific activity falls and remains low during NE infusion. Each symbol represents one heart.

FIG. 4

Left: diagram of interendothelial cleft with uptake on sides. Right: diagram of endothelial cells tiling a capillary. Dimensions are appropriate for the heart. C, concentration; h, depth of cleft; w, width of cleft; x, distance along unit; y, fraction of distance from plasma to interstitial fluid.

FIG. 5

Concentration profiles along cleft of lenght h between plasma (at y = x/h = 0) and interstitium (at y = 1). Fractional uptake is governed by dimensionless parameter βh, ratio of lateral permeation to axial diffusivity in cleft {where β = [PS_cleft/(60V_cleftD)]^1/2}. Expected values for βh are <1. Top: fixed concentrations are C_p = 0.05, C_ec = 0, and C_isf = 1 μM. Bottom: C_p = 1, C_ec = 0.1, C_isf = 0.5 μM. With βh ≥ 3, there is net influx into cleft from both ends.

FIG. 6

Fraction of tracer passing through cleft as function of product of β, permeability-to-diffusivity ratio, and cleft length h. Expected values for βh are <0.2–0.7.