Transmural distribution of extracellular purines in isolated guinea pig heart.

Abstract

The purine adenosine appears to be involved in regulation of coronary vascular tone. Little is known concerning the levels and distribution of adenosine and related purines in the extracellular fluid of the heart. We have measured epicardial and endocardial levels of adenosine, inosine, hypoxanthine, AMP, and IMP in isolated constant flow perfused guinea pig hearts by using a recently developed technique with porous nylon sampling discs. Venous effluent purine levels were also measured. Concentrations of all purines measured, excluding IMP, were significantly higher in endocardial fluid samples than in epicardial fluid samples (P < 0.05). Conversely, IMP levels were significantly lower in endocardial than in epicardial samples. The magnitude of the endocardial/epicardial ratios for adenosine, inosine, hypoxanthine, AMP, and IMP were '12:1, 4:1, 5:1, 4:1, and 1:2, respectively. To assess cellular damage, lactate dehydrogenase activity was measured in all fluid samples and was not significantly different in endocardial and epicardial fluid. These data support the existence of significant transmural gradients for extracellular purine levels in crystalloid perfused guinea pig hearts. Transmural differences in vasoactive adenosine levels may be partially due to the greater endocardial oxygen consumption and metabolism and may be involved in maintaining relatively high subendocardial blood flows in the face of high intramyocardial pressures.Adenosine may be responsible for modulating myocardial blood flow in response to alterations in oxygen supply and demand (1). This hypothesis predicts a consistent relationship between extracellular adenosine levels and coronary vascular tone or blood flow. In examining this hypothesis, the process of myocardial adenosine formation has been intensively studied (1-4). Unfortunately, most indices of adenosine formation do not directly reflect interstitial adenosine levels-the adenosine pool in direct contact with extracellular receptors. Furthermore, these same indices only reflect global changes in adenosine levels and cannot detect regional differences in extracellular adenosine. The mammalian myocardium is a heterogeneous tissue and the subepicardial and subendocardial regions differ physically, metabolically, and functionally (5-10). Little information is available regarding spatial heterogeneities in extracellular metabolite levels. Interestingly, recent studies indicate that transmural differences exist in the distribution of adenosine-regulating enzymes (11) and it has been demonstrated that transmural gradients exist for intracellular adenosine during control perfusion and ischemia in vivo (12). Thus, it is possible that physiological messengers such as adenosine may be unevenly distributed throughout the myocardial interstitial space.Recently, different methods have been developed for sampling interstitial transudate on the epicardial surface of the myocardium (13)(14)(15)(16)(17)(18). In the present study, we measured epicardial and endocardial transudate concentrat...

Stimulation of α1‐adrenoceptors and Protein Kinase C‐mediated Activation of Ecto‐5′‐Nucleotidase in Rat Hearts in vivo

“…Based on our estimate of the recovery rate of tissue adenosine of 18·0%, the concentration of adenosine in the interstitial fluid of the ventricular muscle located adjacent to the dialysis membrane was therefore •2·8 ìÒ, a value comparable to that reported in other studies, i.e. 0·3-3·6 ìÒ, using conventional microdialysis techniques (Van Wylen, Willis, Sodhi, Weiss, Lasley & Menztzer, 1990;Van Wylen, Schmit, Lasley, Gingell & Mentzer, 1992) or porous nylon sampling disc techniques (Zhu, Headrick & Berne, 1991). In contrast, the level of dialysate adenosine measured during perfusion of 100 ìÒ AMP was 9·25 ± 0·46 ìÒ, i.e.…”

The effect of glibenclamide on the production of interstitial adenosine by inhibiting ecto‐5′‐nucleotidase in rat hearts

“…Estimates of myocardial interstitial adenosine concentrations by use of conventional microdialysis techniques have been found to be 0.9–1.3 μm in dog hearts (Van Wylen et al , 1990), 0.5 μm in rat isolated perfused rat hearts (Van Wylen et al , 1992) and 1.6 μm in swine hearts (Schulz et al , 1995). In guinea‐pig isolated perfused hearts, by use of a porous nylon sampling discs technique the interstitial concentration of adenosine was estimated to be 0.3–3.6 μm (Zhu et al , 1991). Thus, the value of 2.8 μm obtained in the present study is within the range obtained in previous studies.…”

Ecto-5?-nucleotidase is expressed by pericytes and fibroblasts in the rat heart

“…The existence of two anatomic compartments, one with a short transit time and low 5'-nucleotidase activity (capillary lumen) and the other with a long transit time and high ecto-5'-nucleotidase activity (interstitium), would explain the lag time, observed until the fractional metabolism increases linearly with time when a large dose of AMP is injected as a bolus (Fleetwood et al 1989). Considering that the concentration of endogenous adenosine is much higher in the interstitial fluid of the myocardium than in the lumen of the capillaries (Wangler et al 1989;Zhu et al 1991), and that the endothelial clefts are permeable to adenosine (Wangler et al 1989), it seems possible that endogenous adenosine in the venous effluent of hearts originates largely from the interstitial space.…”

“…Whether nucleotides can diffuse out of the cardiomyocytes has not been demonstrated but AMP has been found in the interstitial fluid of the myocardium (Imai et al 1989;Zhu et al 1991). Nerve endings (Imai et al 1989) and capillary endothelia (Yang et aI.…”