The results suggest the presence of adenosine A2a receptors in the rat ventricular myocyte that appear to be responsible for an increase in inotropy via cAMP-dependent and -independent mechanisms.
Experiments were performed on isolated frog sartorius muscle and in situ dog skeletal muscle to determine whether adenine nucleotides and their degradation products are released during contraction in concentrations capable of producing arteriolar dilation. ATP was not detectable ( < 1 O -8 M ) in the bathing solution of the resting or contracting frog sartorius muscle. Inorganic phosphate (P,) in the muscle bath increased from 9 x lO" 6 ** to 28 X K Hŵ ith 30 minutes of contraction (2 Hz) or with rest. With the dog hindlimb preparation, ATP, ADP, and AMP were not detectable ( < 5 X 1 0~8 M ) in the venous blood collected after 5 minutes of ischemic contraction whereas P t was present at a concentration of 3.7 X ICMM. Arterial blood levels required to elicit detectable vasodilation for ATP, ADP, AMP, and P, were 28.7 X 1(HM, 27.1 X K H M , 31.4 X K H M and 7.2 X IO^M, respectively. The adenosine concentration in dog muscle increased from 0.7 to 1.5 nmole/g with ischemic contraction, and hypoxanthine and inosine increased from 4.5 to 8.5 nmole/g and 2.0 to 5.5 nmole/g, respectively. The adenosine concentration in venous plasma collected from the hindlimb immediately after termination of the ischemic contraction period was 2.2 X 10~7M as compared to 0.4 X 10~7M in control venous and arterial blood samples. Hypoxanthine and inosine concentrations in venous blood increased 22-and 270-fold, respectively, following ischemic contraction. The calculated interstitial fluid adenosine concentration was twice the arterial concentration of adenosine required to elicit maximal arteriolar dilation. These findings suggest that adenosine may play a role in the metabolic regulation of skeletal muscle blood flow, whereas ATP, ADP, AMP, and P, may not.
KEY WORDSautoregulation of blood flow inosine hypoxanthine skeletal muscle ischemia skeletal muscle contraction peripheral resistance reactive hyperemia functional hyperemia purine derivatives in muscle vascular smooth muscle tone dog frog• For a number of years, the adenine nucleotides have been periodically suggested as mediators of the vasodilation associated with an increase in metabolic activity or
Adenosine per se is a potent vasodilator of vascular smooth muscle. Endothelial cells modulate vascular tone via the release of nitric oxide (NO), which also elicits vasodilation. This study was undertaken to determine whether adenosine could directly stimulate endothelial cells to enhance NO production, which could subsequently reduce vascular tone. NO production was evaluated in porcine carotid artery endothelial cells (PCAEC) and human saphenous vein endothelial cells (HSVEC) seeded on multiwell plates, grown to confluence, and treated with adenosine for 1 h. The bathing medium was collected, and the NO production was determined as reflected by the formation of NO2- and NO3-. NO production by PCAEC was significantly increased by adenosine in a dose-dependent manner, whereas there was only an insignificant tendency for an increase by HSVEC. The addition of the NO synthase competitive inhibitor, NG-monomethyl-L-arginine (NMMA), or the adenosine receptor antagonist, theophylline, prevented the increase in NO production by adenosine. The results suggest that adenosine stimulates, by a receptor-mediated mechanism, the production of NO by arterial, but not by venous, endothelial cells.
The effect of adenosine on proliferation of human endothelial cells was investigated by adding adenosine to the medium of cultures derived from human umbilical veins. Cell counts on cultures grown in 10 microM adenosine for 4-7 days were 41-53% greater than counts from control cultures. In contrast, 10 microM adenosine had no effect on growth of a human fibroblast cell strain (IMR-90). Neither inosine nor 2',5'-dideoxyadenosine influenced endothelial cell growth at concentrations of 0.1 or 10 microM. Addition of adenosine deaminase abolished the proliferative effect of added adenosine and inhibited proliferation by 16% in control cultures, suggesting that endogenous adenosine may enhance proliferation in culture. The adenosine receptor antagonist, 8-phenyltheophylline, at 0.1 and 1.0 microM blocked the enhanced proliferation caused by 10 microM adenosine. Addition of 10 microM adenosine enhanced DNA synthesis in endothelial cell cultures as indicated by an increased incorporation of [3H]thymidine into acid-insoluble cell material. The results indicate that addition of physiological concentrations of adenosine to human umbilical vein endothelial cell cultures stimulates proliferation, possibly via a surface receptor, and suggest that adenosine may be a factor for human endothelial cell growth and possibly angiogenesis.
Adenosine A(2a)-receptor activation enhances shortening of isolated cardiomyocytes. In the present study the effect of A(2a)-receptor activation on the contractile performance of isolated rat hearts was investigated by recording left ventricular pressure (LVP) and the maximal rate of LVP development (+dP/dt(max)). With constant-pressure perfusion, adenosine caused concentration-dependent increases in LVP and +dP/dt(max), with detectable increases of 4.1 and 4.8% at 10(-6) M and maximal increases of 12.0 and 11.1% at 10(-4) M, respectively. The contractile responses were prevented by the A(2a)-receptor antagonists chlorostyryl-caffeine and aminofuryltriazolotriazinyl-aminoethylphenol (ZM-241385) but were not affected by the beta(1)-adrenergic antagonist atenolol. The adenosine A(1)-receptor antagonist dipropylcyclopentylxanthine and pertussis toxin potentiated the positive inotropic effects of adenosine. The A(2a)-receptor agonists ethylcarboxamidoadenosine and dimethoxyphenyl-methylphenylethyl-adenosine also enhanced contractility. With constant-flow perfusion, 10(-5) M adenosine increased LVP and +dP/dt(max) by 5.5 and 6.0%, respectively. In the presence of the coronary vasodilator hydralazine, adenosine increased LVP and +dP/dt(max) by 7.5 and 7.4%, respectively. Dipropylcyclopentylxanthine potentiated the adenosine contractile responses with constant-flow perfusion in the absence and presence of hydralazine. These increases in contractile performance were also antagonized by chlorostyryl-caffeine and ZM-241385. The results indicate that adenosine increases contractile performance via activation of A(2a) receptors in the intact heart independent of beta(1)-adrenergic receptor activation or changes in coronary flow.
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