Edwin K. Jackson

Reduction of myocardial reperfusion injury by intravenous adenosine administered during the early reperfusion period.

Adenosine influences the function of several cell types thought to be involved in the pathogenesis of myocardial reperfusion injury. We have previously demonstrated that intracoronary administration of adenosine enhances myocardial salvage 24 hours after reperfusion. To determine if these beneficial effects could be obtained during a prolonged period of reperfusion using an intravenous route of administration, 22 closed-chest dogs were subjected to 90 minutes of proximal left anterior descending coronary artery occlusion and 72 hours of reperfusion. Animals randomly received either intravenous adenosine (0.15 mg/kg/min) or an equal volume of Ringers lactate during the first 150 minutes of reperfusion. The area at risk was defined in vivo with Monastral blue, and infarct size was measured histologically with Mallorys trichrome stain. Serial global and regional ventricular function were determined with contrast ventriculography and analyzed using a computerized radial shortening method. Biopsies were obtained from the central ischemic zone to assess endothelial ultrastructure and capillary obstruction. No significant effects in heart rate or blood pressure were noted during adenosine infusion. Transmural collateral blood flow during ischemia was similar in the groups. Infarct size expressed as a percentage of the anatomical area at risk was significantly less in the adenosine-treated group (35.3 +/- 4.3% in controls versus 17.1 +/- 4.3% in treated animals, p less than 0.01). A progressive decrease in transmural blood flow was noted in control animals during reperfusion, resulting in a significant reduction at 3 hours compared with the preocclusion value (0.69 +/- 0.11 ml/min/g [at baseline versus 0.45 +/- 0.10 ml/min/g at 3 hours, p less than 0.05]). In contrast, flow in adenosine animals at 3 hours was similar to baseline values (0.91 +/- 0.15 ml/min/g at baseline versus 0.98 +/- 0.14 ml/min/g at 3 hours, p = NS) and was significantly higher (p less than 0.05) than the control group. Radial shortening in the ischemic zone was significantly improved at 3 (-2.6 +/- 2.8% in controls versus 11.6 +/- 3.3% in treated animals, p less than 0.01) and 72 hours (5.5 +/- 2.0% in controls versus 17.3 +/- 3.5% in treated animals, p less than 0.01) after reperfusion in treated animals. Electron microscopy showed reduced neutrophil and erythrocyte plugging of capillaries with relative preservation of endothelial cell structure in the adenosine group.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of intravenous infusions of selective adenosine A1-receptor and A2-receptor agonists on myocardial reperfusion injury

To determine the efficacy of very low doses of adenosine on myocardial reperfusion injury and whether its effect is receptor mediated, 78 rabbits underwent 30 minutes of left circumflex artery occlusion and 48 hours of reperfusion. Animals were randomly assigned to receive one of three doses of adenosine, cyclopentyladenosine (a selective A1-receptor agonist), or CGS 21680C (a selective A2-receptor agonist). The drugs were infused for 65 minutes beginning 5 minutes before reperfusion. A significant reduction in histologically determined infarct size was noted with all three doses of adenosine, intermediate and low doses of the A1-receptor agonist (cyclopentyladenosine), and high and intermediate doses of the A2-receptor agonist (CGS 21680C). Furthermore, all three adenosine receptor agonists afforded similar degrees of protection. Results of this study demonstrate that intravenous infusions of very low doses of adenosine significantly enhance myocardial salvage and this protection is receptor mediated. Furthermore, the administration of the A1-receptor agonist would be clinically appealing, since it would avoid the potential side effects associated with activation of A2 receptors.

Effects of angiotensin subtype 1 and subtype 2 receptor antagonists in normotensive versus hypertensive rats.

The purpose of this study was to examine in vivo the importance of angiotensin subtype 1 (AT1) versus subtype 2 (AT2) receptors in spontaneously hypertensive (hypertensive) versus normotensive Wistar-Kyoto (control) rats. Intravenous infusions of DuP 753, a selective AT1 receptor antagonist, abolished the pressor responses to intravenous infusions of angiotensin II in both strains, and the potency of DuP 753 in this regard was similar in the two strains. DuP 753 also abolished angiotensin II-induced aldosterone release in both strains; however, with respect to inhibiting angiotensin II-induced aldosterone release, DuP 753 was more potent in hypertensive compared with control rats. In hypertensive but not control rats, DuP 753 inhibited angiotensin II-induced aldosterone release at doses lower than required to inhibit angiotensin II-induced pressor responses. Intramesenteric infusions of DuP 753 abolished mesenteric vascular responses to intramesenteric infusions of angiotensin II with a similar potency in both strains. In control but not hypertensive rats, angiotensin II consistently potentiated noradrenergic neurotransmission in the mesenteric vascular bed, and this effect of angiotensin II was abolished by DuP 753. High doses of PD123177, a selective AT2 antagonist, did not influence any of the aforementioned effects of angiotensin II in either strain. These data indicate that 1) in both hypertensive and control rats, AT2 receptors mediate the acute effects of angiotensin II on arterial blood pressure, aldosterone release, and mesenteric vascular resistance; 2) in control rats, AT1 receptors mediate the effects of angiotensin II on noradrenergic neurotransmission; and 3) spontaneously hypertensive rats are more sensitive to DuP 753-induced blockade of angiotensin II-mediated aldosterone release compared with Wistar- Kyoto control rats, at least under the conditions of this study.

Effect of intravenous adenosine on myocardial reperfusion injury in a model with low myocardial collateral blood flow.

The purpose of this study was to investigate the effects of various doses of adenosine administered intravenously on myocardial reperfusion injury in a model with poor collateral blood flow. New Zealand White rabbits were subjected to 30 minutes of occlusion of the obtuse marginal branch of the left circumflex artery and to 48 hours of reperfusion. Animals were randomized to receive intravenous adenosine in doses of 0.1 mg/min (low), 0.3 mg/min (intermediate), or 0.55 mg/min (high), or an equivalent volume of saline (control) commencing 5 minutes prior to reperfusion and continuing through the first 60 minutes of reperfusion. The area at risk was determined in vivo with Monastral blue dye and the area of necrosis was histologically examined with Massons trichrome stain. Both the intermediate and high doses of adenosine, but not the low dose, significantly (p less than 0.05) decreased mean blood pressure. However, all three doses of adenosine produced a significant (p less than 0.05) and comparable decrease in infarct size expressed as a percent of area at risk (control, 52.0 +/- 4.6%; low, 35.3 +/- 4.1%; intermediate, 31.7 +/- 4.6%; high, 31.3 +/- 4.6%). Regional myocardial blood flow was significantly increased and coronary vascular resistance decreased by all three doses of adenosine in a subset of animals that did not undergo coronary occlusion (p less than 0.05). This study demonstrates that intravenous administration of nonhypotensive doses of adenosine given during the early reperfusion period attenuates reperfusion injury in a model with poor collateral blood flow.

Endogenous adenosine restrains renin release in conscious rats.

The purpose of this study was to test the hypothesis that endogenous adenosine functions to restrain the renin release response to pharmacological and pathophysiological stimuli. To achieve this objective, we examined the effects of an adenosine receptor antagonist, 1,3-dipropyl-8-(p-sulfophenyl)xanthine (DPSPX), on the renin release response induced by acute administration of hydralazine or by chronic clipping of the left renal artery (renovascular hypertensive rats). In conscious, unrestrained rats, DPSPX significantly increased plasma renin activity (PRA) in control rats, in rats treated with hydralazine, and in renovascular hypertensive rats. The effect of DPSPX on PRA was significantly greater in rats treated with hydralazine or in renovascular hypertensive rats compared with control rats. DPSPX did not influence arterial blood pressure in any group, did not affect the measurement of PRA, and did not alter the elimination of renin activity from the circulation. Additional experiments were performed in the in situ autoperfused kidney so that the effects of DPSPX on renal hemodynamics and renal excretory function could be assessed. In this experimental model, DPSPX also increased PRA in hydralazine-treated rats and in renovascular hypertensive rats without affecting arterial pressure, renal blood flow, or sodium excretion. In a final set of studies in conscious, unrestrained rats, adenosine deaminase increased PRA in a dose-dependent manner in hydralazine-treated rats and significantly increased the slope of the relation between PRA and the depressor response to hydralazine. We conclude: 1) Although the kidney has both A1 and A2 adenosine receptors mediating inhibitory and stimulatory actions, respectively, on renin release, the dominant effect of endogenous adenosine on renin release is inhibitory. 2) Even under basal physiological conditions, endogenous adenosine tonically inhibits renin release. 3) This inhibitory effect is augmented whenever the renin-angiotensin system is stimulated regardless of the approach used to activate renin release. 4) Endogenous adenosine negatively modulates renin release by a direct effect on juxtaglomerular cells.

Effect of aminophylline on renal vasoconstriction produced by amphotericin B in the rat.

SummaryAdministration of the antifungal agent amphotericin B causes a pronounced reduction in renal blood flow (RBF). Since amphotericin B induced renal vasoconstriction may contribute to the clinical nephrotoxicity of this drug, the purpose of these studies was to investigate the mechanism of amphotericin B induced renal vasoconstriction. To determine if the vascular response to amphotericin B is linked to the intrarenal release of either adenosine or angiotensin II, the effects of aminophylline (5 μmol/kg/min for 10 min followed by 0.5 μmol/kg/min) and saralasin (6 μg/min) on the renal vascular response produced by two 10 min intravenous amphotericin B (0.35 mg/kg) infusions were examined. In the control group, amphotericin B decreased RBF 1.7 ml/min (22%, P<0.01) and 3.5 ml/min (44%, P<0.001) during the 1st and 2nd amphotericin B infusions, respectively. In animals pretreated with aminophylline the decrease in RBF produced by amphotericin B was only 0.4 ml/min (5.5%; N.S.) and 1.3 ml/min (15%, P<0.05) during the 1st and 2nd amphotericin B infusions, respectively. In contrast, neither saralasin nor the direct vasodilator sodium nitroprusside (0.4–2 μg/min) influenced the renal vascular response to amphotericin B. These data suggest that the renal vascular response to amphotericin B is not linked to the formation of angiotensin II, but rather might be mediated by increases in renal adenosine levels.

Attenuation of cisplatinum-induced nephrotoxicity in the rat by high salt diet, furosemide and acetazolamide

SummaryThe influence of variations in sodium chloride diet, furosemide and acetazolamide on nephrotoxicity induced by cisplatinum have been investigated in the rat by measuring serum creatinine concentrations 5 days after cisplatinum (5 mg/kg, ip) administration. Sodium chloride depletion enhanced, while sodium chloride loading minimized changes in renal function. Increases in urine flow rate following a dextrose water load failed to alter the nephrotoxic response. Both furosemide and acetazolamide, given 30 min before cisplatinum, attenuated the nephrotoxic response. In contrast, neither sodium chloride loading, furosemide, nor acetazolamide influenced the change in renal function when given 30 min after cisplatinum. These observations indicate that renal damage due to cisplatinum can be modified by alterations in dietary salt and by diuretics and that the extent of ultimate renal damage is dependent on factors occurring immediately after cisplatinum administration.

Adenosine in renin-dependent renovascular hypertension.

Our previous studies support the hypothesis that activation of the renin-angiotensin system by renal ischemia elevates adenosine levels and that adenosine acts in a negative feedback loop to limit renin release and to mitigate some of the hypertension-producing effects of angiotensin II. To further test this hypothesis, we compared the time course of caffeine-induced Increases in plasma renin activity with the time course of changes in plasma levels of adenosine in two models of renin-dependent renovascular hypertension. Also, we compared the effects of caffeine on plasma renin activity and arterial blood pressure in renin-dependent versus renin-independent renovascular hypertension. In comparison to sham-operated rats, plasma levels of adenosine In the left and right renal veins and aorta were elevated severalfold in two-kidney, one clip rats (2K1C) 1 week after left renal artery clipping. However, adenosine levels declined during the second and third weeks after clipping. In 2K1C rats treated chronically with caffeine, plasma renin activity was markedly elevated during the first week after operation as compared to non-caffeine-treated 2K1C rats. However, during the second and third weeks after clipping, caffeine had lesser effects on plasma renin activity. A temporal relationship between plasma adenosine levels and caffeine-induced hyperreninemia was also observed in rats with aortic ligation. Caffeine accelerated hypertension in 2K1C rats and rats with aortic ligation (renin-dependent renovascular hypertension), but it had no effect on plasma renin activity or blood pressure hi one-kidney, one clip rats (renin-independent renovascular hypertension). These results lend further support to the hypothesis that adenosine functions to mitigate the renin-angiotensin system hi renin-dependent renovascular hypertension.

Comparison of the pulmonary, hepatic and renal extraction of PGI2 and 6-KETO-PGE1

Prostaglandin (PG) I2 and 6-keto-PGE1 were infused into the aortic arch, femoral vein, renal artery and portal vein in anesthetized dogs over a dose range to produce a stable decrease in systemic blood pressure after 7 min of infusion. Mesenteric artery blood flow was measured continuously with a non-cannulating electromagnetic flow probe. Parallel log dose-response relationship were seen for decreases in blood pressure and increases in mesenteric blood flow with both prostaglandins when infused into the aortic arch. The dose responses for intra-aortic and intravenous infusion of PGI2 were not different but were shifted to the right with intrarenal and intraportal infusions. With 6-keto-PGE1 the dose responses after intravenous, intrarenal and intraportal infusions were all shifted to the right. By comparing doses which reduced blood pressure by 10 mm Hg, the extraction of the PGs by the lungs, kidney and liver was calculated PGI2 had no significant pulmonary extraction. 31% renal and 75% hepatic extraction. In contrast, 6-keto-PGE1 had extractions of 40, 66 and 51% for pulmonary, renal, and hepatic vascular beds, respectively. We conclude that 6-keto-PGE1 is reasonably well extracted by the lungs, kidney, and liver.

Relation between renin release and blood pressure response to nonsteroidal anti-inflammatory drugs in hypertension.

This editorial has three goals: 1) to underscore the significance of the work by Imanishi and colleagues to clinical medicine and to basic science; 2) to integrate the present study into a conceptual framework by highlighting certain aspects of the biology of prostaglandins and; 3) to point out the relation of the study by Imanishi et al to earlier observations in both animals and humans