Investigation of mechanisms that mediate reactive hyperaemia in guinea-pig hearts: role of K(ATP) channels, adenosine, nitric oxide and prostaglandins

Metabolic hyperemia requires ATP-sensitive K+ channels and H2O2 but not adenosine in isolated mouse hearts

We have previously demonstrated that adenosine-mediated H2O2 production and opening of ATP-sensitive K(+) (KATP) channels contributes to coronary reactive hyperemia. The present study aimed to investigate the roles of adenosine, H2O2, and KATP channels in coronary metabolic hyperemia (MH). Experiments were conducted on isolated Langendorff-perfused mouse hearts using combined pharmacological approaches with adenosine receptor (AR) knockout mice. MH was induced by electrical pacing at graded frequencies. Coronary flow increased linearly from 14.4 ± 1.2 to 20.6 ± 1.2 ml·min(-1)·g(-1) with an increase in heart rate from 400 to 650 beats/min in wild-type mice. Neither non-selective blockade of ARs by 8-(p-sulfophenyl)theophylline (8-SPT; 50 μM) nor selective A2AAR blockade by SCH-58261 (1 μM) or deletion affected MH, although resting flow and left ventricular developed pressure were reduced. Combined A2AAR and A2BAR blockade or deletion showed similar effects as 8-SPT. Inhibition of nitric oxide synthesis by N-nitro-l-arginine methyl ester (100 μM) or combined 8-SPT administration failed to reduce MH, although resting flows were reduced (by ∼20%). However, glibenclamide (KATP channel blocker, 5 μM) decreased not only resting flow (by ∼45%) and left ventricular developed pressure (by ∼36%) but also markedly reduced MH by ∼94%, resulting in cardiac contractile dysfunction. Scavenging of H2O2 by catalase (2,500 U/min) also decreased resting flow (by ∼16%) and MH (by ∼24%) but to a lesser extent than glibenclamide. Our results suggest that while adenosine modulates coronary flow under both resting and ischemic conditions, it is not required for MH. However, H2O2 and KATP channels are important local control mechanisms responsible for both coronary ischemic and metabolic vasodilation.

A1 adenosine receptor negatively modulates coronary reactive hyperemia via counteracting A2A-mediated H2O2 production and KATP opening in isolated mouse hearts

We previously demonstrated that A2A, but not A2B, adenosine receptors (ARs) mediate coronary reactive hyperemia (RH), possibly by producing H2O2 and, subsequently, opening ATP-dependent K(+) (KATP) channels in coronary smooth muscle cells. In this study, A1 AR knockout (KO), A3 AR KO, and A1 and A3 AR double-KO (A1/A3 DKO) mice were used to investigate the roles and mechanisms of A1 and A3 ARs in modulation of coronary RH. Coronary flow of isolated hearts was measured using the Langendorff system. A1 KO and A1/A3 DKO, but not A3 KO, mice showed a higher flow debt repayment [~30% more than wild-type (WT) mice, P < 0.05] following a 15-s occlusion. SCH-58261 (a selective A2A AR antagonist, 1 μM) eliminated the augmented RH, suggesting the involvement of enhanced A2A AR-mediated signaling in A1 KO mice. In isolated coronary arteries, immunohistochemistry showed an upregulation of A2A AR (1.6 ± 0.2 times that of WT mice, P < 0.05) and a higher magnitude of adenosine-induced H2O2 production in A1 KO mice (1.8 ± 0.3 times that of WT mice, P < 0.05), which was blocked by SCH-58261. Catalase (2,500 U/ml) and glibenclamide (a KATP channel blocker, 5 μM), but not N(G)-nitro-l-arginine methyl ester, also abolished the enhanced RH in A1 KO mice. Our data suggest that A1, but not A3, AR counteracts the A2A AR-mediated CF increase and that deletion of A1 AR results in upregulation of A2A AR and/or removal of the negative modulatory effect of A1 AR, thus leading to an enhanced A2A AR-mediated H2O2 production, KATP channel opening, and coronary vasodilation during RH. This is the first report implying that A1 AR has a role in coronary RH.

Coronary artery reperfusion: The ADP receptor P2Y(1) mediates early reactive hyperemia in vivo in pigs

The physiological mechanisms that regulate reactive hyperemia are not fully understood. We postulated that the endothelial P2Y(1) receptor that release vasodilatory factors in response to ADP might play a vital role in the regulation of coronary flow. Intracoronary flow was measured with a Doppler flow-wire in a porcine model. 2-MeSADP (10(-5) M), ATP (10(-4) M) or UTP (10(-4) M) alone or as co-infusion with a selective P2Y(1) receptor blocker, MRS 2179 (10(-3) M) was locally delivered through the tip of a coronary angioplasty balloon. In separate pigs the coronary artery was occluded with the balloon for 10 min. During the first and tenth minutes of coronary ischemia, 2.5 ml of MRS 2179 (10(-3) M) was delivered distal to the occlusion in 8 pigs, 10 pigs were used as controls. MRS 2179 fully inhibited the 2-MeSADP-mediated coronary flow increase (P < 0.05) with no effect on UTP, indicating selective P2Y(1) inhibition. ATP-mediated flow increase was significantly inhibited by MRS 2179. During reactive hyperemia following coronary occlusion, flow increased by nearly sevenfold. MRS 2179, however, reduced the post-ischemic hyperemia by a mean of 46% during the period 1-2.5 min following balloon deflation (P < 0.05), which corresponds to peak velocity flow during reperfusion. In conclusion, MRS 2179, a selective P2Y(1) receptor blocker, significantly reduces the increased coronary flow caused both by 2-MeSADP and reactive hyperemia in coronary arteries. Thus, ADP acting on the endothelial P2Y(1) receptor may play a major role in coronary flow during post-ischemic hyperemia.

Prenatal hypoxia induces increased cardiac contractility on a background of decreased capillary density

Background

Chronic hypoxia in utero (CHU) is one of the most common insults to fetal development and may be associated with poor cardiac recovery from ischaemia-reperfusion injury, yet the effects on normal cardiac mechanical performance are poorly understood.

Methods

Pregnant female wistar rats were exposed to hypoxia (12% oxygen, balance nitrogen) for days 10–20 of pregnancy. Pups were born into normal room air and weaned normally. At 10 weeks of age, hearts were excised under anaesthesia and underwent retrograde 'Langendorff' perfusion. Mechanical performance was measured at constant filling pressure (100 cm H₂O) with intraventricular balloon. Left ventricular free wall was dissected away and capillary density estimated following alkaline phosphatase staining. Expression of SERCA2a and Nitric Oxide Synthases (NOS) proteins were estimated by immunoblotting.

Results

CHU significantly increased body mass (P < 0.001) compared with age-matched control rats but was without effect on relative cardiac mass. For incremental increases in left ventricular balloon volume, diastolic pressure was preserved. However, systolic pressure was significantly greater following CHU for balloon volume = 50 μl (P < 0.01) and up to 200 μl (P < 0.05). For higher balloon volumes systolic pressure was not significantly different from control. Developed pressures were correspondingly increased relative to controls for balloon volumes up to 250 μl (P < 0.05). Left ventricular free wall capillary density was significantly decreased in both epicardium (18%; P < 0.05) and endocardium (11%; P < 0.05) despite preserved coronary flow. Western blot analysis revealed no change to the expression of SERCA2a or nNOS but immuno-detectable eNOS protein was significantly decreased (P < 0.001) in cardiac tissue following chronic hypoxia in utero.

Conclusion

These data offer potential mechanisms for poor recovery following ischaemia, including decreased coronary flow reserve and impaired angiogenesis with subsequent detrimental effects of post-natal cardiac performance.

Mild hypothermia reduces cardiac post-ischemic reactive hyperemia

Background

In experimentally induced myocardial infarction, mild hypothermia (33–35°C) is beneficial if applied prior to ischemia or reperfusion. Hypothermia, when applied after reperfusion seems to confer little or no benefit. The mechanism by which hypothermia exerts its cell-protective effect during cardiac ischemia remains unclear. It has been hypothesized that hypothermia reduces the reperfusion damage; the additional damage incurred upon the myocardium during reperfusion. Reperfusion results in a massive increase in blood flow, reactive hyperemia, which may contribute to reperfusion damage. We postulated that hypothermia could attenuate the post-ischemic reactive hyperemia.

Methods

Sixteen 25–30 kg pigs, in a closed chest model, were anesthetized and temperature was established in all pigs at 37°C using an intravascular cooling catheter. The 16 pigs were then randomized to hypothermia (34°C) or control (37°C). The left main coronary artery was then catheterized with a PCI guiding catheter. A Doppler flow wire was placed in the mid part of the LAD and a PCI balloon was then positioned proximal to the Doppler wire but distal to the first diagonal branch. The LAD was then occluded for ten minutes in all pigs. Coronary blood flow was measured before, during and after ischemia/reperfusion.

Results

The peak flow seen during post-ischemic reactive hyperemia (during the first minutes of reperfusion) was significantly reduced by 43 % (p < 0.01) in hypothermic pigs compared to controls.

Conclusion

Mild hypothermia significantly reduces post-ischemic hyperemia in a closed chest pig model. The reduction of reactive hyperemia during reperfusion may have an impact on cardiac reperfusion injury.

Metabolic coronary flow regulation--current concepts

The concept of metabolic coronary flow control provides a rationale for the close relationship of coronary flow and myocardial metabolic rate of oxygen. The concept is based on the presence of an oxygen (metabolic) sensor coupled functionally to effector mechanisms, which control vascular tone. Four modes of metabolic control models have been proposed. 1) An oxygen sensor located in the wall of coronary vessels coupling to smooth muscle tension. Endothelial prostaglandin production may support this concept. 2) An oxygen sensing mechanism located in the myocardium and changing metabolism in response to changes of local pO(2). Adenosine is a metabolite produced at an accelerated rate when the supply-to-demand relationship for oxygen falls. 3) Sensing of oxygen turnover may be achieved by carbon dioxide production and, potentially, by mitochondrial production of reactive oxygen species. 4) The red blood cell might serve as an oxygen sensor in response to changes of haemoglobin oxygenation. A potential link to vessel relaxation may be red cell ATP release. A large body of experimental evidence supports the notion that K(ATP) channels play a significant role causing smooth muscle hyper-polarization. However, additional yet unknown effector mechanisms must exist, because block of K(ATP) channels does not lead to deterioration of coronary flow control under conditions of exercise. Thus, although several lines of evidence show that metabolic flow regulation is effective during hypoxic conditions,mechanisms mediating normoxic metabolic flow control still await further clarification.

Coronary reactive hyperaemia and arterial pressure in anaesthetized goats

To study the effects of arterial pressure on coronary reactive hyperaemia, left circumflex coronary artery flow was measured, and reactive hyperaemia was determined after 5, 10 or 20 s of occlusion of this artery in anaesthetized goats during normotension, hypertension and hypotension. During hypertension induced by aortic constriction (mean arterial pressure, MAP = 140 +/- 6 mmHg) coronary vascular resistance (CVR), reactive hyperaemia (ratio of peak in hyperaemic flow to control flow and ratio of repayment to debt) and the decrease in CVR during the peak in hyperaemic flow were comparable to those during normotension. During hypertension induced by noradrenaline (MAP = 144 +/- 6 mmHg) CVR was 16% lower (P < 0.05), reactive hyperaemia was reduced by 14-25% (P < 0.05) and the decrease in CVR during the peak in hyperaemic flow was lower than the values of these parameters during normotension. During hypotension induced by constriction of the caudal vena cava (MAP = 40 +/- 4 mmHg) CVR was 22% lower (P < 0.05), reactive hyperaemia was reduced by 25-65% (P < 0.05) and the decrease in CVR during the peak in hyperaemic flow was less compared to the values of these parameters during normotension. During hypotension induced by isoprenaline (MAP = 45 +/- 4 mmHg) CVR was 59% lower, reactive hyperaemia was reduced by 55-100% (P < 0.01) and the decrease in CVR during the peak in hyperaemic flow was less compared to the values of these parameters during normotension. Arterial pressure is a main determinant of coronary reactive hyperaemia after brief periods of ischaemia, and the relationship between arterial pressure and reactive hyperaemia may depend in part on changes in CVR after variations in arterial pressure. These changes in CVR may be related to the action on coronary vessels of myocardial factors and vascular myogenic mechanisms.

Spontaneous coronary vasospasm in KATP mutant mice arises from a smooth muscle-extrinsic process

In the vasculature, ATP-sensitive potassium channels (KATP) channels regulate vascular tone. Mice with targeted gene disruptions of KATP subunits expressed in vascular smooth muscle develop spontaneous coronary vascular spasm and sudden death. From these models, it was hypothesized that the loss of KATP channel activity in arterial vascular smooth muscle was responsible for coronary artery spasm. We now tested this hypothesis using a transgenic strategy where the full-length sulfonylurea receptor containing exon 40 was expressed under the control of a smooth muscle-specific SM22alpha promoter. Two transgenic founder lines were generated and independently bred to sulfonylurea receptor 2 (SUR2) null mice to generate mice that restored expression of KATP channels in vascular smooth muscle. Transgenic expression of the sulfonylurea receptor in vascular smooth muscle cells was confirmed by detecting mRNA and protein from the transgene. Functional restoration was determined by recording pinacidil-based KATP current by whole cell voltage clamping of isolated aortic vascular smooth muscle cells isolated from the transgenic restored mice. Despite successful restoration of KATP channels in vascular smooth muscle, transgene-restored SUR2 null mice continued to display frequent episodes of spontaneous ST segment elevation, identical to the phenotype seen in SUR2 null mice. As in SUR2 null mice, ST segment elevation was frequently followed by atrioventricular heart block. ST segment elevation and coronary perfusion pressure in the restored mice did not differ significantly between transgene-negative and transgene-positive SUR2 null mice. We conclude that spontaneous coronary vasospasm and sudden death in SUR2 null mice arises from a coronary artery vascular smooth muscle-extrinsic process.

Mediators of coronary reactive hyperaemia in isolated mouse heart

1. Mechanisms regulating coronary tone under basal conditions and during reactive hyperaemia following transient ischaemia were assessed in isolated mouse hearts. 2. Blockade of NO-synthase (50 muM L-NAME), K(ATP) channels (5 muM glibenclamide), A(2A) adenosine receptors (A(2A)ARs; 100 nM SCH58261), prostanoid synthesis (100 muM indomethacin), and EDHF (100 nM apamin+100 nM charybdotoxin) all reduced basal flow approximately 40%. Effects of L-NAME, glibenclamide, and apamin+charybdotoxin were additive, whereas coadministration of SCH58261 and indomethacin with these inhibitors failed to further limit flow. 3. Substantial hyperaemia was observed after 5-40 s occlusions, with flow increasing to a peak of 48+/-1 ml min(-1) g(-1). Glibenclamide most effectively inhibited peak flows (up to 50%) while L-NAME was ineffective. 4. With longer occlusions (20-40 s), glibenclamide alone was increasingly ineffective, reducing peak flows by approximately 15% after 20 s occlusion, and not altering peak flow after 40 s occlusion. However, cotreatment with L-NAME+glibenclamide inhibited peak hyperaemia by 70 and 25% following 20 and 40 s occlusions, respectively. 5. In contrast to peak flow changes, sustained dilation and flow repayment over 60 s was almost entirely K(ATP) channel and NO dependent (each contributing equally) with all occlusion durations. 6. Antagonism of A(2A)ARs with SCH58261 reduced hyperaemia 20-30% whereas inhibition of prostanoid synthesis was ineffective. Effects of A(2A)AR antagonism were absent in hearts treated with L-NAME and glibenclamide, supporting NO and K(ATP)-channel-dependent effects of A(2A)ARs. 7. EDHF inhibition alone exerted minor effects on hyperaemia and only with longer occlusions. However, residual hyperaemia after 40 s occlusion in hearts treated with L-NAME+glibenclamide+SCH58261+indomethacin was abrogated by cotreatment with apamin+charybdotoxin. 8. Data support a primary role for K(ATP) channels and NO in mediating sustained dilation after coronary occlusion. While K(ATP) channels (and not NO) are also important in mediating initial peak flow adjustments after brief 5-10 s occlusions, their contribution declines with longer 20-40 s occlusions. Intrinsic activation of A(2A)ARs is important in triggering K(ATP) channel/NO-dependent hyperaemia. Synergistic effects of combined inhibitors implicate interplay between mediators, with compensatory changes occurring in K(ATP) channel, NO, and/or EDHF responses when one is individually blocked.

Aging-related changes in myocardial purine metabolism and ischemic tolerance

Impaired tolerance to ischemia-reperfusion in older hearts may stem in part from alterations in purine catabolism, impacting on maintenance of energy state and protective signaling via extracellular adenosine. We characterized effects of aging on normoxic and post-ischemic purine metabolism in hearts from young (2-4 month), middle-aged (12 month), old (18 month), and senescent (24-28 month) C57/Bl6 mice. Normoxic function was similar in all age groups while normoxic purine efflux increased gradually with age. This was the result of enhanced efflux of hypoxanthine, xanthine and uric acid, with extracellular accumulation of adenosine and inosine remaining unchanged. While total purine washout during 60 min reperfusion following 20 min global ischemia was unaltered by aging (1057+/-109 nmoles/g in young vs. 1221+/-127 nmoles/g in senescent hearts), selective changes in purine catabolism were evident. Accumulation of adenosine and inosine were reduced by 50 and 80%, respectively, matched by 400 and 300% elevations in hypoxanthine and xanthine accumulation, respectively. Uric acid remained unchanged. Thus, while adenosine and inosine represented 15+/-2 and 47+/-3% of total purine efflux in young hearts, these values decreased to only 6+/-1 and 9+/-2% in senescent hearts. Efflux of IMP also increased 500% with aging whereas 5'-AMP was unaltered. These changes were associated with a substantial fall in ischemic tolerance, with left ventricular developed pressure recovering to 46+/-3% in young hearts vs. only 24+/-6, 16+/-4, and 19+/-4% in middle-age, old and senescent hearts, respectively. Our data collectively support a pronounced shift in purine catabolism, with reduced accumulation of salvageable and cardioprotective adenosine, and enhanced accumulation of poorly salvaged (and potentially injurious) hypoxanthine and xanthine. Mechanisms underlying this shift have yet to be determined. However, this may play a role in the marked decline in myocardial tolerance to ischemia with aging and senescence.