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Cardiovascular Research 1999 42(1):57-64; doi:10.1016/S0008-6363(98)00319-8
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Copyright © 1999, European Society of Cardiology

Ischaemic preconditioning changes the pattern of coronary reactive hyperaemia in the goat: role of adenosine and nitric oxide

Donatella Gattulloa, Ronald J. Lindenb, Gianni Losanoa, Pasquale Pagliaroa,* and Nico Westerhofc

aDipartimento di Scienze Cliniche e Biologiche, Università di Torino, Ospedale S. Luigi, Regione Gonzole, I-10039 Orbassano (TO), Italy
bDivision of Biomedical Science, King’s College London, Strand, London, UK
cLaboratory for Physiology, Institute for Cardiovascular Research, ICaR-VU, Vrije Universiteit, Amsterdam, Netherlands

* Corresponding author. Tel.: +39-11-903-8617; fax: +39-11-903-8639; e-mail: pagliaro@medfarm.unito.it

Received 23 March 1998; accepted 12 October 1998


   Abstract
Top
 Abstract
1 Introduction
 2 Materials
 3 Results
 4 Discussion
 5 Conclusions
 References
 
Objectives: After ischaemic preconditioning (IP), obtained by short episodes of ischaemia, cardiac protection occurs due to a reduction in myocardial metabolism through the activation of A1 adenosine receptors. The antiarrhythmic effect of IP is attributed to an increase in the release of nitric oxide (NO) by the endothelium. On the basis of the above consideration the present investigation studies the changes induced by preconditioning in coronary reactive hyperaemia (RH) and how blockade of A1 receptors and inhibition of NO synthesis can modify these changes. Methods: In anaesthetised goats, an electromagnetic flow-probe was placed around the left circumflex coronary artery. Preconditioning was obtained with two episodes of 2.5 min of coronary occlusion, separated by 5 min of reperfusion. RH was obtained with a 15 s occlusion. In a control group (n=7) RH was studied before and after IP. In a second group (n=7), 0.2 mg kg–1 of 8-cyclopentyl-dipropylxanthine, an A1 receptor blocker, and in a third group (n=7) 10 mg kg–1 of NG-nitro-L-arginine (LNNA), an NO inhibitor, were given before IP. Reactive hyperaemia was again obtained before and after IP. Results: In the control group, after IP, the time to peak hyperaemic flow and total hyperaemic flow decreased by about 50% and 25%, respectively. The A1 receptor blockade alone did not change RH. During A1 blockade, IP reduced the time to peak of RH similar as in control (45%), but did not alter total hyperaemic flow. LNNA alone reduced resting flow and total hyperaemic flow. After NO inhibition, IP only reduced total hyperaemic flow by about 15%, but the time to peak flow was not affected. Conclusions: IP alters RH by decreasing total hyperaemic flow and reducing the time to peak hyperaemic flow. While the former effect is attributed to a reduction in myocardial metabolism through the activation of the A1 receptors, the latter is likely to be due to an increased endothelial release of NO, suggesting that in addition to a protective effect on the myocardium, IP also exerts a direct effect on the responsiveness of the coronary vasculature (vascular preconditioning).

KEYWORDS Ischaemic preconditioning; Coronary flow; Reactive hyperaemia; Adenosine A1 receptors; Endothelium; Nitric oxide


   1 Introduction
Top
 Abstract
 1 Introduction
2 Materials
 3 Results
 4 Discussion
 5 Conclusions
 References
 
Coronary reactive hyperaemia (RH) reflects the degree of myocardial ischaemia and vascular reactivity which occur during and just after a coronary occlusion [1–4], and is used as an index of coronary vascular reserve [3]. In addition, RH is reproducible with constant characteristics if the basal perfusion pressure and flow are unchanged [3]. It is also reported that 5 min of reperfusion after a brief coronary occlusion (4 to 8 s) are sufficient to fully restore both myocardial performance and the conditions to elicit a normal RH [2].

Ischaemic preconditioning (IP) protects the myocardium against damage caused by long periods of coronary occlusion and reperfusion [5]. Recently, several studies on IP obtained by transient coronary occlusions (at least 2 min), report a reduction in myocardial metabolism as the main mechanism of myocardial protection [5–9]. This reduction in metabolism is mediated by the adenosine accumulated during the occlusion, which acts on the A1 myocardial receptors [7], and is reported to be responsible for myocardial protection [5–9]. The protection lasts from 1 to 3 h depending on the manner in which preconditioning is induced [8, 9].

The function of the coronary vascular endothelium can be affected by IP [10, 11]. In particular the protection exerted by ischaemic preconditioning against cardiac arrhythmias during ischaemia-reperfusion is attributed to an activation of L-arginine–nitric oxide pathway [12–14]. It is likely that such an activation will also modify coronary responsiveness to vasodilator stimuli.

Since RH is controlled by both metabolic (e.g. [1]) and endothelial components [15–18], it is intriguing to study the pattern of RH during the period of presumed protection, when the metabolic component is reduced [5, 7–9], and the endothelial release of nitric oxide (NO) is reported to be enhanced [12–14]. On the basis of these considerations, we hypothesised that after IP the reactive hyperaemic response could be affected by the reduced metabolism depending on the activation of the A1 receptors and by the increased synthesis of NO. To test this hypothesis, the magnitude and pattern of the reactive hyperaemic response following a 15-s coronary artery occlusion, was studied before and after IP in a control group (Group 1), after the blockade of the A1 receptors (Group 2) and after the inhibition of the release of NO by the endothelium (Group 3), respectively.


   2 Materials
Top
 Abstract
 1 Introduction
 2 Materials
3 Results
 4 Discussion
 5 Conclusions
 References
 
The experiments were carried out in anaesthetised goats (30–50 kg). After sedation with diazepam (30–45 mg), anaesthesia was induced by i.v. injection of ketamine hydrochloride (Ketalar, Park-Davis, Milan, Italy) in a dose of 15 mg kg–1 [19]. Also atropine was injected i.v. (0.1 mg kg–1) to limit bronchial secretion. The animals were ventilated using a Harvard respiratory pump (Harvard 607, Harvard Apparatus, Dover, MA, USA). The anaesthesia was maintained throughout the experiments by ventilating the animals with a nitrous oxide–oxygen (2:1) mixture and a continuous infusion of ketamine hydrochloride (24 mg kg–1 h–1). Fentanyl (Fentanest, Pharmacia, Milan, Italy) was injected hourly in a dose of 1 µg kg–1. The left external jugular vein and both common carotid arteries were dissected free and cannulated. The venous catheter was used for the administration of drugs and solutions needed to keep the acid–base balance constant and for the maintenance of the anaesthesia. The catheters inserted into the arteries were connected to electromanometers (Monitoring kit mk5-02, DTNFV, Abbot, Milan, Italy). One catheter was used to record the aortic blood pressure (ABP), assumed to be coronary perfusion pressure. The other catheter was pushed into the left ventricle to record left ventricular pressure (LVP). The chest was opened by left lateral thoracotomy and the heart placed in a pericardial cradle. A flow-probe connected to a channel of an electromagnetic flowmeter (BL 613, Biotronics, Silver Spring, MD, USA) was placed around the ascending aorta to record aortic flow. LVP and aortic flow were recorded continuously in order to monitor the ventricular performance.

The proximal part of the left circumflex coronary artery (LCCA) was gently isolated. A flow-probe connected to the other channel of the flowmeter was placed around LCCA to record coronary blood flow (CBF). Distal to this flow-probe a snare was placed around the artery. The snare was used to occlude LCCA for the assessment of the zero flow level and to produce the experimental manoeuvres (see Section 2.1).

The temperature of the animals was monitored through a rectal thermistor probe (Ellab DU-3, Copenhagen, Denmark) and was maintained within normal limits using an electric pad and/or an infrared light lamp. Arterial pH, pO2 and pCO2 were monitored with a Ciba-Corning 280 gas-analyser (Ciba-Corning, Halstead, Essex, UK) and kept in the normal range.

ECG, ABP, LVP, CBF and aortic flow were recorded using a TEAC cassette recorder R-71 (TEAC Corporation, Tokyo, Japan) and reproduced using a Gould electrostatic recorder (Gould, Baillainvilliers, France).

Experiments were performed in accordance with the Guide for the Care and Use of Laboratory Animals, DL 116, January 27, 1992, published in the Gazzetta Ufficiale della Repubblica Italiana, issue No. 40 of Feb. 18, 1992, and conformed to the Guide for the Care and Use of Laboratory Animals (NIH publication N. 85-23, revised 1985).

2.1 Experimental protocol
The animals were divided into three groups: control animals (Group 1, n=7), A1 blocked animals (Group 2, n=7) and NO synthesis inhibited animals (two doses: n=5, and Group 3, n=7).

In the control animals, LCCA was occluded for 15 s to study RH and then released using the snare distal to the flow probe. After RH was produced, two periods of 2.5 min of LCCA occlusion, separated from each other by 5 min of reperfusion, were used to produce IP. An additional period of 5–10 min of reperfusion elapsed between the end of the preconditioning manoeuvres and the production of a second RH with 15 s of occlusion, when the resting flow was at a steady state.

In the A1 blocked animals, after the first RH a 0.2 mg kg–1 i.v. infusion of 8-cyclopentyl-1,3-dipropylxanthine (CPX) (Sigma, St. Louis, MO, USA) (dissolved in dimethylsulphoxide, 0.75% v/v 1 M NaOH in saline) was performed to block the A1 adenosine receptors [20]. A second RH was then induced. Finally a third RH was elicited after the preconditioning manoeuvres. All the RHs were obtained with a 15-s coronary occlusion.

At the end of the experiments, the effectiveness of the A1 blockade was checked by the i.v. bolus injection of 5 µg kg-1 of (–)-N6-(2-phenylisopropyl)-adenosine (R-PIA) (Sigma), a selective A1 receptor agonist, dissolved in absolute ethanol which was subsequently diluted at 10% concentration in saline solution. No change in heart rate was observed, showing the complete blockade of these receptors [21].

In the NO synthesis inhibited animals, after the first RH, 5 and 10 mg kg–1 of NG-nitro-L-arginine (LNNA), an NO synthesis inhibitor [18], were infused intravenously. Depending on the dose, the infusion lasted 10–20 min. A second RH was then induced in these animals too. Finally a third RH was elicited after the preconditioning manoeuvre.

Since the slowing of myocardial metabolism is reported to last from 1 to 3 h depending on the duration of the preconditioning manoeuvre [8, 9], in two goats of Group 1 and in five of Group 2, a second experiment (RH, preconditioning manoeuvres, RH) was performed 1.5 h after the end of the first one.

In order to check whether the dose of CPX was sufficient to completely block A1 receptors, in three additional goats the A1 selective agonist R-PIA was infused before and after the blocking agent. Before CPX, R-PIA reduced heart rate by about 50%, whereas, after CPX, it did not change the heart rate. This result was taken as proof that the dose of CPX used was effective in producing the blockade [20, 21].

In order to check whether the CPX solvent could alter the studied coronary parameters, in three other additional animals RH was produced before and after the intravenous infusion of the solvent alone (dimethylsulphoxide plus NaOH). No changes were obtained in resting coronary flow and RH. It was also found that the solvent did not affect the changes in the reactive hyperaemic response observed after IP in the control animals (see Section 3).

2.2 Data analysis
Resting CBF, and the following variables of RH were determined: CBF at the peak of RH, time to the peak flow of hyperaemia, the average value of the time derivative of CBF (dF/dt) from resting level to the peak of the hyperaemia, duration of the hyperaemia and total hyperaemic flow. Calculation of above variables was done according to Coffmann and Gregg [22]. The time to the peak of RH was taken as the time required for the hyperaemic flow to reach the maximum value starting from the instant of the release of the occlusion. The duration of the ischaemic response was calculated as the time between the release and the moment the flow had returned to 5% of the control. Total hyperaemic flow was defined as the area under the flow tracing over the duration of the response. Data are reported as means±S.D.; ANOVA and Student’s t test for paired data, when appropriate, were used to evaluate the statistical significance of the changes.


   3 Results
Top
 Abstract
 1 Introduction
 2 Materials
 3 Results
4 Discussion
 5 Conclusions
 References
 
Reactive hyperaemia was studied in control, A1 blocked and in NO synthesis inhibited animals, before and after ischaemic preconditioning produced with two periods of 2.5 min of LCCA occlusion separated from each other by 5 min of reperfusion [5–9]. RH was studied before and after the blockade in the A1 blocked animals and before and after inhibition in the NO synthesis inhibited animals. In all groups HR, ABP and basal CBF were not affected by ischaemic preconditioning (Tables 1–3GoGo).


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  		Table 1 Haemodynamic parameters and coronary reactive hyperaemia following a 15-s coronary artery occlusion before and after ischaemic preconditioning

 

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  		Table 2 Haemodynamic parameters and coronary reactive hyperaemia following a 15-s coronary artery occlusion before and after ischaemic preconditioning in the presence of A1 adenosine receptor blockade (0.2 mg kg–1 of CPX)

 

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  		Table 3 Haemodynamic parameters and coronary reactive hyperaemia following a 15-s coronary artery occlusion before and after ischaemic preconditioning in the presence of nitric oxide inhibition (10 mg kg–1 of LNNA)

 
3.1 Reactive hyperaemia in the control animals
An example of the reactive hyperaemic response in baseline and after IP is shown in Fig. 1. The results are summarized in Table 1. In the RH obtained after the preconditioning manoeuvres, in spite of an unchanged peak hyperaemic flow, the total hyperaemic flow was reduced by, on average, 24% (P<0.02), mainly as a result of the reduction in duration of the response (–19%, P<0.05). Preconditioning resulted in a significant (P<0.005) increase, of 105%, in the time derivative of flow from resting level to the peak of RH accompanied by a significant (P<0.005) reduction of 48% in the time to the peak of the RH.


Figure 1
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  		Fig. 1 Effects of ischaemic preconditioning on reactive hyperaemia. The arrows show the peak of hyperaemic flow. The time to the peak is reduced after preconditioning. ABP, aortic blood pressure; CBF, coronary blood flow.

 
The experiments were repeated in two animals after an interval of 1.5 h (data not included in Table 1). The reactive hyperaemic response was similar to the one observed in the first control. The repeat of the preconditioning manoeuvres induced again the same changes in the variables of RH as those observed in the first experiment; for instance, the time to the peak hyperaemic flow was reduced by about 45% after IP.

3.2 Reactive hyperaemia in the A1 blocked animals
An example of the reactive hyperaemic response in baseline, after A1 blockade and subsequent IP is shown in Fig. 2. The results of this group are summarised in Table 2. Resting CBF and the measured variables of RH were not affected by the administration of the A1 blocker only.


Figure 2
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  		Fig. 2 Effects of A1 adenosine receptors blockade (0.2 mg kg–1 of CPX) and ischaemic preconditioning on reactive hyperaemia. The arrows show the peak of hyperaemic flow. The time to the peak is unchanged after A1 blockade, but is reduced after preconditioning. ABP and CBF: see Fig. 1.

 
After the preconditioning manoeuvres, the peak flow, the duration of the response and the total hyperaemic flow were not significantly changed. However, a significant (P<0.01) increase of 73% in the time derivative of the flow from resting level to the peak of RH was observed, which resulted in a significant (P<0.01) reduction of about 44% in the time to the peak of RH, which was not different from the decrease in the control animals.

Also in this group, in the five animals in which the experiments were repeated after an interval of 1.5 h, the entire reactive hyperaemic response (data not included in Table 2) had returned to the first control. In particular the time to the peak of RH and the time derivative of flow from resting level to the peak were not significantly different (+5%, P=0.9 and –15%, P=0.3, respectively) from the values observed before IP in the first part of the experiment. Also the repeat of the preconditioning manoeuvres induced the same changes in RH as those observed in the first part of the experiment. In particular the time derivative of flow from resting level to the peak was increased by about 60% (P<0.05) and the time to the peak was reduced by about 45% (P<0.01), while the total hyperaemic flow was again unchanged (–2%; P=0.7).

The results obtained from the first experimental manoeuvres of control animals vs. A1 blocked animals were compared by the ANOVA test, which revealed that the reduction in the time to the peak (48 vs. 45%) and the increase in the time derivative of flow from resting level to the peak (105 vs. 73%) were not statistically different.

3.3 Reactive hyperaemia in the NO synthesis inhibited animals
An example of the reactive hyperaemic response in the baseline conditions, after LNNA, and after the subsequent IP is shown in Fig. 3. The results are summarised in Table 3. The administration of LNNA at the doses of 5 and 10 mg kg–1 caused an increase in blood pressure of about 14% for the dose of 5 mg kg–1 and about 16% for the dose of 10 mg kg–1 (P<0.05 for both doses), and a decrease in heart rate [23]of about 8% for the lower and of about 15% for the higher dose (P<0.05 for both doses) which were taken as indices of the effectiveness of the compound in inhibiting the release of NO. However, while basal CBF did not change significantly after 5 mg kg–1 of LNNA, it showed a significant average decrease by 11% (P<0.05) at the dose of 10 mg kg–1 (Table 3). Moreover, both doses of LNNA caused a significant reduction in the total flow of the RH of about 21% (P<0.05) for the lower dose and about 27% (P<0.01) for the higher dose. The reduction in total hyperaemic flow mainly resulted from the reduction in duration of the hyperaemic plateau and of the overall duration (not significant) of RH (Fig. 3), as was previously observed in guinea pig and dog hearts [15–18, 24].


Figure 3
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  		Fig. 3 Effects of NO synthesis inhibition (10 mg kg–1 of LNNA) and ischaemic preconditioning on reactive hyperaemia. The arrows show the peak of hyperaemic flow. The time to the peak is unchanged both after NO inhibition and preconditioning. NO inhibition alone causes a reduction in total hyperaemic flow, mainly resulting from the reduction in the duration of hyperaemic plateau. ABP and CBF: see Fig. 1.

 
IP did not cause significant changes in ABP and basal CBF in any of these animals, but reduced the total flow of the RH by 18% (P<0.05) and 15% (P<0.05), respectively for the two doses. However, depending on the dose of LNNA IP had different effects on the rate of vasodilation at the beginning of HR. In the animals treated with 5 mg kg–1 of LNNA a reduction of about 50% (from 9.4±1.3 to 4.8±0.3 s; P<0.001) was observed in the time to the peak flow of RH together with an increase of about 70% (from 11.25±5.5 to 19.37±11.2 ml min–1 s–1; P<0.05) in the time derivative of flow to the peak of RH. On the contrary, in the animals treated with the higher dose (10 mg kg–1) the time to peak flow and the corresponding rate of change of the flow were not significantly altered by preconditioning.

Fig. 4 summarises the reactive hyperaemic responses before and after ischaemic preconditioning in the three groups of animals.


Figure 4
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  		Fig. 4 Schematic representation of mean reactive hyperaemic responses before and after ischaemic preconditioning (IP) in the three groups of animals. Coronary flow was normalized by considering the flow changes as percent variation with respect to the basal flow before the 15-s occlusions.

 

   4 Discussion
Top
 Abstract
 1 Introduction
 2 Materials
 3 Results
 4 Discussion
5 Conclusions
 References
 
The present study reports two major new findings. First, the total hyperaemic flow is reduced by preconditioning, suggesting that preconditioning slows myocardial metabolism in accordance with several reports [5–9]. This suggestion is re-enforced by the fact that the reduction in total hyperaemic flow is not observed in the presence of A1 receptor blockade, i.e., when the effect of ischaemic preconditioning on myocardial metabolism is reported to be prevented [7, 8]. Second, while the total hyperaemic response is reduced by preconditioning, the rate of change of flow in the initial part of the RH, is markedly increased (Fig. 4). This latter change is not affected by the A1 receptor blockade, but is suppressed by the administration of LNNA at the dose of 10 mg kg–1 (Fig. 4), suggesting that, in addition to a protective effect on the myocardium, ischaemic preconditioning also exerts a direct effect on the responsiveness of the coronary vasculature.

Ischaemic preconditioning can be obtained in a variety of animal species with different protocols regarding the duration (from about 2.5 to 5 min) and number (from 1 to 12) of the occlusions [8]. We also found that the duration of the changes induced in the hyperaemic response by our occlusion was of the same order as the duration of protection reported in the literature [25–27]. In the seven goats in which a second experiment was performed 1.5 h after the first one either with or without A1 receptor blockade, the measured variables of RH before the repeated preconditioning were the same as those obtained in the control RH at the start of the experiment. This finding indicates that the changes produced in vascular reactivity by IP have the same time-course as those reported to be induced in myocardial metabolism [5–9]. This finding also shows that the changes observed in RH after the first preconditioning were not the result of a time-dependent deterioration of the cardiovascular system of the animal. In addition, the fact that the new preconditioning produced results identical to those observed after the first one, confirms that the time elapsed from the beginning of the experiments did not alter myocardial performance and vascular responsiveness.

There is evidence that RH consists of both a metabolic and a non-metabolic (i.e. vascular) component [1–4, 22]. Although tissue adenosine has long been considered to be involved in the regulation of coronary flow and RH [28]a number of authors considers its role unimportant in this regulation [29–32]. Since adenosine can mediate a direct dilator response by acting on the A2 vascular receptors and an indirect metabolic vasoconstriction by acting on the A1 myocardial receptors, an effect could be expected on resting flow and RH after A1 receptor blockade. A2 receptor blockade was not induced because it would have altered the metabolic component of RH independently of preconditioning. The unchanged resting flow and the unchanged RH observed after the infusion of the A1 blocker before IP support the idea that the level of adenosine present under resting condition and after the 15 s coronary occlusion plays a minor role in the regulation of the coronary circulation through an action on myocardial metabolism. It may also be argued, in accordance with Duncker et al. [33], that, when adenosine effects are prevented, other compounds can be released and may play a role in the regulation of the coronary flow.

Less controversial is the role of adenosine acting on the A1 receptors as a mediator of the reduction in metabolism observed during the period of myocardial protection obtained with coronary occlusions lasting from 2 to 20 min [5–8]. Our experiments support this point of view. In fact, in the control animals the total hyperaemic flow is reduced after the preconditioning manoeuvres, but is unchanged if the manoeuvres are performed in the presence of A1 receptor blockade which prevents this aspect of preconditioning [7, 8].

Although the two periods of 2.5 min of coronary occlusions performed in the presence of A1 blockade did not alter the total hyperaemic flow, they still caused an increase in the time derivative of flow of coronary RH, with a reduction of the time to peak flow similar to that observed in the control animals. The fact that after ischaemic preconditioning the same increase in the velocity of vasodilation was observed in the reactive hyperaemic response, independently of whether or not A1 receptors were blocked, suggests that this faster vasodilation was not a consequence of the changes in myocardial metabolism, but was a vascular phenomenon only.

An enhanced endothelial release of NO after ischaemic preconditioning is considered to be responsible for the protection against ischaemia-reperfusion arrhythmias [12–14]. It may thus be expected that such an enhanced release could also exert an effect on the changes in coronary vasomotor tone occurring during RH after IP. Although the effective dose of LNNA prevented IP from reducing the time to peak flow of RH, it was unsuccessful in preventing the reduction in total hyperaemic flow (see Fig. 3, Table 3). This finding confirms that this latter reduction depends on the decrease in myocardial metabolism elicited by preconditioning as revealed by the results from A1 blocked animals. It is intriguing that the presumed increase of NO release does not counteract the metabolic reduction in total hyperaemic flow after IP. It is likely that the effect of the decrease in myocardial metabolism is predominant with respect to the vasodilator effect of NO on the overall RH. The ineffectiveness of the dose of 5 mg kg–1 in suppressing the reduction in the time to peak flow of RH can be attributed to the dependence of the effect on the dose of the inhibitor [34].

Another consideration is suggested by the results of the present investigation. After preconditioning the reduction of about 40–50% of the time to the peak flow provides novel insights into the explanation of the delay of the peak observed in the RH. Classically two different mechanisms have been proposed to explain this delay: (1) a time-dependent stretching of viscoelastic elements of the vessel wall [35, 36]and (2) a time-dependent reopening of coronary vessels compressed by ischaemic subendocardial layers of the myocardium, whose relaxation has been impaired by the preceding occlusion [4, 35, 37]. In favour of the latter mechanism is the fact that preconditioning can attenuate the depression of the contractile function [38], thereby diminishing the relaxation impairment and accelerating the rate of reopening of the subendocardial vessels. However, in our experiments, after A1 blockade, i.e., when the myocardial protection does not occur [7, 8], the shortening of the time to peak by IP is the same as in the control. This finding confirms the hypothesis that the mechanism responsible for the reduction in the time to peak flow after preconditioning does not depend on the myocardium but is related to a direct vascular response, i.e., to the enhanced endothelial release of NO.


   5 Conclusions
Top
 Abstract
 1 Introduction
 2 Materials
 3 Results
 4 Discussion
 5 Conclusions
References
 
Our results show that the manoeuvres able to induce ischaemic preconditioning also cause a reduction in the total flow of RH and induce changes in the responsiveness of the coronary bed to the vasodilator stimuli elicited by brief coronary occlusions. We can thus speak of vascular preconditioning. While the reduction in total hyperaemic flow can be explained by a decrease in myocardial metabolism caused by the activation of the A1 receptors [7, 8], the faster vasodilation at the beginning of the RH is attributed to a vascular phenomenon which is likely to depend on the release of nitric oxide.

Time for primary review 25 days.


   Acknowledgements
 
The authors wish to express their gratitude to Mr. Rodolfo Dalla Valle for his excellent technical cooperation and Dr. Silvestro Roatta for his assistance in the preparation of the figures. This research was performed with financial support from the Italian Ministry of the University and Scientific and Technological Research (MURST - quota 40%), the Italian National Research Council (CNR-95. 02316.CT 04 B) and the British Heart Foundation.


   References
Top
 Abstract
 1 Introduction
 2 Materials
 3 Results
 4 Discussion
 5 Conclusions
 References
 

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