Effect of acute exercise on cardiovascular hemodynamic and red blood cell concentrations of purine nucleotides in hypertensive compared with normotensives rats

Abstract

Objective:

The mechanisms of exercise-induced health benefits are complex and not fully understood. This study investigated the effects of exercise and hypertension on cardiovascular hemodynamic responses and red blood cell (RBC) concentrations of purine nucleotides using normotensive and hypertensive rat models in vivo.

Methods:

Sprague Dawley rats (SDRs) and spontaneously hypertensive rats (SHRs) were exercised on a treadmill for 15 min at a speed of 10 m/min and 5% grade. Blood samples were obtained from each rat before, during, and after exercise for measurement of adenosine 5′-triphosphate (ATP) and guanosine 5′-triphosphate (GTP) concentrations in RBCs by a validated high-performance liquid chromatography assay. They were returned to a restrainer after exercise, and hemodynamic recording collected continuously up to 6 h. Two separate groups (SDRs and SHRs) without exercise were used as controls. Biomarker data were compared between SDRs and SHRs using analysis of variance and t test and difference considered significant at p < 0.05.

Results:

The study has demonstrated for the first time a difference in the postexercise effect between SDRs and SHRs. The 15 min of exercise significantly increased systolic blood pressure (SBP) (129 ± 16 to 162 ± 26 mmHg) and heart rate (HR) (416 ± 29 to 491 ± 26 bpm) in SDRs (p < 0.05), but not in SHRs. The postexercise hemodynamic effects were more profound in SHRs. SBP and diastolic blood pressure (DBP) also fell significantly in the control group of SHRs (SBP 184 ± 14 to 152 ± 29 mmHg and DBP 149 ± 9 to 120 ± 14 mmHg, p < 0.05 for both) towards the end of the experiment but not in the SDR group. The RBC concentrations of ATP and GTP increased after exercise in both SDRs and SHRs which were significantly correlated with the postexercise hemodynamic effect (p < 0.05).

Conclusion:

SHRs were more tolerant to increases in HR and SBP induced by exercise, and have more profound postexercise hemodynamic effects than SDRs. The hemodynamic effects were linked closely with RBC concentrations of ATP and GTP in both SDRs and SHRs.

Keywords

adenosine 5′-triphosphate biomarkers exercise guanosine 5′-triphosphate hemodynamics red blood cells spontaneously hypertensive rats Sprague Dawley rats

Introduction

While exercise can trigger neurohormone activation which may induce cardiac adverse events [Hackney, 2006], its benefits to cardiovascular health and metabolic diseases is undeniable [Roine et al. 2009; Blair and Morris, 2009; Vogel et al. 2009]. It is believed that regular exercise is one of the most important nonpharmacological tools in reducing overall cardiometabolic risk since it significantly reduces body weight, blood pressure (BP), blood glucose and lipid levels, and also improves strength flexibility and quality of life and reduces stress [Antic et al. 2009; Keteyian et al. 2010]. Although the mechanism of cardiovascular protection induced by exercise is not fully understood, the BP lowering effect post exercise is believed to be a major contributor [Collins et al. 2001; MacDonald, 2002; Pescatello et al. 2004; Halliwill et al. 2012]. The effect could be mediated via changes in neurohormonal release, oxidant/antioxidant balance, and synthesis of molecular mediators [Di Francescomarino et al. 2009; Kavazis, 2009; Terziotti et al. 2001], improve glucose uptake by a noninsulin-dependent mechanism [Lee-Young et al. 2010] and correcting the imbalance occurring in endothelial dysfunction [Hirata et al. 2010; Lizardo et al. 2008; Lee et al. 2009]. It is possible many or all of these factors are contributing to the cardiovascular health benefits from exercise.

Circulatory concentrations of adenosine and adenosine 5′-triphosphate (ATP) and their metabolites have been implicated as potential biomarkers for cardiovascular protection and as targets for anti-ischemia drugs [Yeung and Feng, 1998; Yeung et al. 2009]. The importance of adenosine and ATP in regulating many biological functions has long been recognized, especially for their effects on the cardiovascular system [Olsson and Pearson, 1990; Burnstock, 2002; Ingwall, 2009; Laubach et al. 2011; Yang et al. 2010]. It is known that adenosine and ATP are key factors in regulation of coronary blood flow [Berne, 1980], inhibiting platelet aggregation [Gerlach et al. 1987], protection of myocardium [Cohen and Downey, 2008], neuromodulation [Burnstock, 2009; Gomes et al. 2011; Laubach et al. 2011], modulating tissue necrosis [Burnstock, 2002], ischemic preconditioning [Donato and Gelpi, 2003; Das and Das, 2008; Yang et al. 2010; Bein and Meybohm, 2010], immunomodulation [McCallion et al. 2004], energy metabolism [Porkka-Heiskanen et al. 2003; Ingwall, 2009], and perhaps other functions as well (e.g. pain mediation) which maintain cardiovascular homeostasis. It has been shown that patients with effort angina and essential hypertension have altered adenosine metabolism compared with healthy individuals [Tykarski et al. 1993; Duthie et al. 1994; Yeung et al. 1997], and that plasma concentrations of adenosine increase in patients with congestive heart failure [Funaya et al. 1997], which could be a physiologic response to heart failure and help to reduce the severity of the disease [Kitakaze et al. 1998]. Adenosine is known also to interact with the rennin angiotensin system to maintain a homeostatic balance between BP, vascular resistance and blood volume in hypertension and endothelial dysfunction [Taddei et al. 1992; Franco et al. 2009; Tang and Vanhoutte, 2010]. Thus it has been postulated that adenosine and ATP may be used as sensitive biomarkers to quantify myocardial and endothelial ischemia [DeJong, 1988; Round et al. 1994]; and for monitoring therapeutic effects of anti-ischemia drugs [Yeung and Feng, 1998; Yeung et al. 1998, 2009].

We have recently shown that in the normotensive Sprague Dawley rat (SDR), exercise improved cardiovascular hemodynamic profiles and increased red blood cell (RBC) concentrations of ATP and guanosine 5′-triphosphate (GTP), which may be key attributing factors for the postexercise effects [Yeung et al. 2010]. It is not clear if a similar response to exercise may also occur in hypertension. The current study investigates further the relationships between hemodynamic profiles and RBC concentrations of ATP and GTP using both SDRs and spontaneously hypertensive rats (SHRs).

Materials and methods

Authentic standards of purine nucleotides including ATP, GTP and other purine nucleotides were purchased from Sigma-Aldrich Chem Co. (St Louis, MO, USA). Solvents were high-performance liquid chromatography assay (HPLC) grade, and all other chemicals were reagent grade (Fisher Scientific, Toronto, ON, Canada).

The protocol followed the Canadian Council on Animal Care guidelines and was approved by the Dalhousie University Committee on Laboratory Animals (UCLA 10-003). Male SDRs and SHRs weighing between 250 and 300 g with an indwelling carotid artery catheter were purchased directly from Charles River Laboratories (Wilmington, MA, USA). Each rat was acclimatized for at least 48 h in the Carleton Animal Care Centre before experiment. The exercise test was performed on a research Exercise Treadmill (Model Exer-4, Columbus Instruments International Corporation, Columbus, OH, USA). After two brief sessions (3–5 min each) of training to acclimatize the rat with the treadmill on the day before the study, each rat (n = 11 for both SDRs and SHRs) was exercised on the treadmill for 15 min at a speed of 10 m/min with a 5% grade after an hour settling down in a restrainer on the experiment day as described previously [Yeung et al. 2010], and then returned to the restrainer after the exercise. Blood samples (0.3 ml each) were collected via the indwelling catheter using a ‘Stopping Solution’ from each rat at 0, 0.25, 1 (onset of exercise), 5, 10 and 15 min after onset of exercise, and at 1, 2, 3, 4 and 5 h after exercise. Each blood sample withdrawn was replenished with the same volume of saline to avoid volume depletion. Hemodynamic recording was interrupted briefly during each blood sample collection, but it did not affect the quality of the tracing recorded. At the end of the experiment, the rat was euthanized by cardiac puncture under anesthesia with isoflurane. The total length of the experiment was about 6 h. Hemodynamic variables [systolic BP (SBP), diastolic BP (DBP) and heart rate (HR)] were continuously recorded (except interrupted briefly during each blood sample collection) via the intravascular catheter using a TruWave disposable pressure transducer (Model PX601, Edwards Lifesciences Canada, Inc., Mississauga, ON, Canada) coupled to a Siemens hemodynamic monitor (Sirecust 400) and chart recorder (Siredoc) (Erlangen, Germany) as previously described [Yeung et al. 2010]. The hemodynamic data presented were averages of 10–15 s recording. The RBC samples collected were processed and lysed immediately using an ice cold 10% trichloroacetic acid. The lysate samples were stored at −80^oC, and concentrations of ATP and other purine nucleotides in the RBCs were determined by a validated HPLC assay [Yeung et al. 2008]. Two separate groups using SDRs (n = 11) and SHRs (n = 8) handled the same way but without the exercise were used as controls. They were housed in the same location as the rats undergoing the exercise experiment, and a similar number of blood samples (n = 9) were collected from each rat except for the three samples taken during exercise. This should minimize the effect of blood loss that may confound interpretation of the results for the effect of exercise and strain difference. Hemodynamic and biomarker variables at baseline, before exercise, at the peak of exercise, after exercise and at the end of the experiment were used for comparison between groups (Figure 1). Data between groups were analyzed by analysis of variance followed by Tukey’s multiple comparison, and also by student’s paired and unpaired t test, and differences between the control and exercise groups, and between pre and post exercise were considered significant when p < 0.05. In addition, possible relationships between biomarkers from the group mean data were assessed using both Pearson and Spearman correlations and considered significant at p < 0.05 (Minitab Inc., Release 15.1, State College, PA, USA).

Figure 1.

Hemodynamic response to exercise in spontaneously hypertensive rats. Each point represents mean ± standard error of the mean (n = 11 each). DBP, diastolic blood pressure; HR, heart rate; SBP, systolic blood pressure.

Results

The baseline SBP and DBP were significantly higher in SHRs (SBP 199 ± 24 versus 129 ± 16 mmHg; DBP 169 ± 18 versus 117 ± 18 mmHg, p < 0.05 for both), but there was no difference in the HR between SDRs and SHRs (414 ± 34 versus 416 ± 29 bpm, p > 0.05). A 15 min exercise under the described condition significantly increased mean SBP to 162 ± 26 mmHg (+26 ± 18%) and HR to 491 ± 26 (+18 ± 8%) in SDRs (p < 0.05), but not DBP (+16 ± 16%) in SDRs (p > 0.05). The same level of exercise also increased SBP, DBP and HR in SHRs, but the increase was not statistically significant (Table 1). The mean maximum values of SBP and HR were achieved at or near the end of the exercise for both SHRs and SDRs. However, the DBP appeared to reach a maximum before the end of exercise (Figure 2). The DBP and HR but not SBP were significantly lower post exercise in SDRs, but they were all significantly lower in SHRs (p < 0.05). At the end of the 6 h experiment, the mean DBP fell below the baseline in SDRs and SHRs by −22% and −31% respectively. The HR also fell significantly after exercise to 358 ± 35 bpm in SDRs (−25% from peak exercise), and 303 ± 46 in SHRs (−31% from peak exercise) (p < 0.05), and slowly returned to baseline level at the end of the experiment, which was longer than 4 h after exercise (Figure 2). Both SBP and DBP (not HR) at the end of the experiment were significantly lowered in SHRs (p < 0.05) not engaged in exercise, but the effects were minimal in the nonexercised SDRs (< 10%) and only the decrease in DBP was significant (p < 0.05) (Table 1 and Figure 3).

Table 1.

Hemodynamic variables in rats with and without exercise.^$

	Exercise SDR (n = 11)	Control SDR (n = 11)	Exercise SHR (n = 11)	Control SHR (n = 8)
SBP baseline (mmHg)	129 ± 16*	131 ± 13*	199 ± 24	184 ± 14
SBP maxEx^‡ (mmHg)	162 ± 26***	NA	237 ± 66	NA
SBP afterEx^§ (mmHg)	140 ± 27*	NA	189 ± 65	NA
SBP end (mmHg)	125 ± 17*	129 ± 13	166 ± 24***	152 ± 29***
DBP baseline (mmHg)	117 ± 18*	109 ± 10*	169 ± 18	149 ± 9
DBP maxEx (mmHg)	136 ± 18	NA	191 ± 59	NA
DBP afterEx (mmHg)	115 ± 21	NA	144 ± 57	NA
DBP end (mmHg)	91 ± 14,**	100 ± 11,**	116 ± 18***	120 ± 14***
HR baseline (bpm)	416 ± 29	409 ± 37	414 ± 34	448 ± 15
HR maxEx (bpm)	491 ± 26***	NA	458 ± 20	NA
HR afterEx (bpm)	358 ± 35***	NA	303 ± 46***	NA
HR end (bpm)	428 ± 32	427 ± 40	402 ± 79	416 ± 35

Each value represents mean ± standard deviation.

‡

Maximum value recorded during exercise.

Average value over 1 h after exercise.

p < 0.05 versus strain; **p < 0.05 versus exercise group; ***p < 0.05 versus baseline.

DBP, diastolic blood pressure; HR, heart rate; NA, not applicable; SBP, systolic blood pressure; SDR, Sprague Dawley rat; SHR, spontaneously hypertensive rat.

Figure 2.

Hemodynamic effects of exercise in Sprague Dawley rats and spontaneously hypertensive rats. Each point represents mean ± standard error of the mean (n = 11 each).

Figure 3.

Hemodynamic effects in Sprague Dawley rats (n = 11) and spontaneously hypertensive rats (n = 8) in control group (no exercise). Each point represents mean ± standard error of the mean.

The baseline RBC concentrations of GTP (but not ATP) in the control SHRs were higher (p < 0.05) than in SDRs (Table 2). The concentrations of ATP and GTP in the RBCs increased towards the end of the experiments in both SHRs and SDRs (Figure 4). Exercise increased RBC concentrations of ATP and GTP from 1.14 ± 0.48 and 0.069 ± 0.045 mM to 1.95 ± 0.99 and 0.17 ± 0.06 mM respectively in SDRs (p < 0.05 for both). The increase was much greater in the exercised SDRs than the control (no exercise) group (Table 2). The RBC concentrations of ATP and GTP in SHRs also increased significantly after exercise (p < 0.05), as well as in SHRs kept in the restrainer without exercise (Table 2). There was an abrupt decline of ATP and GTP concentrations in the RBCs during the 15 min of exercise in SHRs, which was not observed in SDRs (Figure 4).

Table 2.

Purine nucleotide concentrations in red blood cells in rats with and without exercise.^$

	Exercise SDR (n = 11)	Control SDR (n = 11)	Exercise SHR (n = 11)	Control SHR (n = 8)
ATP baseline (mM)	1.14 ± 0.48	1.11 ± 0.23	0.94 ± 0.28	1.28 ± 0.52
ATP end (mM)	1.95 ± 0.99***	1.32 ± 0.22	1.82 ± 0.47***	2.19 ± 0.89***
GTP baseline (mM)	0.069 ± 0.045	0.072 ± 0.021*	0.035 ± 0.025	0.12 ± 0.037**
GTP end (mM)	0.17 ± 0.078***	0.11 ± 0.050,**	0.17 ± 0.060***	0.25 ± 0.11***

Each value represents mean ± standard deviation.

p < 0.05 versus strain; **p < 0.05 versus exercise group; ***p < 0.05 versus baseline (paired t test).

ATP, adenosine 5′-triphosphate; GTP, guanosine 5′-triphosphate; SDR, Sprague Dawley rat; SHR, spontaneously hypertensive rat.

Figure 4.

Red blood cell concentrations of adenosine 5′-triphosphate and guanosine 5′-triphosphate in Sprague Dawley rats and spontaneously hypertensive rats. Each point represents mean ± standard error of the mean.

There were good correlations between mean values of DBP and RBC concentration of ATP and GTP in exercise SDRs and SHRs (r > −0.8 and p < 0.05) (Table 3). Significant correlations with SBP were also obtained in exercise SHRs, but not in SDRs, although the correlation was not as strong as with DBP (Table 3). The correlations with HR were not significant except for ATP in SDRs (r = 0.631 and p = 0.037). In the nonexercise rats (control), significant correlations were obtained for ATP and GTP with the hemodynamic parameters (SBP, DBP and HR) in SHRs, but not in SDRs (Table 3). The correlations between ATP and GTP with DBP are presented in Figure 5. In light of the relatively small sample size for the mean values, the data were also analyzed using Spearman correlation and the results were similar.

Table 3.

Pearson correlations between red blood cell concentrations of ATP and GTP with cardiovascular hemodynamics.*

	ATP	GTP
Exercise SDR (n = 11)
SBP	−0.301 (p > 0.05)	−0.233 (p > 0.05)
DBP	−0.812 (p = 0.002)	−0.823 (p = 0.002)
HR	0.631 (p = 0.037)	0.552 (p > 0.05)
Exercise SHR (n = 11)
SBP	−0.696 (p = 0.017)	−0.671 (p = 0.024)
DBP	−0.817 (p = 0.002)	−0.820 (p = 0.002)
HR	0.161 (p > 0.05)	0.096 (p > 0.05)
Nonexercise SDR (n = 11)
SBP	−0.044 (p > 0.05)	ND
DBP	−0.566 (p > 0.05)	0.276 (p > 0.05)
HR	0.506 (p > 0.05)	0.057 (p > 0.05)
Nonexercise SHR (n = 8)
SBP	−0.761 (p = 0.028)	−0.775 (p = 0.024)
DBP	−0.877 (p = 0.004)	−0.873 (p = 0.005)
HR	−0.874 (p = 0.005)	−0.800 (p = 0.017)

Determined from mean data from each group

ATP, adenosine 5′-triphosphate; DBP, diastolic blood pressure; GTP, guanosine 5′-triphosphate; HR, heart rate; ND, not determined; SDR, Sprague Dawley rat; SHR, spontaneously hypertensive rat.

Figure 5.

Pearson correlations between adenosine 5′-triphosphate and guanosine 5′-triphosphate with diastolic blood pressure. Each point represents the mean of the group data.

Discussion

As reported in our previous communications and by other investigators, the RBCs in SDRs have abundant amount of ATP in the mM range which are typically 10–20 times higher than GTP concentrations [Yeung et al. 2008, 2009, 2010; Chiao et al. 1987]. Similar concentration ratios are found in SHRs in the current study (Table 2). We had earlier shown that exercise improved hemodynamic profiles and increased RBC concentrations of ATP in normotensive SDRs which may be an important mechanism for postexercise hypotension. This was evidenced by a strong correlation between the fall in DBP and increase in RBC concentrations of ATP and GTP [Yeung et al. 2010]. However, there are other mechanisms which also contribute to postexercise hypotension. It has been shown that nitric oxide synthase activity in the endothelium and the rennin angiotensin system are activated during exercise [Lee et al. 2009; Wan et al. 2007]. Blockade of angiotensin II receptor has been shown to increase the effect of exercise to limit cardiac postinfarct ventricular remodeling in rats [Xu et al. 2008]. In contrast, blocking nitric oxide synthase activity was shown to attenuate postexercise hypotension [Lizardo et al. 2008]. There are probably other additional mechanisms which work together that are responsible for the health benefits associated with cardiovascular exercise [Halliwill et al. 2012].

The current study has demonstrated for the first time a similar hemodynamic and biochemical effect on RBC concentrations of ATP and GTP from exercise in a hypertension model using SHRs, albeit that there were some notable differences. First, the same level of exercise (15 min at a speed of 10 m/min with a 5% grade) provoked considerably less HR response in SHRs than SDRs (11% versus 18%). The increase in SBP during exercise was also not statistically significant in SHRs because of greater variability (Table 1). While the increase in the mean SBP at the end of exercise (SBP maxEx) from baseline was comparable between SHRs (+38 mmHg or 19% increase) and SDRs (+33 mmHg or 26% increase), the percentage increase was more variable in SHRs (CV = 119%) than in SDRs (CV = 70%), which may explain the statistical difference. Second, the hemodynamic effects post exercise were greater in SHRs (Figure 2). While we observed a 3% and 22% decrease in mean SBP and DBP respectively in exercise SDRs at the end of the experiment, the decrease was even greater in SHRs (−17% and −31% respectively for SBP and DBP). Similarly, the mean HR was decreased by 14% after exercise in SDRs compared with the 27% decline in SHRs, although it returned to the baseline value at the end of the experiment (Table 1). Third, when the rats were not exercised and kept in a restrainer as described in an earlier communication [Yeung et al. 2011], both SBP and DBP in SHRs fell throughout the experiment (p < 0.05). The BP also fell in SDRs, but only the decline in DBP was significant (p < 0.05). There was no significant change in the HR for both SDRs and SHRs in the control (i.e. nonexercise) group (Table 1 and Figure 3). The current study is the first to report a difference in hemodynamic response to treadmill exercise between SDRs and SHRs. A strain difference in neurohormone response to restraining and tail cuff BP measurement has been reported in an earlier study, which showed neurohormone response was greater in SHRs than normotensive Wistar Kyoto rats [Grundt et al. 2009]. The strain difference observed during exercise could be related to greater tolerance to exercise-induced stress secondary to the higher basal BP in SHRs. The reason for the difference observed in restraining is not clear, although it could be related to the greater response in SHRs to the stress induced by restraining and from BP measurement by the tail cuff method [Grundt et al. 2009 ]. It will be very exciting to follow up the exercise study using another cardiovascular disease model such as spontaneous hypertensive heart failure (congenic) or the coronary artery ligation myoinfarct model to explore if they also have an altered response to exercise.

As reported previously, exercise also increased ATP and GTP concentrations in the RBCs in SDRs, and the effects lasted several hours after the exercise [Yeung et al. 2010]. It is important to note that while the concentrations of ATP and GTP increased during exercise in SDRs, the concentrations in SHRs decreased during exercise (Figure 4). It is not clear whether or not these differences during exercise were responsible for the differences observed for the postexercise effects between SHRs and SDRs (Figure 2). In addition, we found the RBC concentrations of ATP and GTP increased in SHRs kept in a restrainer towards the end of the experiment (p < 0.05), but there was no significant increase in SDRs (Table 2 and Figure 4). Although the underlying mechanism for the difference is not known, it is possible that basal energy metabolism is greater in SHRs, which is required to meet the inherent demand of energy for maintaining cardiovascular homeostasis. Energy demand may be reduced when the rats are kept in a restrainer resulting in higher RBC concentrations of the nucleotides. However, the demand for energy and ATP is augmented during exercise, leading to a decrease in RBC concentrations of ATP and GTP as a consequence of reduced reserves of the purine nucleotides in the cardiovascular system. Previous study has shown that plasma concentrations of adenosine are higher in SHRs compared with normotensive rats [Yamada et al. 1992], which supports the higher demand for adenosine and ATP in SHRs. There is also evidence to suggest that ATP turnover and supply of ADP in myocardial tissue is more restricted in SHRs, particularly when energy demand is increased in hard work conditions [Hickey et al. 2009]. This could be partly attributed to dysfunctional ATP synthesis in mitochondria in SHRs [Doroshchuk et al. 2004], which has also been shown to be more susceptible to systolic dysfunction after myocardial ischemia insult [Norton et al. 2008]. These data together suggest that energy and ATP metabolism and the response to exercise are different in SHRs as demonstrated in the current study, and may potentially be used for cardiovascular disease management.

We have previously postulated that circulatory concentrations of ATP and GTP may be key factors for the observed postexercise effects in SDRs [Yeung et al. 2010]. The current study has demonstrated again highly significant correlations between postexercise hypotension and RBC concentrations of ATP and GTP in both SDRs and SHRs. Significant correlations were also found in SHRs kept in a restrainer, but there were no correlations for the restrained SDRs. Further, while there was no correlation between RBC nucleotide concentrations and HR in the exercise SHRs, a highly significant correlation was observed in the restrained SHRs (Table 3). These data further support that there is an inherent difference in energy and ATP metabolism such that the ATP and GTP reserve in the cardiovascular system is limited in SHRs. This would suggest SHRs may be more vulnerable to exercise-induced stress and sympathetic activation, and less resistant to myocardial injury in the event of an ischemic insult, and hence it may be a more sensitive model for cardiovascular injury and protection. However it is possible that the same intensity of exercise may induce different level of stress in the two strains of rats. Based on our experience, exercise at a speed of 10 m/min and 5% grade for 15 min was a relatively mild condition for SDRs [Yeung et al. 2010]. It was difficult to assess if SHRs had experienced the same intensity because the rats were not exercised with other conditions, and there were no other biomarkers (e.g. plasma lactate concentration) to assess the exercise intensity besides RBC concentrations of ATP and cardiovascular hemodynamic. Further study comparing the cardiovascular response between SDRs and SHRs using more vigorous exercise conditions to exhaust more energy may be warranted. Although it is not clear whether the results from the experimental study could be extrapolated directly to patients with hypertension as the pathophysiology for the elevated BP may be different, the finding does imply that patients with such underlying conditions should be more cautious when exercising compared with healthy individuals.

Although clinical significance of the change of nucleotide concentrations in the RBCs induced by exercise is not clear, it is possible that it may be a measure for cardiovascular events in vivo. It has been shown that ATP is released from human RBCs and myocardium in response to a brief period of hypoxia, which is subsequently broken down to adenosine diphosphate and adenosine monophosphate in vitro [Bergfeld and Forrester, 1992; Watts, 1986]. Similar breakdown may also occur for GTP, although little is known of the physiologic functions of this nucleotide in the RBCs and how they may be different from ATP. While there is no direct evidence to indicate a similar response to ischemia or exercise may also occur in vivo, the idea of the RBC being an oxygen sensor as suggested by other workers needs further investigation [Ellsworth, 2000; Jensen, 2009]. It is known that RBCs are capable of releasing increased amounts of ATP as oxygen content falls and its hemoglobin becomes desaturated [Bergfeld and Forrester, 1992]. It has been hypothesized that RBCs may sense tissue oxygen requirements when they travel through the microcirculation and release vasodilatory compounds such as ATP that enhance blood flow in hypoxic tissues [Jensen, 2009]. The released ATP would help to increase blood supply to the tissue and preserve an optimum balance between oxygen supply and demand, thereby modulating the concentrations of tissue ATP within the cardiovascular system. Such a mechanism would eliminate the requirement for a diverse network of sensing sites throughout the vasculature, and should provide a more efficient means of appropriately matching oxygen supply with demand, and allow an immediate switch to alternative energy sources under hypoxia condition [Lopez-Barneo et al. 2010]. We have recently shown that preconditioning with a brief 15 min treadmill exercise significantly reduced mortality induced by isoproterenol in SDRs in an acute myocardial infarction model in vivo [Yeung and Seeto, 2011]. Thus we advocate the beneficial effects of exercise are at least in part attributable to BP and HR lowering effects occurring after exercise, which are due to an increase in RBC concentrations of ATP and GTP. If this is proven, purine nucleotide concentrations and their metabolism may be used as surrogate biomarkers for management of cardiovascular diseases, which would be an exciting topic with important clinical implications for further studies.

Conclusion

The current study has demonstrated an inherent difference in ATP metabolism between SDRs and SHRs. SHRs were more tolerant to the increase in HR and SBP induced by exercise. The postexercise effect was more profound in SHRs, albeit that the ATP reserves in the cardiovascular system may be more restricted in SHRs. Further study to evaluate ATP metabolism as a relevant biomarker for cardiovascular protection using both SDRs and SHRs is warranted.

Footnotes

Acknowledgements

The paper was precharmaceutical Sciences (CSPS) Symposium, Vancouver, BC, Canada 2–5 June 2010.

Funding

The study was supported in part by Canadian Institute of Health Research (ROP86932), Nova Scotia Health Research Foundation (MED-MAT-2007-3546) and Dalhousie Pharmacy Endowment Foundation.

Conflict of interest statement

The authors declare that there are no conflicts of interest.

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