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Exp Physiol 99.12 (2014) pp 1552–1558
Hot Topic Review
Regulation of the skeletal muscle blood flow in humans Stefan P. Mortensen1 and Bengt Saltin2 † 1 2
Department of Cardiovascular and Renal Research, University of Southern Denmark, Odense, Denmark Copenhagen Muscle Research Centre, Rigshospitalet, Copenhagen, Denmark
Experimental Physiology
New Findings r What is the topic of this review? This review highlights recent advances in our knowledge about the control of skeletal muscle blood flow during exercise in humans. r What advances does it highlight? In recent years, it has become evident that the control of skeletal muscle blood flow is an interaction between various vasodilator agents, including nitric oxide, prostaglandins and adenosine. Adenosine triphosphate could play multiple roles by inducing local vasodilatation, overriding local sympathetic vasoconstriction and stimulating the exercise pressor reflex.
In humans, skeletal muscle blood flow is regulated by an interaction between several locally formed vasodilators, including NO and prostaglandins. In plasma, ATP is a potent vasodilator that stimulates the formation of NO and prostaglandins and, very importantly, can offset local sympathetic vasoconstriction. Adenosine triphosphate is released into plasma from erythrocytes and endothelial cells, and the plasma concentration increases in both the feed artery and the vein draining the contracting skeletal muscle. Adenosine also stimulates the formation of NO and prostaglandins, but the plasma adenosine concentration does not increase during exercise. In the skeletal muscle interstitium, there is a marked increase in the concentration of ATP and adenosine, and this increase is tightly coupled to the increase in blood flow. The sources of interstitial ATP and adenosine are thought to be skeletal muscle cells and endothelial cells. In the interstitium, both ATP and adenosine stimulate the formation of NO and prostaglandins, but ATP has also been suggested to induce vasoconstriction and stimulate afferent nerves that signal to increase sympathetic nerve activity. Adenosine has been shown to contribute to exercise hyperaemia, whereas the role of ATP remains uncertain due to lack of specific purinergic receptor blockers for human use. The purpose of this review is to address the interaction between vasodilator systems and to discuss the multiple proposed roles of ATP in human skeletal muscle blood flow regulation. (Received 27 June 2014; accepted after revision 1 September 2014; first published online 5 September 2014) Corresponding author S. P. Mortensen: Department of Cardiovascular and Renal Research, Institute of Molecular Medicine, University of Southern Denmark, J. B. Winsløws Vej 21-3, 5000 Odense C, Denmark. Email: [email protected]
Introduction During exercise, skeletal muscle perfusion increases in direct proportion to the metabolic demand. When the †
In memory of Bengt Saltin, who passed away on 12 September 2014.
DOI: 10.1113/expphysiol.2014.081620
active muscle mass is small and the heart is therefore not a limitation, skeletal muscle blood flow can increase 100-fold from resting conditions and reach perfusion rates of 300–400 ml min−1 kg−1 (Andersen & Saltin, 1985; Richardson et al. 1993). Studies manipulating arterial O2 content in humans have provided clear evidence that it is C 2014 The Authors. Experimental Physiology C 2014 The Physiological Society
Exp Physiol 99.12 (2014) pp 1552–1558
Regulation of the skeletal muscle blood flow in humans
not blood flow per se that is regulated but rather the O2 delivery matching the metabolic demand. In recent years, it has become apparent that several vasoactive compounds interact and function in synergy to contribute to the regulation of skeletal muscle blood flow (Hellsten et al. 2012). Apart from substances that can induce local smooth muscle cell relaxation, it has been demonstrated that some substances have the ability to inhibit the local vasoconstrictor effect of the increased sympathetic nerve activity during exercise. This phenomenon, termed functional sympatholysis, is known to occur during exercise (Remensnyder et al. 1962; Rosenmeier et al. 2004), but its importance for the blood flow regulation and the substance(s) involved in the regulation is debated (Hansen et al. 2000; Saltin & Mortensen, 2012). This review highlights the formation, interaction and effect of vasodilator substances that are likely candidates to be important for blood flow regulation in human skeletal muscle. Due to constraints on space, the review is mainly focused on experimental work undertaken in human subjects and not work from animal studies; for this, we refer the reader to comprehensive reviews published previously (Sarelius & Pohl, 2010; Marshall & Ray, 2012). Nitric oxide, prostaglandins and endothelium-derived hyperpolarization factors (EDHFs)
Nitric oxide is formed by conversion of L-arginine by nitric oxide synthase (NOS). In human skeletal muscle, mainly two isoforms (eNOS and nNOS) are localized in the vascular endothelium and in the skeletal muscle cells, respectively (Frandsen et al. 1996; Grozdanovic et al. 1996). Erythrocytes are also a potential source of NO by formation of S-nitrosohaemoglobin (Stamler et al. 1997). Nitric oxide plays an important role for the regulation of blood flow at rest and during the recovery from exercise, but when NOS is inhibited during leg exercise, there is no change in exercise hyperaemia (R˚adegran & Saltin, 1999; Bradley et al. 1999; Frandsen et al. 2001), suggesting that NO is not obligatory for exercise hyperaemia in humans. The enzyme cyclo-oxygenase (COX) catalyses the conversion of arachidonic acid to prostaglandin H2 , from which the vasodilators prostaglandin E2 and prostacyclin (PGI2 ) are derived. The potential sources of prostaglandins are endothelial and skeletal muscle cells (Vandenburgh et al. 1995; Davidge, 2001). The interstitial concentration of PGI2 (Frandsen et al. 2000) increases during exercise, and both vasodilators are also increased in the venous efflux of contracting muscle (Wilson & Kapoor, 1993). Inhibition of COX does not reduce exercise hyperaemia (Mortensen et al. 2007; Schrage et al. 2004). Nevertheless, when COX is inhibited simultaneously with NOS, blood flow is reduced by 30% in the exercising leg (Mortensen et al. 2007, 2009b) and forearm (Schrage et al. 2004; C 2014 The Authors. Experimental Physiology C 2014 The Physiological Society
1553
Saunders et al. 2005). These observations indicate that there is a compensatory formation of other vasodilators when one is inhibited, so that an adequate blood flow is achieved. It is important to note that these observations were performed at low and moderate workloads and that the reduction in exercise hyperaemia appears to be lower at higher exercise intensities (Christensen et al. 2013). Whether this reflects a lower contribution of NO and COX systems at higher exercise intensities or a lower effect of the blockers when blood flow is high (Schrage et al. 2010) remains unclear. The concept of EDHFs is based on the observation that specific substances can induce hyperpolarization of smooth muscle cells in the presence of COX and NOS inhibitors. Cytochrome P450 2C9 (CYP2C9) has been suggested to be an EDHF in skeletal muscle, but blockade of CYP2C9 with sulfaphenazole does not reduce exercise hyperaemia (Hillig et al. 2003). Yet, combined inhibition of NOS and CYP2C9 lowers exercise hyperaemia by 15%, suggesting an interaction between these two vasodilator systems, similar to the interaction observed between NOS and COX. Nevertheless, when EDHFs are blocked with the non-specific potassium channel blocker TEA in combination with inhibition of NOS and COX, exercise hyperaemia is not reduced beyond that of combined NOS and COX inhibition (Mortensen et al. 2007). This could indicate that TEA may not be sufficiently specific to examine an EDHF effect or that EDHF-induced vasodilatation cannot compensate for the impaired NO and prostaglandin systems. Adenosine triphosphate
Adenosine triphosphate is thought to play an important role in blood flow regulation by inducing local vasodilatation (Gonz´alez-Alonso, 2012), but direct evidence for a role of ATP in blood flow regulation is lacking because of the absence of a selective receptor antagonist for human use. Levels of ATP increase in both the feed artery and the vein draining contracting muscle (Mortensen et al. 2011). Given that the estimated half-life of ATP is
Exp Physiol 99.12 (2014) pp 1552–1558
Hot Topic Review
Regulation of the skeletal muscle blood flow in humans Stefan P. Mortensen1 and Bengt Saltin2 † 1 2
Department of Cardiovascular and Renal Research, University of Southern Denmark, Odense, Denmark Copenhagen Muscle Research Centre, Rigshospitalet, Copenhagen, Denmark
Experimental Physiology
New Findings r What is the topic of this review? This review highlights recent advances in our knowledge about the control of skeletal muscle blood flow during exercise in humans. r What advances does it highlight? In recent years, it has become evident that the control of skeletal muscle blood flow is an interaction between various vasodilator agents, including nitric oxide, prostaglandins and adenosine. Adenosine triphosphate could play multiple roles by inducing local vasodilatation, overriding local sympathetic vasoconstriction and stimulating the exercise pressor reflex.
In humans, skeletal muscle blood flow is regulated by an interaction between several locally formed vasodilators, including NO and prostaglandins. In plasma, ATP is a potent vasodilator that stimulates the formation of NO and prostaglandins and, very importantly, can offset local sympathetic vasoconstriction. Adenosine triphosphate is released into plasma from erythrocytes and endothelial cells, and the plasma concentration increases in both the feed artery and the vein draining the contracting skeletal muscle. Adenosine also stimulates the formation of NO and prostaglandins, but the plasma adenosine concentration does not increase during exercise. In the skeletal muscle interstitium, there is a marked increase in the concentration of ATP and adenosine, and this increase is tightly coupled to the increase in blood flow. The sources of interstitial ATP and adenosine are thought to be skeletal muscle cells and endothelial cells. In the interstitium, both ATP and adenosine stimulate the formation of NO and prostaglandins, but ATP has also been suggested to induce vasoconstriction and stimulate afferent nerves that signal to increase sympathetic nerve activity. Adenosine has been shown to contribute to exercise hyperaemia, whereas the role of ATP remains uncertain due to lack of specific purinergic receptor blockers for human use. The purpose of this review is to address the interaction between vasodilator systems and to discuss the multiple proposed roles of ATP in human skeletal muscle blood flow regulation. (Received 27 June 2014; accepted after revision 1 September 2014; first published online 5 September 2014) Corresponding author S. P. Mortensen: Department of Cardiovascular and Renal Research, Institute of Molecular Medicine, University of Southern Denmark, J. B. Winsløws Vej 21-3, 5000 Odense C, Denmark. Email: [email protected]
Introduction During exercise, skeletal muscle perfusion increases in direct proportion to the metabolic demand. When the †
In memory of Bengt Saltin, who passed away on 12 September 2014.
DOI: 10.1113/expphysiol.2014.081620
active muscle mass is small and the heart is therefore not a limitation, skeletal muscle blood flow can increase 100-fold from resting conditions and reach perfusion rates of 300–400 ml min−1 kg−1 (Andersen & Saltin, 1985; Richardson et al. 1993). Studies manipulating arterial O2 content in humans have provided clear evidence that it is C 2014 The Authors. Experimental Physiology C 2014 The Physiological Society
Exp Physiol 99.12 (2014) pp 1552–1558
Regulation of the skeletal muscle blood flow in humans
not blood flow per se that is regulated but rather the O2 delivery matching the metabolic demand. In recent years, it has become apparent that several vasoactive compounds interact and function in synergy to contribute to the regulation of skeletal muscle blood flow (Hellsten et al. 2012). Apart from substances that can induce local smooth muscle cell relaxation, it has been demonstrated that some substances have the ability to inhibit the local vasoconstrictor effect of the increased sympathetic nerve activity during exercise. This phenomenon, termed functional sympatholysis, is known to occur during exercise (Remensnyder et al. 1962; Rosenmeier et al. 2004), but its importance for the blood flow regulation and the substance(s) involved in the regulation is debated (Hansen et al. 2000; Saltin & Mortensen, 2012). This review highlights the formation, interaction and effect of vasodilator substances that are likely candidates to be important for blood flow regulation in human skeletal muscle. Due to constraints on space, the review is mainly focused on experimental work undertaken in human subjects and not work from animal studies; for this, we refer the reader to comprehensive reviews published previously (Sarelius & Pohl, 2010; Marshall & Ray, 2012). Nitric oxide, prostaglandins and endothelium-derived hyperpolarization factors (EDHFs)
Nitric oxide is formed by conversion of L-arginine by nitric oxide synthase (NOS). In human skeletal muscle, mainly two isoforms (eNOS and nNOS) are localized in the vascular endothelium and in the skeletal muscle cells, respectively (Frandsen et al. 1996; Grozdanovic et al. 1996). Erythrocytes are also a potential source of NO by formation of S-nitrosohaemoglobin (Stamler et al. 1997). Nitric oxide plays an important role for the regulation of blood flow at rest and during the recovery from exercise, but when NOS is inhibited during leg exercise, there is no change in exercise hyperaemia (R˚adegran & Saltin, 1999; Bradley et al. 1999; Frandsen et al. 2001), suggesting that NO is not obligatory for exercise hyperaemia in humans. The enzyme cyclo-oxygenase (COX) catalyses the conversion of arachidonic acid to prostaglandin H2 , from which the vasodilators prostaglandin E2 and prostacyclin (PGI2 ) are derived. The potential sources of prostaglandins are endothelial and skeletal muscle cells (Vandenburgh et al. 1995; Davidge, 2001). The interstitial concentration of PGI2 (Frandsen et al. 2000) increases during exercise, and both vasodilators are also increased in the venous efflux of contracting muscle (Wilson & Kapoor, 1993). Inhibition of COX does not reduce exercise hyperaemia (Mortensen et al. 2007; Schrage et al. 2004). Nevertheless, when COX is inhibited simultaneously with NOS, blood flow is reduced by 30% in the exercising leg (Mortensen et al. 2007, 2009b) and forearm (Schrage et al. 2004; C 2014 The Authors. Experimental Physiology C 2014 The Physiological Society
1553
Saunders et al. 2005). These observations indicate that there is a compensatory formation of other vasodilators when one is inhibited, so that an adequate blood flow is achieved. It is important to note that these observations were performed at low and moderate workloads and that the reduction in exercise hyperaemia appears to be lower at higher exercise intensities (Christensen et al. 2013). Whether this reflects a lower contribution of NO and COX systems at higher exercise intensities or a lower effect of the blockers when blood flow is high (Schrage et al. 2010) remains unclear. The concept of EDHFs is based on the observation that specific substances can induce hyperpolarization of smooth muscle cells in the presence of COX and NOS inhibitors. Cytochrome P450 2C9 (CYP2C9) has been suggested to be an EDHF in skeletal muscle, but blockade of CYP2C9 with sulfaphenazole does not reduce exercise hyperaemia (Hillig et al. 2003). Yet, combined inhibition of NOS and CYP2C9 lowers exercise hyperaemia by 15%, suggesting an interaction between these two vasodilator systems, similar to the interaction observed between NOS and COX. Nevertheless, when EDHFs are blocked with the non-specific potassium channel blocker TEA in combination with inhibition of NOS and COX, exercise hyperaemia is not reduced beyond that of combined NOS and COX inhibition (Mortensen et al. 2007). This could indicate that TEA may not be sufficiently specific to examine an EDHF effect or that EDHF-induced vasodilatation cannot compensate for the impaired NO and prostaglandin systems. Adenosine triphosphate
Adenosine triphosphate is thought to play an important role in blood flow regulation by inducing local vasodilatation (Gonz´alez-Alonso, 2012), but direct evidence for a role of ATP in blood flow regulation is lacking because of the absence of a selective receptor antagonist for human use. Levels of ATP increase in both the feed artery and the vein draining contracting muscle (Mortensen et al. 2011). Given that the estimated half-life of ATP is
