Cardiovascular autonomic neuropathy and left ventricular dysfunction

Coronary artery disease and hypertension are associated with LV dysfunction (Ref. Reference Debono and Cachia43). Cardiovascular autonomic neuropathy, through alteration in myocardial blood flow and sympathetic denervation, and changes in myocardial neurotransmitters of the neuropeptidergic system (such as SP, CGRP, vasoactive intestinal peptide and NPY) can cause LV dysfunction (Ref. Reference Debono and Cachia43). Cardiovascular autonomic neuropathy correlates with LV diastolic dysfunction (Ref. Reference Uusitupa, Mustonen and Airaksinen44) in type 2 diabetes, and is associated with LV hypertrophy or higher LV mass index, and diastolic and systolic dysfunction in type 1 diabetes (Ref. Reference Taskiran45). Other studies in cultured rat myocytes (Ref. Reference Rapacciuolo46) and canines (Ref. Reference King47) have shown that hypertrophy or higher LV mass index might be due to sympathetic stimulation or prolonged infusion of norepinephrine.

Cardiovascular autonomic neuropathy and neuropeptides

There is increasing evidence that reduction or loss of neural activation is associated with cardiovascular autonomic neuropathy in diabetes (Ref. Reference Pop-Busui48). The level of pancreatic polypeptide and NPY is absent or low in type 1 diabetic patients with cardiovascular autonomic neuropathy than in individuals without it (Ref. Reference Bolinder8). It has been shown that the level of NPY in diabetic patients might be a useful marker of sympathetic nerve failure (Ref. Reference Sundkvist49). The expression of autonomic neuropeptides such as NPY and vasoactive intestinal peptide is significantly reduced in cardiovascular autonomic neuropathy patients with rheumatic diseases (Ref. Reference El-Sayed50). Moreover, acrylamide-induced autonomic neuropathy of rat mesenteric vessels shows the greatest reduction in intensity and number of CGRP and SP nerves (Ref. Reference Ralevic, Aberdeen and Burnstock51). Reduced expression of SP and CGRP is also observed in cardiovascular autonomic neuropathy patients (Ref. Reference Pittenger and Vinik23).

NPY and diabetes

NPY levels are decreased in patients with type 1 diabetes with cardiac autonomic neuropathy (Ref. Reference Bolinder8). NPY expression is also reduced in the heart, hippocampus and frontal cortex tissue of streptozotocin (STZ)-induced diabetic rats (Ref. Reference Kuncova70). Conversely, in the arcuate nucleus of rats, insulin and hypoglycaemia increase the transcription of the gene encoding NPY (Ref. Reference Watanabe71).

NPY, when acting through its receptor NPY1R, causes vasoconstriction. In alloxan-induced diabetic rabbits, the contractile effect of NPY is reduced in cerebral and coronary vessels (Ref. Reference Andersson72). Clinically, vascular smooth muscle contractile response to NPY is markedly attenuated in diabetic patients (Refs Reference Lind73, Reference Zhang, Han and Zhou74). Thus, in diabetes, it is not just the expression, but also the effect of exogenous NPY, that is diminished in the vasculature.

In contrast to the above studies, in a recent study, plasma NPY levels were increased in patients with type 2 diabetes (Ref. Reference Matyal75). As mentioned earlier, non-neuronal cells such as smooth muscle cells, endothelial cells, pancreatic acinar cells and vas deferens cells can produce NPY. In patients with chronic type 2 diabetes, a compensatory increase in these extraneuronal sources of NPY might explain the discrepancy.

NPY and the heart

NPY is located in the sympathetic nerve endings of the coronary vasculature and cardiac myocytes of humans and many other species. Functional NPY receptors in the heart are NPY1R, NPY2R and NPY5R (Ref. Reference Pedrazzini, Pralong and Grouzmann67). In rats, NPY is co-stored and co-released with norepinephrine and ATP in the postsympathetic nerve terminal and with epinephrine in the adrenal medulla (Ref. Reference Olcese76). However, at presynaptic nerve terminals, NPY inhibits the release of norepinephrine by a negative-feedback mechanism (Ref. Reference Olcese76). NPY, acting through NPY1R and NPY5R, has a pathogenic role by inducing cardiac hypertrophy and vasoconstriction of coronary vessels (Refs Reference Zukowska65, Reference Allen77). In rat models, NPY leads to vasoconstriction indirectly by potentiating the effect of norepinephrine-induced calcium signalling (Ref. Reference Wier78) and by a direct effect of NPY on increasing intracellular calcium ([Ca²⁺]_i) (Ref. Reference Prieto79). NPY induces angiogenesis through NPY2R and NPY5R, and thus has an imperative role in ischaemic revascularisation and wound healing (Refs Reference Meit Björndahl, Luxun and Yihai52, Reference Pradhan56). Therefore, in diabetic patients with reduced NPY expression or activity, there could be a diminished angiogenic response after ischaemia or after wounding, seriously affecting recovery.

Although plasma NPY levels have also been shown to be increased in diabetic patients, there is a decrease in neuronal NPY expression in the right atrium of diabetic patients undergoing coronary artery bypass surgery (Ref. Reference Matyal75). The same study showed that NPY2R and NPY5R expression is increased within the diabetic right atrium. This increase in receptor expression could be counter-regulatory to the reduced NPY expression in the myocardium. In a rat model of long-term type 1 diabetes, similar results are observed, with a reduced expression of NPY and an increased expression of NPY1R in the myocardium of diabetic rats (Ref. Reference Chottova Dvorakova80). Although NPY is a vasoconstrictor in most vascular beds, in the myocardium it decreases contractility. In isolated perfused rat myocardium, NPY decreases the positive chronotropic effect of phenylephrine and isoprotrerenol by decreasing the activity of sarcolemmal Na⁺/K⁺ ATPase (Ref. Reference Woo81). In a similar rat model, NPY decreases the positive inotropic effects induced by agonists without having a direct effect (Ref. Reference Woo and Ganguly59).

NPY also stimulates protein synthesis in adult rat cardiomyocytes by activation of pertussis toxin-sensitive G-proteins and phosphoinositol 3-kinase, and induces the fetal-type creatinine kinase-BB by activation of PKC and mitogen-activated protein kinase (Ref. Reference Goldberg82). Exposure of mouse neonatal cardiomyocytes to NPY induces rapid phosphorylation of the extracellular responsive kinase the Jun N-terminal kinase and the p38 kinase, as well as activation of PKC (Ref. Reference Pellieux83).

In a swine model of chronic myocardial ischaemia, exogenous addition of NPY increases mean arterial pressure and improves LV function without changing blood flow (Ref. Reference Robich84). However, there is an increase in capillary and arteriole formation, along with upregulation of NPY1R, NPY2R, NPY5R, vascular endothelial growth factor (VEGF), endothelial nitric oxide synthase (eNOS), phospho-eNOS (p-eNOS) on Ser1177 and platelet-derived growth factor (PDGF), and downregulation of antiangiogenic factors endostatin and angiostatin in the NPY-treated group (Ref. Reference Robich84). This large animal study emphasises the role of NPY in ischaemic angiogenesis wherein all the important angiogenic (pro- and anti-) factors are affected by NPY. However, the study does not address the connection or lack thereof between increased ischaemic myocardial angiogenesis, with unchanged blood flow achieved by NPY treatment. In addition to its effects on angiogenesis and myocardial function, NPY also activates diuretic and natriuretic properties in vivo (Refs Reference Meit Björndahl, Luxun and Yihai52, Reference Waeber85, Reference Winaver and Abassi86) and promotes the clearance of water and electrolytes by increasing the release of ANP from isolated rat left atrium (Ref. Reference Piao87). NPY has a prominent role in the kidneys, as demonstrated in an isolated rat kidney study, where it induces renal vasoconstriction and inhibits renin release by inhibiting adenylate cyclase activity in vascular smooth muscle and renin-producing cells (Ref. Reference Hackenthal61). In a rat myocardial infarction model of moderate compensated congestive heart failure, continuous infusion with NPY led to a decrease in renin levels (Ref. Reference Zelis88).

Thus, NPY and its receptors have a diverse role in the cardiovascular system by affecting almost all components of this system; therefore, any perturbations in its expression or signalling can have serious effects. There does not seem to be a single common pathway that it evokes. However, its role as a proangiogenic molecule seems to be the most consistent of its cardiovascular effects, with serious implications in diabetes-associated heart failure and the reparative response to a myocardial infarction. Its role in cardiac hypertrophy and vasoconstriction, although not clearly understood, can serve as a contributory factor towards development of diabetic heart failure.

SP and diabetes

SP expression is decreased in the CNS and PNS of patients with type 1 and type 2 diabetes (Ref. Reference Kunt9). In the nonobese type 1 diabetic mouse model, administration of SP impedes chronic, progressive β-cell stress, reduces islet cell inflammation, reduces insulin resistance and restores normoglycaemia (Ref. Reference Tsui95). Similarly, skin biopsies from both type 1 and type 2 diabetes patients show a reduced density of SP nerve fibres (Ref. Reference Lindberger96). In type 2 diabetic patients, decreased expression of neuronal NOS and SP is accompanied by a deficiency of gastric interstitial cells of Cajal associated with diabetic gastroparesis (Ref. Reference Iwasaki97). NK1R is the predominant receptor through which SP is known to exert its effect, and signalling through this receptor is affected in most diseases whenever SP is implicated. However, there are a few studies suggesting a change in NK1R expression itself. In one study, a decrease in NK1R expression was observed in the dorsal root ganglia of diabetic rats (Ref. Reference Boer98). These studies suggest that, unlike NPY whose expression could be up- or downregulated in diabetes, SP expression is always downregulated. This downregulation is the effect of diabetes and hyperglycaemia that might lead to sensory denervation and neuropathy.

SP and the heart

Although not an autonomic neuropeptide, SP is significantly associated with cardiovascular regulation (Ref. Reference Paulus99). Most of its effects counteract the effect of autonomic effectors such as norepinephrine and epinephrine. In addition to enteroendocrine cells, other major sources of circulating SP are perivascular nerves. SP release through perivascular nerves contributes to the regulation of vascular tone and acts as a vasodilator, in addition to reducing the effect of vasoconstrictive factors in patients with congestive heart failure (Ref. Reference Tingberg100). Our lab has recently shown that SP increases human dermal microvascular endothelial cell proliferation and tube formation, but the exact mechanism is not clear (Ref. Reference Jain57). Also, SP expression is reduced in cutaneous wounds of diabetic rabbits (Ref. Reference Pradhan101). Several reports have shown that SP enhances the release of NO, which preserves the expression of sarcoplasmic reticular Ca²⁺ ATPase by opposing norepinephrine. This effect of SP protects myocardial calcium homeostasis and heart function (Refs Reference Paulus99, Reference Calderone102). A recent canine study shows that atrial fibrillation leads to ventricular remodelling with a decrease in SP-positive nerves (Ref. Reference Yu103). Also, blockade of NK1R deteriorates postischaemic recovery of heart in a mouse model (Ref. Reference Wang and Wang104). Furthermore, increasing expression of SP protects the heart from injury by enhancing NK1R expression in guinea pigs and rats (Ref. Reference Wharton105). Another recent rat study shows that loss of cardioprotection by ischaemic postconditioning during diabetes is partly associated with failure to increase the release of CGRP and SP in diabetic hearts. This study also shows that SP and CGRP induce ischaemic postconditioning in both nondiabetic and diabetic hearts, preventing ischaemia–reperfusion injury by improving cardiac function as a consequence of lower levels of creatinine kinase and cardiac troponin I (Ref. Reference Ren106). Treatment with SP reverses LV hypertrophy in patients with essential hypertension (Ref. Reference Yan, Gao and Deng107). Plasma levels of SP are increased in heart failure patients receiving ACE inhibitors compared with those given a placebo, suggesting that ACE breaks down SP (Ref. Reference Valdemarsson108). Thus, ACE inhibitors, in addition to their effect on blocking the expression of angiotensin II, can increase the expression of SP and therefore add to their therapeutic effects. In contrast to the above studies, which show cardioprotective effects of SP, there are fewer studies implicating SP in the pathophysiology of cardiomyopathy and heart failure. Whether the increase in SP is a cause or effect of the underlying pathology is not clear from these studies. In a randomised, double-blind, placebo-controlled study using the NK1R antagonist aprepitant, SP causes a tonic enhancement of sympathetic outflow to some cardiovascular structures by modulating NK1R (Ref. Reference Dzurik109).

In another study of acute myocardial infarction, within 15 min of coronary artery occlusion, there was an increase in SP in the rat myocardium (Ref. Reference Wang110). Moreover, this study also showed that SP regulates cardiac function through NK1R by modulating adrenergic actions through the protein kinase A pathway (Ref. Reference Wang110). In a rodent model, SP induced parasitic dilated cardiomyopathy (Ref. Reference D'Souza111) and, similarly, SP was increased in patients with moderate and severe congestive heart failure (Ref. Reference Edvinsson112).

Because of its vasodilatory and angiogenic effects, SP seems to have a cardioprotective role that can be compromised in diabetes where SP expression is diminished. In cardiovascular diseases where SP is implicated, its increased expression might be a counter-regulatory effect to negate the effect of sympathetic effectors. In diabetes, where such compensation by SP cannot be achieved as a result of the lack of SP-positive nerves, therapies should be designed for supplementation with SP. Therefore, to determine the ‘cause’ or ‘consequence’ role of SP, additional studies are warranted to perform a more in-depth analysis of this molecule in animal models of diabetes-associated cardiac dysfunction.

CGRP and diabetes

CGRP shows about 50% homology with amylin, colocalises with insulin (Ref. Reference Adeghate117) and somatostatin (Ref. Reference Saffir118) in the islets of Langerhans, and might be involved in the synthesis or release of insulin from secretory granules (Ref. Reference Adeghate117). It is a circulating hormone that modulates peripheral insulin sensitivity (Ref. Reference Parlapiano119). Studies have also shown that CGRP has trophic effects on the pancreas, myotubules, motor neurons and peripheral nerves (Ref. Reference Ponery10). In rodent models, CGRP ameliorates β-cell destruction and reduces the occurrence of type 1 diabetes (Ref. Reference Sun120). The number of CGRP-immunoreactive cells was significantly decreased in a rat model of type 1 diabetes (Ref. Reference Wharton105). CGRP levels are also lowered in the serum of STZ-induced hyperglycaemic rodents (Ref. Reference Boer98). Impaired CGRP expression has also been shown in the hearts of diabetic STZ mice (Ref. Reference Morrison, Dhanasekaran and Howarth121).

CGRP and the heart

CGRP is found in the human heart, where it exerts complex cardiovascular effects including a protective effect against heart failure by curtailing the workload of the heart (Ref. Reference Creager122). CGRP supports the heart by inducing potent vasorelaxation by the release of NO and cyclic guanosine monophosphate (cGMP) (Ref. Reference Parlapiano119), which protects myocytes and endothelial cells from damage (Ref. Reference Clementi123), and modulating the immune system to reduce inflammation (Ref. Reference Wu124). In rats, the cardiovascular protection of nitroglycerin is partly modulated by the release of endogenous CGRP (Ref. Reference Li125). In a recent rat cardiac ischaemia-reperfusion study, pretreatment with CGRP improved cardiac contractility and overall outcome (Ref. Reference Liu126). Similarly, as mentioned earlier in a rat study, SP- and CGRP-induced postconditioning significantly prevented both nondiabetic and diabetic hearts from ischaemia–reperfusion injury. Ischaemic postconditioning markedly increased CGRP and SP release in nondiabetic hearts, but failed to affect CGRP and SP release in diabetic hearts, indicating that the loss of cardioprotection by ischaemic postconditioning during diabetes is partly associated with failure to release CGRP and SP (Refs Reference Ren106, Reference Covino and Spadaccio127). Clinical studies show that, in addition to cardiac effects, CGRP increases angiogenesis by increasing VEGF-A, basic FGF (bFGF) and transforming growth factor-β (TGF-β) in a hind-limb ischaemia model (Ref. Reference Mishima128). Thus, CGRP not only improves cardiac function, but can also facilitate recovery postischaemia by increasing capillary formation and cardiac blood flow.

These studies show that in the heart and vasculature, CGRP is similar to SP in expression and activity and can affect the cardiovascular system in a positive manner. Therefore, in diabetes, where CGRP is found to be decreased, the effects on the cardiovascular system and, more specifically, recovery post-ischaemia can be significantly hampered. There are fewer studies of CGRP in diabetes models compared with studies on SP; however, similarities in the effects of CGRP and SP suggests that CGRP supplementation could prove to be beneficial in the treatment of cardiovascular dysfunction of diabetes.

NPs and diabetes

NPs have antioxidant activity that inhibits oxidative stress and ameliorates the endothelial dysfunction of diabetes in a rat model (Ref. Reference Woodman137). In several studies, dysfunction of natriuretic peptide receptors has been observed in STZ diabetic rats (Refs Reference McKenna11, Reference Chattington138, Reference Sechi139, Reference Walther, Schuitheiss and Tschope140). Impairment of NPRA uncoupling is involved in diabetic heart failure (Ref. Reference Ganguly141), and the expression of NPRA receptors is reduced in diabetic rat kidneys (Ref. Reference Sechi139). BNP has been shown to be a reliable predictor of patients with type 2 diabetes and is a potential marker of diastolic heart failure (Ref. Reference Bhalla142). Heart failure patients with diabetes have elevated BNP levels compared with patients without diabetes (Ref. Reference van der Horst143). In a recent study, the gene encoding NPRC was found to have different single-nucleotide polymorphisms in diabetic patients, one of which significantly correlated with increased systolic blood pressure in this population (Ref. Reference Saulnier144).

These studies indicate that not only NPs, but also their receptors, are significantly affected in diabetes. Although related, ANP and BNP have very distinct profiles and yet both are good predictive tools in diabetic heart failure patients.

NPs and the heart

NPs have a vital role in congestive heart failure (Ref. Reference D'Souza111). Natriuretic peptide receptors (NPRs) are located on the endocardial endothelium and might be involved in regulating myocardial contractility (Ref. Reference Kim, Cho and Kim58). It has also been shown that NPs reduce myocardial load and enhance systolic and diastolic function in normal hearts and in failing hearts of canines (Ref. Reference Lainchbury145). ANP exerts cardioprotective functions, not only as a circulating hormone, but also as a local autocrine or paracrine factor as shown in an in vitro study of endothelial and vascular smooth muscle cells (Ref. Reference Morishita146). In vitro models demonstrate that it has antihypertrophic and antifibrotic functions and that it inhibits apoptosis and proliferation (Ref. Reference Thomas, Head and Woods147). These results are corroborated in a mouse model that lacks NPRA and hence develops cardiac hypertrophy and fibrosis (Ref. Reference Oliver148). Moreover, ANP attenuates RAAS, sympathetic nerve activity and reperfusion injury, averts LV remodelling and improves LV function in patients with acute myocardial infarction (Ref. Reference Kasama149). In addition to inhibiting RAAS, ANP also inhibits NEP, the enzyme that breaks down SP (Ref. Reference Struthers135). Thus ANP can indirectly increase the levels of SP. Omapatrilat, a pharmacological dual inhibitor of NEP and ACE, is a promising agent for the treatment of hypertension, heart failure and renal failure, as demonstrated in a rat study (Ref. Reference Backlund150). This suggests that ANP, which is an endogenous dual inhibitor, might have the same beneficial effects. In canines, blockade of ANP accelerates the progression of heart failure and postmyocardial infarction (Refs Reference Stevens151, Reference Lohmeier152).

In patients, plasma BNP is an important prognostic marker for the outcome of acute congestive heart failure and is a biomarker for myocardial ischaemia (Refs Reference Barrios153, Reference Nadir154). The level of BNP is generally higher in patients with worse outcomes (Ref. Reference Atisha155). Moreover, the plasma concentration of BNP is also typically increased in patients with asymptomatic or symptomatic LV dysfunction (Ref. Reference Atisha155). In contrast to these studies, short-term administration of subcutaneous BNP during the evolution of LV dysfunction in a canine model resulted in considerable improvement in cardiovascular haemodynamics (Ref. Reference Chen156).

Therefore, similarly to NPY, it is not clear whether low or high levels of ANP and BNP are desired for cardioprotection. In diabetes, there is decreased expression and activity of ANP, and thereby a loss of cardioprotection; therefore, supplementing exogenous ANP might be cardioprotective. In most studies of diabetes and cardiovascular disease, there is increased expression and activity of BNP. However, because of contradictory results, it is not clear whether inhibition or supplementation of BNP will have a positive effect on the cardiovascular system. Therefore, more in vivo studies need to be performed to analyse the role of BNP in diabetes and associated cardiovascular dysfunctions.