Gender-modulated endogenous baseline neuropeptide Y Y1-receptor activation in the hindlimb of Sprague-Dawley rats

Sympathetic vasoconstriction in skeletal muscle: Modulatory effects of aging, exercise training, and sex

The sympathetic nervous system (SNS) is a critically important regulator of the cardiovascular system. The SNS controls cardiac output and its distribution, as well as peripheral vascular resistance and blood pressure at rest and during exercise. Aging is associated with increased blood pressure and decreased skeletal muscle blood flow at rest and in response to exercise. The mechanisms responsible for the blunted skeletal muscle blood flow response to dynamic exercise with aging have not been fully elucidated; however, increased muscle sympathetic nerve activity (MSNA), elevated vascular resistance and a decline in endothelium-dependent vasodilation are commonly reported in older adults. In contrast to aging, exercise training has been shown to reduce blood pressure and enhance skeletal muscle vascular function. Exercise training has been shown to enhance nitric oxide-dependent vascular function and may improve the vasodilatory capacity of the skeletal muscle vasculature; however, surprisingly little is known about the effect of exercise training on the neural control of circulation. The control of blood pressure and skeletal muscle blood flow also differs between males and females. Blood pressure and MSNA appear to be lower in young females compared to males. However, females experience a larger increase in MSNA with aging compared to males. The mechanism(s) for the altered SNS control of vascular function in females remain to be determined. Novelty: • This review will summarize our current understanding of the effects of aging, exercise training and sex on sympathetic vasoconstriction at rest and during exercise. • Areas where additional research is needed are also identified.

Dipeptidyl Peptidase IV as a Muscle Myokine

Dipeptidyl peptidase IV (DPP-IV) is a unique serine protease that exists in a membrane bound state and in a soluble state in most tissues in the body. DPP-IV has multiple targets including cytokines, neuropeptides, and incretin hormones, and plays an important role in health and disease. Recent work suggests that skeletal muscle releases DPP-IV as a myokine and participates in control of muscle blood flow. However, few of the functions of DPP-IV as a myokine have been investigated to date and there is a poor understanding about what causes DPP-IV to be released from muscle.

The gender-specific expression of neuropeptide Y and neuropeptide Y receptors in human atrial tissue during cardiopulmonary bypass surgery

Background

Cardiac sympathetic nervous system is usually activated in cardiopulmonary bypass (CPB) surgery, accompanied by excessive release of norepinephrine (NE). Neuropeptide Y (NPY) has been shown to regulate NE release in the terminal of sympathetic fiber, which is a target for regulating heart function. The expression of NPY and NPY receptor (NPYR) genes in the human atrial tissues during CPB in cardiac surgery was investigated in the present study.

Methods

A few discarded atrial tissues before and after CPB were collected in 22 patients with rheumatic cardiac valve diseases. The transcriptional levels of NPY and NPYRs were monitored by real-time quantitative polymerase chain reaction (RT-qPCR) method. Moreover, the correlation between the mRNA levels of NPY/NPYRs and the clinical data were investigated in detail.

Results

The mRNA levels of NPY Y1 and NPY Y5 genes were statistically attenuated in male patients after CPB. Conversely, the expression of NPY, NPY Y1 and NPY Y5 genes were enhanced in female patients. Correlation analysis suggested that there was a significant negative correlation between cardiac ejection fraction (EF) after CPB with the atrial transcriptional level of NPY in male patients.

Conclusions

These results suggested that the expression of NPY/NPYRs in human atrial tissue during CPB was gender specific and activated NPY signaling was only identified in female patients. The elevated expression level of NPY in male patients was correlated with lower cardiac EF after CPB.

VPAC1 receptors play a dominant role in PACAP-induced vasorelaxation in female mice

Background

PACAP and VIP are closely related neuropeptides with wide distribution and potent effect in the vasculature. We previously reported vasomotor activity in peripheral vasculature of male wild type (WT) and PACAP-deficient (KO) mice. However, female vascular responses are still unexplored. We hypothesized that PACAP-like activity is maintained in female PACAP KO mice and the mechanism through which it is regulated differs from that of male PACAP KO animals.

Methods

We investigated the vasomotor effects of VIP and PACAP isoforms and their selective blockers in WT and PACAP KO female mice in carotid and femoral arteries. The expression and level of different PACAP receptors in the vessels were measured by RT-PCR and Western blot.

Results

In both carotid and femoral arteries of WT mice, PACAP1-38, PACAP1-27 or VIP induced relaxation, without pronounced differences between them. Reduced relaxation was recorded only in the carotid arteries of KO mice as compared to their WT controls. The specific VPAC1R antagonist completely blocked the PACAP/VIP-induced relaxation in both arteries of all mice, while PAC1R antagonist affected relaxation only in their femoral arteries.

Conclusion

In female WT mice, VPAC1 receptors appear to play a dominant role in PACAP-induced vasorelaxation both in carotid and in femoral arteries. In the PACAP KO group PAC1R activation exerts vasorelaxation in the femoral arteries but in carotid arteries there is no significant effect of the activation of this receptor. In the background of this regional difference, decreased PAC1R and increased VPAC1R availability in the carotid arteries was found.

Endogenous dipeptidyl peptidase IV modulates skeletal muscle arteriolar diameter in rats

The purpose of this study is to investigate that dipeptidyl peptidase IV (DPP‐IV) released from skeletal and vascular smooth muscle can increase arteriolar diameter in a skeletal muscle vascular bed by reducing neuropeptide Y (NPY)‐mediated vasoconstriction. We hypothesized that the effect of myokine DPP‐IV would be greatest in the smallest and least in the largest arterioles. Eight male Sprague Dawley rats (age 7–9 weeks; mass, mean ± SD: 258 ± 41 g) were anesthetized and the gluteus maximus dissected in situ for intravital microscopy analysis of arteriolar diameter of the vascular network. Computational modeling was performed on the diameter measurements to evaluate the overall impact of diameter changes on network resistance and flow distribution. In the first set of experiments, whey protein isolate powder was added to physiological saline solution, put in a heated reservoir, and applied to the preparation to induce release of DPP‐IV from the muscle. This resulted in an order‐dependent increase in arteriolar diameter, with the largest change in the 6A arterioles (63% more reactive than 1A arterioles; P < 0.05). This effect was abolished by adding the DPP‐IV inhibitor, Diprotin A. To test if the DPP‐IV released was affecting NPY‐mediated vasoconstriction, we applied NPY and whey protein, which resulted in attenuated vasoconstriction. These findings suggest that DPP‐IV is released from muscle and has a unique effect on blood flow, which appears to act on NPY to attenuate vasoconstriction. The findings suggest that DPP‐IV released from the skeletal or smooth muscle can alter muscle blood flow.

Neuropeptide Y1 and alpha-1 adrenergic receptor-mediated decreases in functional vasodilation in gluteus maximus microvascular networks of prediabetic mice

Prediabetes is associated with impaired contraction‐evoked dilation of skeletal muscle arterioles, which may be due to increased sympathetic activity accompanying this early stage of diabetes disease. Herein, we sought to determine whether blunted contraction‐evoked vasodilation resulted from enhanced sympathetic neuropeptide Y1 receptor (Y1R) and alpha‐1 adrenergic receptor (α1R) activation. Using intravital video microscopy, second‐, third‐, and fourth‐order (2A, 3A, and 4A) arteriolar diameters were measured before and following electrical field stimulation of the gluteus maximus muscle (GM) in prediabetic (PD, Pound Mouse) and control (CTRL, c57bl6, CTRL) mice. Baseline diameter was similar between groups; however, single tetanic contraction (100 Hz; 400 and 800 msec) and sustained rhythmic contraction (2 and 8 Hz, 30 sec) evoked rapid onset vasodilation and steady‐state vasodilatory responses that were blunted by 50% or greater in PD versus CTRL. Following Y1R and α1R blockade with sympathetic antagonists BIBP3226 and prazosin, contraction‐evoked arteriolar dilation in PD was restored to levels observed in CTRL. Furthermore, arteriolar vasoconstrictor responses to NPY (10⁻¹³–10⁻⁸ mol/L) and PE (10⁻⁹–10⁻⁵ mol/L) were greater in PD versus CTRL at higher concentrations, especially at 3A and 4A. These findings suggest that contraction‐evoked vasodilation in PD is blunted by Y1R and α1R receptor activation throughout skeletal muscle arteriolar networks.

Neuropeptide Y not involved in cutaneous vascular control in young human females taking oral contraceptive hormones

We previously reported that the cutaneous vasodilator response to local warming in males required noradrenaline (NA) and neuropeptide-Y (NPY). Animal work has shown no role for NPY in female vascular control. We investigated the contribution of NA and NPY in human female cutaneous vascular control. Nine female and nine male participants volunteered. To elucidate whether synthetic oestrogen and progesterone altered cutaneous vascular responses, females were tested in high-hormone (HH) and low-hormone (LH) phases of oral contraceptive pill (OCP). Skin blood flow was assessed by laser-Doppler flowmetry and expressed as cutaneous vascular conductance (CVC). Treatments were: control, combined yohimbine and propranolol (YP), BIBP-3226, and bretylium tosylate (BT). YP and BT increased basal CVC (p<0.05) relative to control sites in both HH and LH phases; though, BIBP-3226 had no effect in either phase (both p>0.05). Males basal CVC was increased at all treated sites compared to control sites (all p<0.05). YP and BT treated sites were higher in HH compared to LH (p<0.05). YP and BT treatment reduced the local warming-induced vasodilatation compared to control sites (p>0.05) in both HH and LH phases; whereas, BIBP-3226 treatment had no effect (p>0.05). In males, the vasodilatation achieved at all treated sites was reduced compared to the untreated control site (p<0.05). Data indicate that NA, not NPY, regulates basal skin blood flow and contributes to the vasodilator response to local warming in young females; however, both NA and NPY play a role in both basal and heat-induced cutaneous responses in males.

Exercise training and α1-adrenoreceptor-mediated sympathetic vasoconstriction in resting and contracting skeletal muscle

Exercise training (ET) increases sympathetic vasoconstrictor responsiveness and enhances contraction‐mediated inhibition of sympathetic vasoconstriction (i.e., sympatholysis) through a nitric oxide (NO)‐dependent mechanism. Changes in α₂‐adrenoreceptor vasoconstriction mediate a portion of these training adaptations, however the contribution of other postsynaptic receptors remains to be determined. Therefore, the purpose of this study was to investigate the effect of ET on α₁‐adrenoreceptor‐mediated vasoconstriction in resting and contracting muscle. It was hypothesized that α₁‐adrenoreceptor‐mediated sympatholysis would be enhanced following ET. Male Sprague Dawley rats were randomized to sedentary (S; n = 12) or heavy‐intensity treadmill ET (n = 11) groups. Subsequently, rats were anesthetized and instrumented for lumbar sympathetic chain stimulation and measurement of femoral vascular conductance (FVC) at rest and during muscle contraction. The percentage change in FVC in response to sympathetic stimulation was measured in control, α₁‐adrenoreceptor blockade (Prazosin; 20 μg, IV), and combined α₁ and NO synthase (NOS) blockade (l‐NAME; 5 mg·kg⁻¹IV) conditions. Sympathetic vasoconstrictor responsiveness was increased (P < 0.05) in ET compared to S rats at low, but not high (P > 0.05) stimulation frequencies at rest (S: 2 Hz: −25 ± 4%; 5 Hz: −45 ± 5 %; ET: 2 Hz: −35 ± 7%, 5 Hz: −52 ± 7%), whereas sympathetic vasoconstrictor responsiveness was not different (P > 0.05) between groups during contraction (S: 2 Hz: −11 ± 8%; 5 Hz: −26 ± 11%; ET: 2 Hz: −10 ± 7%, 5 Hz: −27 ± 12%). Prazosin blunted (P < 0.05) vasoconstrictor responsiveness in S and ET rats at rest and during contraction, and abolished group differences in vasoconstrictor responsiveness. Subsequent NOS blockade increased vasoconstrictor responses (P < 0.05) in S at rest and during contraction, whereas in ET vasoconstriction was increased (P < 0.05) in response to sympathetic stimulation at 2 Hz at rest and unchanged (P > 0.05) during contraction. ET enhanced (P < 0.05) sympatholysis, however the training‐mediated improvements in sympatholysis were abolished by α₁‐adrenoreceptor blockade. Subsequent NOS inhibition did not alter (P > 0.05) sympatholysis in S or ET rats. In conclusion, ET augmented α₁‐adrenoreceptor‐mediated vasoconstriction in resting skeletal muscle and enhanced α₁‐adrenoreceptor‐mediated sympatholysis. Furthermore, these data suggest that NO is not required to inhibit α₂‐adrenoreceptor‐ and nonadrenoreceptor‐mediated vasoconstriction during exercise.

Neural Control of Vascular Function in Skeletal Muscle

The sympathetic nervous system represents a fundamental homeostatic system that exerts considerable control over blood pressure and the distribution of blood flow. This process has been referred to as neurovascular control. Overall, the concept of neurovascular control includes the following elements: efferent postganglionic sympathetic nerve activity, neurotransmitter release, and the end organ response. Each of these elements reflects multiple levels of control that, in turn, affect complex patterns of change in vascular contractile state. Primarily, this review discusses several of these control layers that combine to produce the integrative physiology of reflex vascular control observed in skeletal muscle. Beginning with three reflexes that provide somewhat dissimilar vascular patterns of response despite similar changes in efferent sympathetic nerve activity, namely, the baroreflex, chemoreflex, and muscle metaboreflex, the article discusses the anatomical and physiological bases of postganglionic sympathetic discharge patterns and recruitment, neurotransmitter release and management, and details of regional variations of receptor density and responses within the microvascular bed. Challenges are addressed regarding the fundamentals of measurement and how conclusions from one response or vascular segment should not be used as an indication of neurovascular control as a generalized physiological dogma. Whereas the bulk of the article focuses on the vasoconstrictor function of sympathetic neurovascular integration, attention is also given to the issues of sympathetic vasodilation as well as the impact of chronic changes in sympathetic activation and innervation on vascular health. © 2016 American Physiological Society.

Potential Interactions between the Autonomic Nervous System and Higher Level Functions in Neurological and Neuropsychiatric Conditions

The autonomic nervous system (ANS) maintains the internal homeostasis by continuously interacting with other brain structures. Its failure is commonly observed in many neurological and neuropsychiatric disorders, including neurodegenerative and vascular brain diseases, spinal cord injury, and peripheral neuropathies. Despite the different underlying pathophysiological mechanisms, ANS failure associates with various forms of higher level dysfunctions, and may also negatively impact on patients’ clinical outcome. In this review, we will discuss potential relationships between ANS and higher level dysfunctions in a selection of neurological and neuropsychiatric disorders. In particular, we will focus on the effect of a documented fall in blood pressure fulfilling the criteria for orthostatic hypotension and/or autonomic-reflex impairment on cognitive performances. Some evidence supports the hypothesis that cardiovascular autonomic failure may play a negative prognostic role in most neurological disorders. Despite a clear causal relationship between ANS involvement and higher level dysfunctions that is still controversial, this might have implications for neuro-rehabilitation strategies aimed at improving patients’ clinical outcome.

Effect of dipeptidyl peptidase 4 inhibition on arterial blood pressure is context dependent

Because the effects of dipeptidyl peptidase 4 (DPP4) inhibitors on blood pressure are controversial, we examined the long-term effects of sitagliptin (80 mg/kg per day) on blood pressure (radiotelemetry) in spontaneously hypertensive rats (SHR), Wistar-Kyoto rats, and Zucker Diabetic-Sprague Dawley rats (metabolic syndrome model). In SHR, chronic (3 weeks) sitagliptin significantly increased systolic, mean, and diastolic blood pressures by 10.3, 9.2, and 7.9 mm Hg, respectively, a response abolished by coadministration of BIBP3226 (2 mg/kg per day; selective Y1-receptor antagonist). Sitagliptin also significantly increased blood pressure in SHR treated with hydralazine (vasodilator; 25 mg/kg per day) or enalapril (angiotensin-converting enzyme inhibitor; 10 mg/kg per day). In Wistar-Kyoto rats, chronic sitagliptin slightly decreased systolic, mean, and diastolic blood pressures (-1.8, -1.1, and -0.4 mm Hg, respectively). In Zucker Diabetic-Sprague Dawley rats, chronic sitagliptin decreased systolic, mean, and diastolic blood pressures by -7.7, -5.8, and -4.3 mm Hg, respectively, and did not alter the antihypertensive effects of chronic enalapril. Because DPP4 inhibitors impair the metabolism of neuropeptide Y1-36 (NPY1-36; Y1-receptor agonist) and glucagon-like peptide (GLP)-1(7-36)NH2 (GLP-1 receptor agonist), we examined renovascular responses to NPY1-36 and GLP-1(7-36)NH2 in isolated perfused SHR and Zucker Diabetic-Sprague Dawley kidneys pretreated with norepinephrine (to induce basal tone). In Zucker Diabetic-Sprague Dawley kidneys, NPY1-36 and GLP-1(7-36)NH2 exerted little, if any, effect on renovascular tone. In contrast, in SHR kidneys, both NPY1-36 and GLP-1(7-36)NH2 elicited potent and efficacious vasoconstriction.

IN CONCLUSION

(1) The effects of DPP4 inhibitors on blood pressure are context dependent; (2) The context-dependent effects of DPP4 inhibitors are due in part to differential renovascular responses to DPP4’s most important substrates (NPY1–36 and GLP-1(7–36)NH2) [corrected]; (3) Y1 receptor antagonists may prevent the prohypertensive and possibly augment the antihypertensive effects of DPP4 inhibitors.

Noradrenaline and neuropeptide Y contribute to initial, but not sustained, vasodilatation in response to local skin warming in humans

NEW FINDINGS

What is the central question of this study? Previous work has produced the counterintuitive finding that the vasoconstrictor neurotransmitters noradrenaline and neuropeptide Y are involved in vasodilatation. We aimed to discover whether sympathetic neurotransmitters are required for the sustained vasodilatation in response to local skin warming, as has been previously suggested, and to determine whether noradrenaline and neuropeptide Y are 'mediating' the sustained vasodilator response directly or acting to 'prime' (or kick-start) it. What is the main finding and its importance? We have found that noradrenaline and neuropeptide Y are required at the initiation of vasodilatation in response to local skin warming, if a complete vasodilator response is to be achieved; however, they are not required once vasodilatation has begun. In a three-part study, we examined whether noradrenaline, neuropeptide Y (NPY) and endothelial nitric oxide synthase (eNOS) were involved in the sustained vasodilatation in response to local skin warming. Forearm skin sites were instrumented with intradermal microdialysis fibres, local skin heaters and laser-Doppler flow probes. Local skin temperature (T(loc)) was increased from 34 to 42°C at a rate of 0.5°C (10 s)(-1). Laser-Doppler flow was expressed as cutaneous vascular conductance (CVC; laser-Doppler flow/mean arterial pressure). In part 1, three skin sites were prepared; two were treated with the study vehicle (lactated Ringer solution), while the third site was treated with yohimbine and propranolol to antagonize α- and β-receptors, and 10 min of baseline data were record at a T(loc) of 34°C. Receptor antagonism was confirmed via infusion of clonidine. The T(loc) was increased to 42°C at all sites. Once CVC had stabilized, site 2 was treated with yohimbine and propranolol to examine the effect of adrenergic receptor blockade on sustained vasodilatation of the skin. Receptor antagonism was again confirmed via infusion of clonidine. All sites were treated with sodium nitroprusside, and T(loc) was increased to 43°C to elicit maximal vasodilatation. In parts 2 and 3, the general protocol was the same, except that BIBP-3226 was used to antagonize Y(1)-receptors, NPY to test the efficacy of the antagonism, N(G)-amino-l-arginine to inhibit eNOS and ACh to test the adequacy of inhibition. Compared with control conditions, antagonism of α- and β-receptors, Y(1)-receptors and eNOS before local skin warming reduced the initial and sustained vasodilatation in response to increased T(loc). However, treatment with yohimbine and propranolol or BIBP-3226 after local skin warming did not affect the sustained vasodilatation [CVC, 90 ± 3 versus 89 ± 3%max (control vs. yohimbine and propranolol) and 88 ± 5 versus 87 ± 4%max (control vs. BIBP-3226); P > 0.05]. N(G)-Amino-l-arginine perfusion caused a large reduction in CVC during this phase (89 ± 5 versus 35 ± 4%max; P < 0.05). These data indicate that if their actions are antagonized after local warming and cutaneous vasodilatation has occurred, noradrenaline and NPY play little, if any, role in the sustained vasodilatation in response to local skin warming. However, eNOS contributes markedly to the sustained vasodilatation regardless of when it is inhibited.

Perivascular innervation: a multiplicity of roles in vasomotor control and myoendothelial signaling

The control of vascular resistance and tissue perfusion reflect coordinated changes in the diameter of feed arteries and the arteriolar networks they supply. Against a background of myogenic tone and metabolic demand, vasoactive signals originating from perivascular sympathetic and sensory nerves are integrated with endothelium-derived signals to produce vasodilation or vasoconstriction. PVNs release adrenergic, cholinergic, peptidergic, purinergic, and nitrergic neurotransmitters that lead to SMC contraction or relaxation via their actions on SMCs, ECs, or other PVNs. ECs release autacoids that can have opposing actions on SMCs. Respective cell layers are connected directly to each other through GJs at discrete sites via MEJs projecting through holes in the IEL. Whereas studies of intercellular communication in the vascular wall have centered on endothelium-derived signals that govern SMC relaxation, attention has increasingly focused on signaling from SMCs to ECs. Thus, via MEJs, neurotransmission from PVNs can evoke distinct responses from ECs subsequent to acting on SMCs. To integrate this emerging area of investigation in light of vasomotor control, the present review synthesizes current understanding of signaling events that originate within SMCs in response to perivascular neurotransmission in light of EC feedback. Although often ignored in studies of the resistance vasculature, PVNs are integral to blood flow control and can provide a physiological stimulus for myoendothelial communication. Greater understanding of these underlying signaling events and how they may be affected by aging and disease will provide new approaches for selective therapeutic interventions.

Contributions of endothelial nitric oxide synthase, noradrenaline, and neuropeptide Y to local warming-induced cutaneous vasodilatation in men

We performed a two-part study to determine the roles of endothelial nitric oxide synthase (eNOS) and the vasoconstrictor nerves neurotransmitters noradrenaline (NA) and neuropeptide Y (NPY) in the cutaneous vasodilator response to local skin warming. Forearm skin sites were instrumented with intradermal microdialysis fibres, local heaters, and laser-Doppler flow (LDF) probes. Sites were locally heated from 34 to 42°C. LDF was expressed as cutaneous vascular conductance (CVC; LDF/mean arterial pressure). In Part I, we tested whether sympathetic noradrenergic nerves acted via eNOS. In 8 male participants, treatments were as follows: 1) untreated; 2) bretylium tosylate (BT), preventing sympathetic neurotransmitter release; 3) l-NAA to inhibit eNOS; and 4) combined BT+l-NAA. At treated sites, the initial peak response was markedly reduced, and the plateau phase response to 35min of local warming was also reduced (P<0.05), which was not different among those sites (P>0.05). In Part II, we tested whether NA and NPY were involved in the vasodilator response to local warming. In Part IIa, treatments were: 1) untreated; 2) propranolol and yohimbine to antagonize α- and β-receptors; 3) l-NAA; and 4) combined propranolol, yohimbine, and l-NAA. In Part IIb, conditions were: 1) untreated; 2) BIBP to antagonize Y1-receptors; 3) l-NAA; and 4) combined BIBP and l-NAA. All treatments caused a reduction in the initial peak and plateau responses to local skin warming (P<0.05). The results of Part II indicate that both NA and NPY play roles in the cutaneous vasodilator response and their actions are achieved via eNOS. These data indicate that NA and NPY are involved in the initial, rapid rise in skin blood flow at the onset of local skin warming. However, their vasodilator actions in response to local skin warming appears to be manifested through eNOS.

Pre-diabetes augments neuropeptide Y1- and α1-receptor control of basal hindlimb vascular tone in young ZDF rats

BACKGROUND

Peripheral vascular disease in pre-diabetes may involve altered sympathetically-mediated vascular control. Thus, we investigated if pre-diabetes modifies baseline sympathetic Y(1)-receptor (Y(1)R) and α(1)-receptor (α(1)R) control of hindlimb blood flow (Q(fem)) and vascular conductance (VC).

METHODS

Q(fem) and VC were measured in pre-diabetic ZDF rats (PD) and lean controls (CTRL) under infusion of BIBP3226 (Y(1)R antagonist), prazosin (α(1)R antagonist) and BIBP3226+prazosin. Neuropeptide Y (NPY) concentration and Y(1)R and α(1)R expression were determined from hindlimb skeletal muscle samples.

RESULTS

Baseline Q(fem) and VC were similar between groups. Independent infusions of BIBP3226 and prazosin led to increases in Q(fem) and VC in CTRL and PD, where responses were greater in PD (p<0.05). The percent change in VC following both drugs was also greater in PD compared to CTRL (p<0.05). As well, Q(fem) and VC responses to combined blockade (BIBP3226+prazosin) were greater in PD compared to CTRL (p<0.05). Interestingly, an absence of synergistic effects was observed within groups, as the sum of the VC responses to independent drug infusions was similar to responses following combined blockade. Finally, white and red vastus skeletal muscle NPY concentration, Y(1)R expression and α(1)R expression were greater in PD compared to CTRL.

CONCLUSIONS

For the first time, we report heightened baseline Y(1)R and α(1)R sympathetic control of Q(fem) and VC in pre-diabetic ZDF rats. In support, our data suggest that augmented sympathetic ligand and receptor expression in pre-diabetes may contribute to vascular dysregulation.

Influence of estradiol supplementation on neuropeptide Y neurotransmission in skeletal muscle arterioles of F344 rats

The effects of estradiol on neuropeptide Y (NPY) neurotransmission in skeletal muscle resistance vessels have not been described. The purpose of this study was to determine the effects of long-term estradiol supplementation on NPY overflow, degradation, and vasoconstriction in gastrocnemius first-order arterioles of adult female rats. Female rats (4 mo; n = 34) were ovariectomized (OVX) with a subset (n = 17) receiving an estradiol pellet (OVE; 17β-estradiol, 4 μg/day). After conclusion of the treatment phase (8 wk), arterioles were excised, placed in a physiological saline solution (PSS) bath, and cannulated with micropipettes connected to albumin reservoirs. NPY-mediated vasoconstriction via a Y(1)-agonist [Leu31Pro34]NPY decreased vessel diameter 44.54 ± 3.95% compared with baseline; however, there were no group differences in EC(50) (OVE: -8.75 ± 0.18; OVX: -8.63 ± 0.10 log M [Leu31Pro34]NPY) or slope (OVE: -1.11 ± 0.25; OVX: -1.65 ± 0.34% baseline/log M [Leu31Pro34]NPY). NPY did not potentiate norepinephrine-mediated vasoconstriction. NPY overflow experienced a slight increase following field stimulation and significantly increased (P < 0.05) over control conditions in the presence of a DPPIV inhibitor (diprotin A). Estradiol status did not affect DPPIV activity. These data suggest that NPY can induce a moderate decrease in vessel diameter in skeletal muscle first-order arterioles, and DPPIV is active in mitigating NPY overflow in young adult female rats. Long-term estradiol supplementation did not influence NPY vasoconstriction, overflow, or its enzymatic breakdown in skeletal muscle first-order arterioles.

Neuropeptide Y overflow and metabolism in skeletal muscle arterioles

The purpose of this study was to characterize neuropeptide Y (NPY) overflow and metabolism from isolated skeletal muscle arterioles of female rats. Gastrocnemius first-order arterioles were removed from young (2 months), young adult (6 months) and middle-aged (12 months) F344 female rats. Arterioles were isolated, cannulated and pressurized in a microvessel bath with field stimulation electrodes. NPY overflow from isolated arterioles was assessed at 0 s and 30 s post-field stimulation. Dipeptidyl peptidase IV (DPPIV) activity was quantified via fluorometric assay of whole vessel homogenate. In young adult and middle-aged rats, NPY overflow increased 0 s and 30 s following field stimulation. In young adult rats, DPPIV inhibition resulted in an increase in NPY overflow at 30 s, while middle-aged rats had no increase in NPY overflow with DPPIV inhibition (P <0.05). DPPIV activity was influenced by factors such as age, vessel type, and endothelium (P <0.05). The present data suggest that DPPIV plays a significant role in modulating the actions of NPY in arterioles of young adult females; however, this role appears to diminish with age.

Blunting of rapid onset vasodilatation and blood flow restriction in arterioles of exercising skeletal muscle with ageing in male mice

Exercise capacity and skeletal muscle blood flow are diminished with ageing but little is known of underlying changes in microvascular haemodynamics. Further, it is not clear how the sympathetic nervous system affects the microcirculation of skeletal muscle with ageing or whether sex differences prevail in the regulation of arteriolar diameter in response to muscle contractions. In the gluteus maximus muscle of C57BL/6 mice, we tested the hypothesis that ageing would impair 'rapid onset vasodilatation' (ROV) in distributing arterioles (second-order, 2A) of old (20-month) males (OM) and females (OF) relative to young (3-month) males (YM) and females (YF). Neither resting (approximately 17 microm) nor maximum (approximately 30 microm) 2A diameters differed between groups. In response to single tetanic contractions at 100 Hz (duration, 100-1000 ms), ROV responses were blunted by half in OM relative to OF, YM or YF. With no effect in YM, blockade of alpha-adrenoreceptors with phentolamine (1 mum) restored ROV in OM. Topical noradrenaline (1 nM) blunted ROV in YM and YF to levels seen in OM and further suppressed ROV in OM (P < 0.05). To evaluate arteriolar blood flow, red blood cell velocity was measured in 2A of OM and YM; respective heart rates (353 +/- 22 vs. 378 +/- 15 beats min(1)) and carotid arterial blood pressures (76 +/- 3 vs. 76 +/- 1 mmHg) were not different. Blood flows at rest (0.6 +/- 0.1 vs. 1.6 +/- 0.2 nl s(1)) and during maximum dilatation (2.0 +/- 0.8 vs. 5.4 +/- 0.8 nl s(1)) with sodium nitroprusside (10 microM) were attenuated >60% (P < 0.05) in OM. Blood flow at peak ROV was blunted by 75-80% in OM vs. YM (P < 0.05). In response to 30 s of rhythmic contractions at 2, 4 and 8 Hz, progressive dilatations did not differ with age or sex. Nevertheless, resting and peak blood flows in YM were 2- to 3-fold greater (P < 0.05) than OM. We suggest that ageing blunts ROV and restricts blood flow to skeletal muscle of OM through subtle activation of alpha-adrenoreceptors in microvascular resistance networks.

Estrogen modulates the contribution of neuropeptide Y to baseline hindlimb blood flow control in female Sprague-Dawley rats

The purpose of this study was to determine the role of estrogen in neuropeptide Y (NPY) and Y(1) receptor (Y(1)R)-mediated vascular responses in female rats. Based on earlier work from our laboratory that female rats lacked an NPY contribution to hindlimb vascular conductance relative to males, we tested the hypothesis that estrogen modulates Y(1)R-mediated hindlimb blood flow control. Thus it was expected that ovariectomy would: 1) increase skeletal muscle Y(1)R expression, 2) decrease skeletal muscle Y(2) receptor (Y(2)R) expression, 3) decrease peptidase activity, and/or 4) increase overall skeletal muscle NPY concentration. Separate groups of control (CTL), ovariectomized (OVX), and OVX + 17beta-estradiol replacement (OVX + E(2); 21-day pellet) rats were studied. Animals were anesthetized and given localized hindlimb delivery of BIBP-3226 (Y(1)R antagonist), while femoral artery blood flow and blood pressure were recorded. Tissue samples from the white and red vastus lateralis muscle were extracted to examine Y(1)R and Y(2)R expression, peptidase activity, and NPY concentration. We found that Y(1)R blockade resulted in increased baseline hindlimb blood flow and vascular conductance in OVX rats, whereas no change was noted in CTL or OVX + E(2) groups (P < 0.05). This enhanced functional effect in the OVX group aligned with greater skeletal muscle Y(1)R expression in white vastus muscle and a substantial increase in NPY concentration in both white and red vastus muscle compared with CTL and OVX + E(2) groups. There was no change in Y(2)R expression or peptidase activity among the groups. These data support the hypothesis that estrogen blunts Y(1)R activation in the rat hindlimb through an effect on Y(1)R expression and NPY concentration.

Neuropeptide Y and neurovascular control in skeletal muscle and skin

Neuropeptide Y (NPY) is a ubiquitous peptide with multiple effects on energy metabolism, reproduction, neurogenesis, and emotion. In addition, NPY is an important sympathetic neurotransmitter involved in neurovascular regulation. Although early studies suggested that the vasoactive effects of NPY were limited to periods of high stress, there is growing evidence for the involvement of NPY on baseline vasomotor tone and sympathetically evoked vasoconstriction in vivo in both skeletal muscle and the cutaneous circulation. In Sprague-Dawley rat skeletal muscle, Y(1)-receptor activation appears to play an important role in the regulation of basal vascular conductance, and this effect is similar in magnitude to the alpha(1)-receptor contribution. Furthermore, under baseline conditions, agonist and receptor-based mechanisms for Y(1)-receptor-dependent control of vascular conductance in skeletal muscle are greater in male than female rats. In skin, there is Y(1)-receptor-mediated vasoconstriction during whole body, but not local, cooling. As with the NPY system in muscle, this neural effect in skin differs between males and females and in addition, declines with aging. Intriguingly, skin vasodilation to local heating also requires NPY and is currently thought to be acting via a nitric oxide pathway. These studies are establishing further interest in the role of NPY as an important vasoactive agent in muscle and skin, adding to the complexity of neurovascular regulation in these tissues. In this review, we focus on the role of NPY on baseline vasomotor tone in skeletal muscle and skin and how NPY modulates vasomotor tone in response to stress, with the aim of compiling what is currently known, while highlighting some of the more pertinent questions yet to be answered.

Functional neuroanatomy of autonomic regulation

Considerable effort has been put into animal studies establishing the sites in the brain that are responsible for control of the autonomic nervous system. These studies relied on an electrophysiological or neurochemical response to the activation of peripheral autonomic receptors or chemical or electrical stimulation of central sites. A large number of excellent reviews summarize the results of these studies. More recently, functional imaging has been used to not only confirm the electrophysiological and anatomical studies in animals, but has allowed a more complete understanding of how the brain responds as a whole for effecting autonomic control. The earliest studies to examine forebrain control during functional imaging utilized tests that involved active participation of the subjects and included maximal inspiration, Valsalva manoeuvre, isometric handgrip and cold compress application. There were a few issues that arose from these studies. First, they involved areas of the brain that included active decision making, they were more prone to inducing movement artefact, and some of these tests could activate noxious regions in the brain in addition to autonomic sites. In fact, this dual modality activation represented a more severe complication for investigators determining nociceptive sites in the brain, since virtually all of their stimuli had concomitant autonomic responses. More recent investigations attempted to resolve these issues with more selective passive and active stimuli. In spite of the very different approaches taken to visceral activation in functional imaging studies, a consistent picture of the key areas involved in autonomic control has emerged.

Sex differences in forebrain and cardiovagal responses at the onset of isometric handgrip exercise: a retrospective fMRI study

In general, cardiac regulation is dominated by the sympathetic and parasympathetic nervous systems in men and women, respectively. Our recent study had revealed sex differences in the forebrain network associated with sympathoexcitatory response to baroreceptor unloading. The present study further examined the sex differences in forebrain modulation of cardiovagal response at the onset of isometric exercise. Forebrain activity in healthy men (n = 8) and women (n = 9) was measured using functional magnetic resonance imaging during 5 and 35% maximal voluntary contraction handgrip exercise. Heart rate (HR), mean arterial pressure (MAP), and muscle sympathetic nerve activity (MSNA) were collected in a separate recording session. During the exercise, HR and MAP increased progressively, while MSNA was suppressed (P < 0.05). Relative to men, women demonstrated smaller HR (8 +/- 2 vs. 18 +/- 3 beats/min) and MAP (3 +/- 2 vs. 11 +/- 2 mmHg) responses to the 35% maximal voluntary contraction trials (P < 0.05). Although a similar forebrain network was activated in both groups, the smaller cardiovascular response in women was reflected in a weaker insular cortex activation. Nevertheless, men did not show a stronger deactivation at the ventral medial prefrontal cortex, which has been associated with modulating cardiovagal activity. In contrast, the smaller cardiovascular response in women related to their stronger suppression of the dorsal anterior cingulate cortex activity, which has been associated with sympathetic control of the heart. Our findings revealed sex differences in both the physiological and forebrain responses to isometric exercise.

Neuropeptide Y bioavailability is suppressed in the hindlimb of female Sprague-Dawley rats

We recently reported that male, but not female, rats exhibit basal endogenous neuropeptide Y Y(1)-receptor modulation of hindlimb vasculature. The lack of baseline endo-genous Y(1)-receptor control in females was evident despite the expression of Y(1)-receptors and neuropeptide Y in hindlimb skeletal muscle tissue. The following study addressed the hypothesis that neuropeptide Y bioavailability is blunted in female rats under baseline conditions. It was further hypothesized that enhanced prejunctional autoinhibitory neuropeptide Y Y(2)-receptor expression and/or proteolytic processing of released neuropeptide Y may persist in female rats. Using western blot analysis, it was observed that females had greater overall neuropeptide Y Y(2)-receptor expression in skeletal muscle compared to males (P < 0.05). To address the prevalence/impact of baseline endogenous Y(2)-receptor activation on neuropeptide Y release in hindlimb vasculature, an arterial infusion of BIIE0246 (specific non-peptide Y(2)-receptor antagonist; 170 microg kg(-1)) was carried out on female and male rats. Y(2)-receptor blockade resulted in a decrease in hindlimb vascular conductance in females and males (P < 0.05). However, the BIIE0246-induced decrease in vascular conductance was Y(1)-receptor dependent in females, but not males (P < 0.05). In addition, compared to baseline, BIIE0246 infusion resulted in increased plasma neuropeptide Y concentration in females (P < 0.05), while there was no observable change in males. In a final experiment, systemic inhibition of proteolytic enzymes dipeptidylpeptidase IV (via 500 nM diprotin A) and aminopeptidase P (via 180 nM 2-mercaptoethanol) elicited a Y(1)-receptor-dependent decrease in hindlimb vascular conductance in females (P < 0.05). It was concluded that our previously reported lack of basal endogenous Y(1)-receptor activation in female hindlimb vasculature was (at least partially) due to prejunctional Y(2)-receptor autoinhibition and proteolytic processing of neuropeptide Y.