Cardiovascular and cerebral vascular health in females with postacute sequelae of COVID-19.

Many individuals who had coronavirus disease 2019 (COVID-19) develop detrimental persistent symptoms, a condition known as postacute sequelae of COVID-19 (PASC). Despite the elevated risk of cardiovascular disease following COVID-19, limited studies have examined vascular function in PASC with equivocal results reported. Moreover, the role of PASC symptom burden on vascular health has not been examined. We tested the hypothesis that peripheral and cerebral vascular function would be blunted and central arterial stiffness would be elevated in patients with PASC compared with age-matched controls. Furthermore, we hypothesized that impairments in vascular health would be greater in those with higher PASC symptom burden. Resting blood pressure (BP; brachial and central), brachial artery flow-mediated dilation (FMD), forearm reactive hyperemia, carotid-femoral pulse wave velocity (PWV), and cerebral vasodilator function were measured in 12 females with PASC and 11 age-matched female controls without PASC. The severity of persistent symptoms in those with PASC was reported on a scale of 1-10 (higher score: greater severity). Brachial BP (e.g., systolic BP, 126 ± 19 vs.109 ± 8 mmHg; P = 0.010), central BP (P < 0.050), and PWV (7.1 ± 1.2 vs. 6.0 ± 0.8 m/s; P = 0.015) were higher in PASC group compared with controls. However, FMD, reactive hyperemia, and cerebral vasodilator function were not different between groups (P > 0.050 for all). Total symptom burden was not correlated with any measure of cardiovascular health (P > 0.050 for all). Collectively, these findings indicate that BP and central arterial stiffness are elevated in females with PASC, whereas peripheral and cerebral vascular function appear to be unaffected, effects that appear independent of symptom burden.NEW & NOTEWORTHY We demonstrate for the first time that resting blood pressure (BP) and central arterial stiffness are higher in females with PASC compared with controls. In contrast, peripheral and cerebral vascular functions appear unaffected. Moreover, there was no relationship between total PASC symptom burden and measures of BP, arterial stiffness, or vascular function. Collectively, these findings suggest that females with PASC could be at greater risk of developing hypertension, which appears independent of symptom burden.

Altered Cardiovascular Responses to Exercise in Type 1 Diabetes.

Exaggerated cardiovascular responses to exercise increase the risk of myocardial infarction and stroke in individuals with type 1 diabetes (T1D); however, the underlying mechanisms remain largely elusive. This review provides an overview of the altered exercise pressor reflex in T1D, with an emphasis on the mechanical component of the reflex.

Impact of COVID-19 on cardiac autonomic function in healthy young adults: potential role of symptomatology and time since diagnosis.

Emerging evidence suggests that COVID-19 may affect cardiac autonomic function; however, the limited findings in young adults with COVID-19 have been equivocal. Notably, symptomology and time since diagnosis appear to influence vascular health following COVID-19, but this has not been explored in the context of cardiac autonomic regulation. Therefore, we hypothesized that young adults who had persistent symptoms following COVID-19 would have lower heart rate variability (HRV) and cardiac baroreflex sensitivity (BRS) compared with those who had COVID-19 but were asymptomatic at testing and controls who never had COVID-19. Furthermore, we hypothesized that there would be relationships between cardiac autonomic function measures and time since diagnosis. We studied 27 adults who had COVID-19 and were either asymptomatic (ASYM; n = 15, 6 females); 21 ± 4 yr; 8.4 ± 4.0 wk from diagnosis) or symptomatic (SYM; n = 12, 9 females); 24 ± 3 yr; 12.3 ± 6.2 wk from diagnosis) at testing, and 20 adults who reported never having COVID-19 (24 ± 4 yr, 11 females). Heart rate and beat-to-beat blood pressure were continuously recorded during 5 min of rest to assess HRV and cardiac BRS. HRV [root mean square of successive differences between normal heartbeats (RMSSD); control, 73 ± 50 ms; ASYM, 71 ± 47 ms; and SYM, 84 ± 45 ms; P = 0.774] and cardiac BRS (overall gain; control, 22.3 ± 10.1 ms/mmHg; ASYM, 22.7 ± 12.2 ms/mmHg; and SYM, 24.3 ± 10.8 ms/mmHg; P = 0.871) were not different between groups. However, we found correlations with time since diagnosis for HRV (e.g., RMSSD, r = 0.460, P = 0.016) and cardiac BRS (overall gain, r = 0.470, P = 0.014). These data suggest a transient impact of COVID-19 on cardiac autonomic function that appears mild and unrelated to persistent symptoms in young adults.NEW & NOTEWORTHY The potential role of persistent COVID-19 symptoms on cardiac autonomic function in young adults was investigated. We observed no differences in heart rate variability or cardiac baroreflex sensitivity between controls who never had COVID-19 and those who had COVID-19, regardless of symptomology. However, there were significant relationships between measures of cardiac autonomic function and time since diagnosis, suggesting that COVID-19-related changes in cardiac autonomic function are transient in young, otherwise healthy adults.

Impact of COVID-19 on ambulatory blood pressure in young adults: a cross-sectional analysis investigating time since diagnosis.

Previous studies have reported detrimental effects of COVID-19 on the peripheral vasculature. However, reports on blood pressure (BP) are inconsistent, and measurements are made only in the laboratory setting. To date, no studies have measured ambulatory BP. In addition, in previous studies, time since COVID-19 diagnosis among participants varied across a wide range, potentially contributing to the inconsistent BP results. Thus, we aimed to perform a comprehensive assessment of BP and BP variability using ambulatory and laboratory (brachial and central) measurements in young adults who had COVID-19. We hypothesized that ambulatory BP would be elevated post-COVID-19 and that measures of BP would be inversely related with time since diagnosis. Twenty-eight young adults who had COVID-19 [11 ± 6 (range 3-22) wk since diagnosis] and 10 controls were studied. Ambulatory daytime, nighttime, and 24-h systolic BP, diastolic BP, and mean BP were not different between the control and COVID groups (e.g., daytime systolic BP: control, 122 ± 12 mmHg; COVID, 122 ± 10 mmHg; P = 0.937). Similar results were observed for laboratory BPs (all P > 0.05). However, ambulatory daytime, nighttime, and 24-h BPs as well as laboratory brachial BPs were inversely correlated with time since COVID-19 diagnosis (e.g., daytime systolic BP: r = -0.444; P = 0.044, nighttime systolic BP: r = -0.518; P = 0.016). Ambulatory and laboratory-measured BP variability were not different between groups nor correlated with time since diagnosis. Collectively, these data suggest that adverse effects of COVID-19 on BP in young adults are minimal and likely transient.NEW & NOTEWORTHY We report for the first time that ambulatory daytime, nighttime, and 24-h blood pressure (BP), as well as laboratory BP, were not different between control and COVID participants. However, a significant inverse relationship with time since COVID-19 diagnosis was found (i.e., greater BP with more recent infection). Ambulatory and laboratory BP variability were unaffected and not related with diagnosis time. These findings suggest that COVID-19 may exert only short-lasting effects on BP in young adults.

Impact of breakthrough COVID-19 cases during the omicron wave on vascular health and cardiac autonomic function in young adults.

We and others have previously shown that COVID-19 results in vascular and autonomic impairments in young adults. However, the newest variant of COVID-19 (Omicron) appears to have less severe complications. Therefore, we investigated whether recent breakthrough infection with COVID-19 during the Omicron wave impacts cardiovascular health in young adults. We hypothesized that measures of vascular health and indices of cardiac autonomic function would be impaired in those who had the Omicron variant of COVID-19 when compared with controls who never had COVID-19. We studied 23 vaccinated adults who had COVID-19 after December 25, 2021 (Omicron; age, 23 ± 3 yr; 14 females) within 6 wk of diagnosis compared with 13 vaccinated adults who never had COVID-19 (age, 26 ± 4 yr; 7 females). Macro- and microvascular function were assessed using flow-mediated dilation (FMD) and reactive hyperemia, respectively. Arterial stiffness was determined as carotid-femoral pulse wave velocity (cfPWV) and augmentation index (AIx). Heart rate (HR) variability and cardiac baroreflex sensitivity (BRS) were assessed as indices of cardiac autonomic function. FMD was not different between control (5.9 ± 2.8%) and Omicron (6.1 ± 2.3%; P = 0.544). Similarly, reactive hyperemia (P = 0.884) and arterial stiffness were not different between groups (e.g., cfPWV; control, 5.9 ± 0.6 m/s and Omicron, 5.7 ± 0.8 m/s; P = 0.367). Finally, measures of HR variability and cardiac BRS were not different between groups (all, P > 0.05). Collectively, these data suggest preserved vascular health and cardiac autonomic function in young, otherwise healthy adults who had breakthrough cases of COVID-19 during the Omicron wave.NEW & NOTEWORTHY We show for the first time that breakthrough cases of COVID-19 during the Omicron wave does not impact vascular health and cardiac autonomic function in young adults. These are promising results considering earlier research showing impaired vascular and autonomic function following previous variants of COVID-19. Collectively, these data demonstrate that the recent Omicron variant is not detrimental to cardiovascular health in young, otherwise healthy, vaccinated adults.

Augmented T-cell mitochondrial reactive oxygen species in adults with major depressive disorder.

The prevalence of major depressive disorder (MDD) is highest in young adulthood, an effect that has been magnified by the COVID-19 pandemic. Importantly, individuals with MDD are at a greater risk of developing cardiovascular disease (CVD). Accumulating evidence supports immune system dysregulation as a major contributor to the elevated CVD risk in older adults with MDD; however, whether this is present in young adults with MDD without comorbid disease remains unclear. Interestingly, recent data suggest augmented T-cell mitochondrial reactive oxygen species (T-cell mitoROS) as a potent driver of immune dysregulation in animal models of psychiatric disease. With this background in mind, we tested the hypothesis that young adults with MDD would have augmented T-cell mitoROS and circulating proinflammatory cytokines compared with healthy young adults without MDD (HA). Whole blood was drawn from 14 young adults with MDD (age: 23 ± 2 yr) and 11 HA (age: 22 ± 1 yr). T-cell mitoROS (MitoSOX red; total: CD3+, T-helper: CD4+, T cytotoxic: CD8+) and serum cytokines were assessed by flow cytometry. Total T-cell mitoROS was significantly greater in adults with MDD compared with HA [median: 14,089 arbitrary units (AU); median: 1,362 AU, P = 0.01]. Likewise, both T-helper and T-cytotoxic cell mitoROS were significantly greater in adults with MDD compared with HA (both: P < 0.05). There were no differences in circulating cytokines between groups (all cytokines: P > 0.05). Collectively, these findings suggest that elevated T-cell mitoROS may represent an early marker of immune system dysregulation in young, otherwise healthy, adults with MDD.NEW & NOTEWORTHY To our knowledge, we provide the first evidence of augmented T-cell mitochondrial reactive oxygen species (T-cell mitoROS) in young, otherwise healthy adults with MDD. Although the elevated T-cell mitoROS did not correspond to a proinflammatory profile, these findings suggest that elevated T-cell mitoROS may be an early marker of immune system dysregulation in young adults with MDD.

Exaggerated exercise pressor reflex in male UC Davis type 2 diabetic rats is due to the pathophysiology of the disease and not aging.

Introduction: Studies in humans and animals have found that type 2 diabetes mellitus (T2DM) exaggerates the blood pressure (BP) response to exercise, which increases the risk of adverse cardiovascular events such as heart attack and stroke. T2DM is a chronic disease that, without appropriate management, progresses in severity as individuals grow older. Thus, it is possible that aging may also exaggerate the BP response to exercise. Therefore, the purpose of the current study was to determine the effect of the pathophysiology of T2DM on the exercise pressor reflex independent of aging. Methods: We compared changes in peak pressor (mean arterial pressure; ΔMAP), BP index (ΔBPi), heart rate (ΔHR), and HR index (ΔHRi) responses to static contraction, intermittent contraction, and tendon stretch in UCD-T2DM rats to those of healthy, age-matched Sprague Dawley rats at three different stages of the disease. Results: We found that the ΔMAP, ΔBPi, ΔHR, and ΔHRi responses to static contraction were significantly higher in T2DM rats (ΔMAP: 29 ± 4 mmHg; ΔBPi: 588 ± 51 mmHg•s; ΔHR: 22 ± 5 bpm; ΔHRi: 478 ± 45 bpm•s) compared to controls (ΔMAP: 10 ± 1 mmHg, p < 0.0001; ΔBPi: 121 ± 19 mmHg•s, p < 0.0001; ΔHR: 5 ± 2 bpm, p = 0.01; ΔHRi: 92 ± 19 bpm•s, p < 0.0001) shortly after diabetes onset. Likewise, the ΔMAP, ΔBPi, and ΔHRi to tendon stretch were significantly higher in T2DM rats (ΔMAP: 33 ± 7 mmHg; ΔBPi: 697 ± 70 mmHg•s; ΔHRi: 496 ± 51 bpm•s) compared to controls (ΔMAP: 12 ± 5 mmHg, p = 0.002; ΔBPi: 186 ± 30 mmHg•s, p < 0.0001; ΔHRi: 144 ± 33 bpm•s, p < 0.0001) shortly after diabetes onset. The ΔBPi and ΔHRi, but not ΔMAP, to intermittent contraction was significantly higher in T2DM rats (ΔBPi: 543 ± 42 mmHg•s; ΔHRi: 453 ± 53 bpm•s) compared to controls (ΔBPi: 140 ± 16 mmHg•s, p < 0.0001; ΔHRi: 108 ± 22 bpm•s, p = 0.0002) shortly after diabetes onset. Discussion: Our findings suggest that the exaggerated exercise pressor reflex and mechanoreflex seen in T2DM are due to the pathophysiology of the disease and not aging.

Blunted peripheral but not cerebral vasodilator function in young otherwise healthy adults with persistent symptoms following COVID-19.

Recent findings suggest that COVID-19 causes vascular dysfunction during the acute phase of the illness in otherwise healthy young adults. To date, to our knowledge, no studies have investigated the longer-term effects of COVID-19 on vascular function. Herein, we hypothesized that young, otherwise healthy adults who are past the acute phase of COVID-19 would exhibit blunted peripheral [brachial artery flow-mediated dilation (FMD) and reactive hyperemia] and cerebral vasodilator function (cerebral vasomotor reactivity to hypercapnia; CVMR) and increased central arterial stiffness. Sixteen young adults who were at least 4 wk past a COVID-19 diagnosis and 12 controls who never had COVID-19 were studied. Eight subjects with COVID-19 were symptomatic (SYM) and eight were asymptomatic (ASYM) at the time of testing. FMD and reactive hyperemia were not different between COVID and control groups. However, FMD was lower in SYM (3.8 ± 0.6%) compared with ASYM (6.8 ± 0.9%; P = 0.007) and control (6.8 ± 0.6%; P = 0.003) with no difference between ASYM and control. Similarly, peak blood velocity following cuff release was lower in SYM (47 ± 8 cm/s) compared with ASYM (64 ± 19 cm/s; P = 0.025) and control (61 ± 14 cm/s; P = 0.036). CVMR and arterial stiffness were not different between any groups. In summary, peripheral macrovascular and microvascular function, but not cerebral vascular function or central arterial stiffness were blunted in young adults symptomatic beyond the acute phase of COVID-19. In contrast, those who were asymptomatic had similar vascular function compared with controls who never had COVID-19.NEW & NOTEWORTHY This study was the first to investigate the persistent effects of COVID-19 on vascular function in otherwise healthy young adults. We demonstrated that peripheral macrovascular and microvascular vasodilation was significantly blunted in young adults still symptomatic from COVID-19 beyond the acute phase (>4 wk from diagnosis), whereas those who become asymptomatic have similar vascular function compared with controls who never had COVID-19. In contrast, cerebral vascular function and central arterial stiffness were unaffected irrespective of COVID-19 symptomology.

Neurovascular Dysregulation During Exercise in Type 2 Diabetes.

Emerging evidence suggests that type 2 diabetes (T2D) may impair the ability to properly adjust the circulation during exercise with augmented blood pressure (BP) and an attenuated contracting skeletal muscle blood flow (BF) response being reported. This review provides a brief overview of the current understanding of these altered exercise responses in T2D and the potential underlying mechanisms, with an emphasis on the sympathetic nervous system and its regulation during exercise. The research presented support augmented sympathetic activation, heightened BP, reduced skeletal muscle BF, and impairment in the ability to attenuate sympathetically mediated vasoconstriction (i.e., functional sympatholysis) as potential drivers of neurovascular dysregulation during exercise in T2D. Furthermore, emerging evidence supporting a contribution of the exercise pressor reflex and central command is discussed along with proposed future directions for studies in this important area of research.

GsMTx-4 normalizes the exercise pressor reflex evoked by intermittent muscle contraction in early stage type 1 diabetic rats.

Emerging evidence suggests the exercise pressor reflex is exaggerated in early stage type 1 diabetes mellitus (T1DM). Piezo channels may play a role in this exaggeration, as blocking these channels attenuates the exaggerated pressor response to tendon stretch in T1DM rats. However, tendon stretch constitutes a different mechanical and physiological stimuli than that occurring during muscle contraction. Therefore, the purpose of this study was to determine the contribution of Piezo channels in evoking the pressor reflex during an intermittent muscle contraction in T1DM. In unanesthetized decerebrate rats, we compared the pressor and cardioaccelerator responses to intermittent muscle contraction before and after locally injecting grammostola spatulata mechanotoxin 4 (GsMTx-4, 0.25 µM) into the hindlimb vasculature. Although GsMTx-4 has a high potency for Piezo channels, it has also been suggested to block transient receptor potential cation (TRPC) channels. We, therefore, performed additional experiments to control for this possibility by also injecting SKF 96365 (10 µM), a TRPC channel blocker. We found that local injection of GsMTx-4, but not SKF 96365, attenuated the exaggerated peak pressor (ΔMAP before: 33 ± 3 mmHg, after: 22 ± 3 mmHg, P = 0.007) and pressor index (ΔBPi before: 668 ± 91 mmHg·s, after: 418 ± 81 mmHg·s, P = 0.021) response in streptozotocin (STZ) rats (n = 8). GsMTx-4 attenuated the exaggerated early onset pressor and the pressor response over time, which eliminated peak differences as well as those over time between T1DM and healthy controls. These data suggest that Piezo channels are an effective target to normalize the exercise pressor reflex in T1DM.NEW & NOTEWORTHY This is the first study to demonstrate that blocking Piezo channels is effective in ameliorating the exaggerated exercise pressor reflex evoked by intermittent muscle contraction, commonly occurring during physical activity, in T1DM. Thus, these findings suggest Piezo channels may serve as an effective therapeutic target to reduce the acute and prolonged cardiovascular strain that may occur during dynamic exercise in T1DM.

Effects of acute hyperglycemia on the exercise pressor reflex in healthy rats.

The exercise pressor reflex is exaggerated in type 2 diabetes mellitus (T2DM). Hyperglycemia, a main characteristic of T2DM, likely contributes to this exaggerated response. However, the isolated effect of acute hyperglycemia, independent of T2DM, on the exercise pressor reflex is not known. Therefore, the purpose of this study was to determine the effect of acute, local exposure to hyperglycemia on the exercise pressor reflex and its two components, namely the mechanoreflex and the metaboreflex, in healthy rats. To accomplish this, we determined the effect of an acute locol intra-arterial glucose infusion (0.25 g/mL) on cardiovascular responses to static contraction (i.e., exercise pressor reflex) and tendon stretch (i.e., mechanoreflex) for 30 s, as well as hindlimb intra-arterial lactic acid (24 mM) injection (i.e., metaboreflex) in fasted unanesthetized, decerebrated Sprague-Dawley rats. We measured and compared changes in mean arterial pressure (MAP) and heart rate (HR) before and after glucose infusion. We found that acute glucose infusion did not affect the pressor response to static contraction (ΔMAP: before: 15 ± 2 mmHg, after: 12 ± 2 mmHg; n = 8, p > 0.05), tendon stretch (ΔMAP: before: 12 ± 1 mmHg, after: 12 ± 3 mmHg; n = 8, p > 0.05), or lactic acid injection (ΔMAP: before: 13 ± 2 mmHg, after: 17 ± 3 mmHg; n = 9, p > 0.05). Likewise, cardioaccelerator responses were unaffected by glucose infusion, p > 0.05 for all. In conclusion, these findings suggest that acute, local exposure to hyperglycemia does not affect the exercise pressor reflex or either of its components.

Recent advances in exercise pressor reflex function in health and disease.

Autonomic alterations at the onset of exercise are critical to redistribute cardiac output towards the contracting muscles while preventing a fall in arterial pressure due to excessive vasodilation within the contracting muscles. Neural mechanisms responsible for these adjustments include central command, the exercise pressor reflex, and arterial and cardiopulmonary baroreflexes. The exercise pressor reflex evokes reflex increases in sympathetic activity to the heart and systemic vessels and decreases in parasympathetic activity to the heart, which increases blood pressure (BP), heart rate, and total peripheral resistance through vasoconstriction of systemic vessels. In this review, we discuss recent advancements in our understanding of exercise pressor reflex function in health and disease. Specifically, we discuss emerging evidence suggesting that sympathetic vasoconstrictor drive to the contracting and non-contracting skeletal muscle is differentially controlled by central command and the metaboreflex in healthy conditions. Further, we discuss evidence from animal and human studies showing that cardiovascular diseases, including hypertension, diabetes, and heart failure, lead to an altered exercise pressor reflex function. We also provide an update on the mechanisms thought to underlie this altered exercise pressor reflex function in each of these diseases. Although these mechanisms are complex, multifactorial, and dependent on the etiology of the disease, there is a clear consensus that several mechanisms are involved. Ultimately, approaches targeting these mechanisms are clinically significant as they provide alternative therapeutic strategies to prevent adverse cardiovascular events while also reducing symptoms of exercise intolerance.

Effects of type 1 diabetes on reflexive cardiovascular responses to intermittent muscle contraction.

Studies have shown that early-stage type 1 diabetes mellitus (T1DM) leads to an exaggerated reflex pressor response to both static muscle contraction and tendon stretch. However, whether similar responses are present during dynamic exercise (i.e., intermittent contraction) is not known. Therefore, the purpose of this study was to determine whether T1DM leads to an exaggerated reflex pressor response to intermittent muscle contraction. We measured the exercise pressor reflex in unanesthetized, decerebrated T1DM (50 mg/kg streptozotocin; STZ) and healthy control (CTL) Sprague-Dawley rats by intermittently contracting the hindlimb muscles for 30 s while measuring mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA), and heart rate (HR). Intermittently contracting the hindlimb muscles evoked exaggerated mean RSNA (STZ: Δ109 ± 21%, n = 4 rats; CTL: Δ61 ± 8%, n = 5 rats, P < 0.05), peak MAP (STZ: Δ32 ± 2 mmHg, n = 9 rats; CTL: Δ12 ± 2 mmHg, n = 6 rats, P < 0.05), blood pressure index (STZ: Δ625 ± 60 mmHg/s, n = 9 rats; CTL: Δ241 ± 46 mmHg/s, n = 6 rats, P < 0.05), and HR (STZ: Δ24 ± 3 beats/min, n = 9 rats; CTL: Δ9 ± 3 beats/min, n = 6 rats, P < 0.05) responses to similar developed tensions (P > 0.05) in T1DM compared with CTL rats. T1DM rats also exhibited exaggerated early-onset sympathetic (onset: 1 s) and pressor (onset: 5 s) responses. These data show that early-stage T1DM leads to an exaggerated pressor reflex evoked by intermittent muscle contraction. The early onset and greater blood pressure index suggest that cardiovascular strain during dynamic exercise may be significantly higher in individuals with T1DM.NEW & NOTEWORTHY This is the first study to provide evidence that early-stage type 1 diabetes mellitus (T1DM) leads to an exaggerated exercise pressor reflex evoked by intermittent muscle contraction, resulting in substantially higher cardiovascular strain. These findings are significant as they indicate that interventions targeting the exercise pressor reflex may work to alleviate the increased cardiovascular strain and overall burden during exercise in T1DM.

Exaggerated exercise pressor reflex in type 2 diabetes: Potential role of oxidative stress.

Type 2 diabetes mellitus (T2DM) leads to exaggerated cardiovascular responses to exercise, in part due to an exaggerated exercise pressor reflex. Accumulating data suggest excessive oxidative stress contributes to an exaggerated exercise pressor reflex in cardiovascular-related diseases. Excessive oxidative stress is also a primary underlying mechanism for the development and progression of T2DM. However, whether oxidative stress plays a role in mediating the exaggerated exercise pressor reflex in T2DM is not known. Therefore, this review explores the potential role of oxidative stress leading to increased activation of the afferent arm of the exercise pressor reflex. Several lines of evidence support direct and indirect effects of oxidative stress on the exercise pressor reflex. For example, intramuscular ROS may directly and indirectly (by attenuating contracting muscle blood flow) increase group III and IV afferent activity. Oxidative stress is a primary underlying mechanism for the development of neuropathic pain, which in turn is associated with increased group III and IV afferent activity. These are the same type of afferents that evoke muscle pain and the exercise pressor reflex. Furthermore, oxidative stress-induced release of inflammatory mediators may modulate afferent activity. Collectively, these alterations may result in a positive feedback loop that further amplifies the exercise pressor reflex. An exaggerated reflex increases the risk of adverse cardiovascular events. Thus, identifying the contribution of oxidative stress could provide a potential therapeutic target to reduce this risk in T2DM.

Exaggerated cardiovascular responses to muscle contraction and tendon stretch in UCD type-2 diabetes mellitus rats.

Patients with type-2 diabetes mellitus (T2DM) have exaggerated sympathetic activity and blood pressure responses to exercise. However, the underlying mechanisms for these responses, as well as how these responses change throughout disease progression, are not completely understood. For this study, we examined the effect of the progression of T2DM on the exercise pressor reflex, a critical neurocardiovascular mechanism that functions to increase sympathetic activity and blood pressure during exercise. We also aimed to examine the effect of T2DM on reflexive cardiovascular responses to static contraction, as well as those responses to tendon stretch when an exaggerated exercise pressor reflex was present. We evoked the exercise pressor reflex and mechanoreflex by statically contracting the hindlimb muscles and stretching the Achilles tendon, respectively, for 30 s. We then compared pressor and cardioaccelerator responses in unanesthetized, decerebrated University of California Davis (UCD)-T2DM rats at 21 and 31 wk following the onset of T2DM to responses in healthy nondiabetic rats. We found that the pressor response to static contraction was greater in the 31-wk T2DM [change in mean arterial pressure (∆MAP) = 39 ± 5 mmHg] but not in the 21-wk T2DM (∆MAP = 24 ± 5 mmHg) rats compared with nondiabetic rats (∆MAP = 18 ± 2 mmHg; P < 0.05). Similarly, the pressor and the cardioaccelerator responses to tendon stretch were significantly greater in the 31-wk T2DM rats [∆MAP = 69 ± 6 mmHg; change in heart rate (∆HR) = 28 ± 4 beats/min] compared with nondiabetic rats (∆MAP = 14 ± 2 mmHg; ∆HR = 5 ± 3 beats/min; P < 0.05). These findings suggest that the exercise pressor reflex changes as T2DM progresses and that a sensitized mechanoreflex may play a role in exaggerating these cardiovascular responses.NEW & NOTEWORTHY This is the first study to provide evidence that as type-2 diabetes mellitus (T2DM) progresses, the exercise pressor reflex becomes exaggerated, an effect that may be due to a sensitized mechanoreflex. Moreover, these findings provide compelling evidence suggesting that impairments in the reflexive control of circulation contribute to exaggerated blood pressure responses to exercise in T2DM.

Exaggerated mechanoreflex in early-stage type 1 diabetic rats: role of Piezo channels.

Recent findings have shown that muscle contraction evokes an exaggerated pressor response in type 1 diabetes mellitus (T1DM) rats; however, it is not known whether the mechanoreflex, which is commonly stimulated by stretching the Achilles tendon, contributes to this abnormal response. Furthermore, the role of mechano-gated Piezo channels, found on thin-fiber afferent endings, in evoking the mechanoreflex in T1DM is also unknown. Therefore, in male and female streptozotocin (STZ, 50 mg/kg)-induced T1DM and healthy control (CTL) rats, we examined the pressor and cardioaccelerator responses to tendon stretch during the early stage of the disease. To determine the role of Piezo channels, GsMTx-4, a selective Piezo channel inhibitor, was injected into the arterial supply of the hindlimb. At 1 wk after STZ injection in anesthetized, decerebrate rats, we stretched the Achilles tendon for 30 s and measured pressor and cardioaccelerator responses. We then compared pressor and cardioaccelerator responses to tendon stretch before and after GsMTx-4 injection (10 µg/100 ml). We found that the pressor (change in mean arterial pressure) response [41 ± 5 mmHg (n = 15) for STZ and 18 ± 3 mmHg (n = 11) for CTL (P < 0.01)] and cardioaccelerator (change in heart rate) response [18 ± 4 beats/min for STZ (n = 15) and 8 ± 2 beats/min (n = 11) for CTL (P < 0.05)] to tendon stretch were exaggerated in STZ rats. Local injection of GsMTx-4 attenuated the pressor [55 ± 7 mmHg (n = 6) before and 27 ± 9 mmHg (n = 6) after GsMTx-4 (P < 0.01)], but not the cardioaccelerator, response to tendon stretch in STZ rats and had no effect on either response in CTL rats. These data suggest that T1DM exaggerates the mechanoreflex response to tendon stretch and that Piezo channels play a role in this exaggeration.

Temporal changes in the exercise pressor reflex in type 1 diabetic rats.

Previous studies have shown that diabetic peripheral neuropathy affects both unmyelinated and myelinated afferents, similar to those evoking the exercise pressor reflex. However, the effect of type 1 diabetes (T1DM) on this reflex is not known. We examined, in decerebrate male and female T1DM [streptozotocin (STZ)] and healthy control (CTL) rats, pressor and cardioaccelerator responses to isometric contraction of the hindlimb muscles during the early and late stages of the disease. STZ (50 mg/kg) was injected to induce diabetes, and experiments were conducted at 1, 3, and 6 wk after injection. On the day of the experiment, we statically contracted the hindlimb muscles by stimulating the sciatic nerve and measured changes in mean arterial pressure and heart rate. We found that the pressor but not cardioaccelerator response was exaggerated in STZ rats at 1 wk (STZ: 21 ± 3 mmHg, n = 10, and CTL: 14 ± 2 mmHg, n = 10, P < 0.05) and at 3 wk (STZ: 26 ± 5 mmHg, n = 10, and CTL: 17 ± 3 mmHg, n = 11, P < 0.05) after injection. However, at 6 wk, and only in male rats, both the pressor (STZ: 13 ± 3 mmHg, n = 12, and CTL: 17 ± 3 mmHg, n = 13, P < 0.05) and cardioaccelerator responses (STZ: 7 ± 3 beats/min, n = 12, and CTL: 10 ± 3 beats/min, n = 13, P < 0.05) to contraction were significantly attenuated in STZ rats compared with CTL rats. These data indicate that T1DM exaggerates the exercise pressor reflex during the early stages of the disease in both male and female rats. Conversely, T1DM attenuates this reflex in the late stage of the disease in male but not female rats.NEW & NOTEWORTHY This is the first study to provide evidence that the pressor and cardioaccelerator responses to skeletal muscle contraction vary depending on the duration of type 1 diabetes.