Exercise Pressor Reflex in Type 1 Diabetic Rats is Not Different Between Sexes

Diabetes is a major risk factor for cardiovascular disease and is associated with complications such as autonomic and peripheral neuropathy. The pathophysiology and prevalence of cardiovascular disease differ between sexes. Diabetic patients have an exaggerated pressor and sympathetic response to exercise compared to healthy individuals, which may increase the risk of myocardial infarction and stroke during physical activity. PURPOSE: The purpose of this study was to determine whether the cardioaccelerator and pressor responses to static contraction and tendon stretch differ between sexes in type 1 diabetic (T1DM) rats. METHODS: We injected 50 mg/kg Streptozotocin (STZ) or the vehicle (CTL) i.p in fasted female and male Sprague Dawley rats and waited 10.5±2.5 days (female CTL: BW=274±5 g, glucose=195±12 mg/dL, HbA1C=4.4±0.07%; male CTL: BW=384±4 g, glucose=189±3 mg/dL, HbA1C=4.7±0.1%; female STZ: BW=259±9 g, glucose=485±20 mg/dL, HbA1C=7.3±0.4%; male STZ: BW=294±10 g, glucose=458±17 mg/dL, HbA1C=9.6±0.6%) before performing experiments. All experiments were performed on unanaesthetized, decerebrated rats. We either statically contracted the hind limb muscles or stretched the Achilles tendon for 30 s and measured changes in mean arterial pressure (MAP) and heart rate (HR). RESULTS: We found that the pressor (female CTL: ΔMAP=15±1 mmHg, n=8; male CTL: ΔMAP=16±1 mmHg, n=9; female STZ: ΔMAP=29±6 mmHg, n=8; male STZ: ΔMAP=25±3 mmHg, n=9, p=0.62) and cardioaccelerator (female CTL: ΔHR=17±2 bpm, n=8; male CTL: ΔHR=12±1 bpm, n=9; female STZ: ΔHR=13±5 bpm, n=8; male STZ: ΔHR=24±5 bpm, n=9, p=0.051) responses to static contraction were not significantly different between sexes in T1DM rats. Likewise, the pressor (female CTL: ΔMAP=21±6 mmHg, n=8; male CTL: ΔMAP=33±2 mmHg, n=9; female STZ: ΔMAP=37±8 mmHg, n=10; male STZ: ΔMAP=31±5 mmHg, n=12, p=0.11) and cardioaccelerator (female CTL: ΔHR=9±2 bpm, n=8; male CTL: ΔHR=12±1 bpm, n=9; female STZ: ΔHR=12±4 bpm, n=10; male STZ: ΔHR=14±3 bpm, n=12, p=0.33) responses to tendon stretch were not different between sexes in T1DM rats. The developed tensions from contraction or tendon stretch were similar within each comparison (p\u3e0.05). CONCLUSION: We conclude that the pressor and cardioaccelerator responses to static contraction and tendon stretch are not different between female and male T1DM rats

Augmented Mechanoreflex in Type 2 Diabetic Rats: Piezo Channels, an Important Part of the Puzzle?

Type 2 diabetics (T2DM) have an abnormal cardiovascular response to exercise. The exercise pressor reflex, which is evoked by metabolic and mechanical stimuli arising from the contracting muscle, is a critical cardiovascular regulatory mechanism during exercise and is exaggerated in hypertension, heart failure, and peripheral artery disease. A recent study found that T2DM patients have an augmented metaboreflex. However, whether the mechanoreflex is also augmented in T2DM is not known. PURPOSE: The purpose of the study was to test whether the mechanoreflex is exaggerated in T2DM. Furthermore, we tested the contribution of mechano-gated Piezo 1 and 2 channels to the mechanoreflex in T2DM. METHODS: In unanaesthetized, decerebrated rats we stretched the Achilles tendon for 30 s and measured changes in mean arterial pressure (MAP) and heart rate (HR) in 12 mo old male T2DM rats (BW=546±26 g, glucose=549±28 mg/dl, HbA1c=12.82±0.18%) and healthy male controls (CTL: BW=453±22 g, glucose=229±31 mg/dl, HbA1c=4.6±0.1%). To test the contribution of Piezo channels, we injected GsMTx-4 (10 mg), a known antagonist of Piezo 1 and 2 channels, into the arterial supply of the hindlimb and repeated the stretch maneuver. RESULTS: We found that the pressor (T2DM: ΔMAP=69±6 mmHg, n=5; CTL: ΔMAP=13±2 mmHg, n=5) and cardioaccelerator (T2DM: ΔHR=28±4 bpm, n=5; CTL: ΔHR=5±3 bpm, n=5) responses to tendon stretch were significantly greater in T2DM rats compared to CTL; pCONCLUSION: We conclude that T2DM significantly exaggerates the pressor and cardioaccelerator response to mechanoreflex activation and that Piezo channels play a significant role in evoking the mechanoreflex in T2DM rats

Acute Effect of Hyperglycemia on the Mechanoreflex and Metaboreflex

Recent studies in both humans and rodents have shown that the mechanoreflex and metaboreflex are exaggerated in type 2 diabetes mellitus (T2DM). Hyperglycemia is a main characteristic of T2DM and is known to cause damage to both cardiovascular and nervous system structures. However, the acute effect of the presence of hyperglycemia on the mechanoreflex and metaboreflex are not known. PURPOSE: To determine the acute effect of hyperglycemia on the mechanoreflex and metaboreflex. METHODS: Experiments were conducted after an overnight fast in unanesthetized, decerebrated healthy male and female Sprague-Dawley rats. The mechanoreflex was evoked by stretching the Achilles tendon for 30 s whereas the metaboreflex was evoked by locally injecting lactic acid (0.2ml, 24mM) into the hindlimb. Time and dosage for glucose infusion were selected based on a preliminary study that showed infusing 250 mg/ml of glucose solution for 15 min into the hindlimb circulation, with blood flow to and from the hindlimb restricted, would elevate local blood glucose concentration to the same degree as that seen in T2DM rats with an exaggerated exercise pressor reflex. To elicit an acute local hyperglycemic environment, while preventing an endogenous insulin response, somatostatin (3.9 ug/100 ul) was infused systemically and simultaneously with the local glucose infusion. Changes in mean arterial pressure (ΔMAP) and heart rate (ΔHR) in response to tendon stretch and lactic acid injection were measured and compared before and after infusion. RESULTS: We found that the peak pressor and cardioaccelerator responses to tendon stretch were not significantly affected by hyperglycemia (ΔMAP before: 12 ± 2 mmHg, after: 12 ± 3 mmHg, n=6, p\u3e0.05; ΔHR before: 10 ± 3 bpm; after: 10 ± 3 bpm, n=6, p\u3e0.05). Likewise, the pressor and cardioaccelerator responses to lactic acid were not significantly affected by hyperglycemia (ΔMAP before: 13 ± 2 mmHg, after: 16 ± 3 mmHg, n=10, p\u3e0.05; ΔHR before: 10 ± 2 bpm, after: 12 ± 5 bpm, n=10, p\u3e0.05). CONCLUSION: The acute presence of hyperglycemia in the local circulation of the hindlimb likely does not contribute to the exaggerated mechanoreflex or metaboreflex

Effects of type 1 diabetes mellitus on the exercise pressor reflex during simulated dynamic muscle contraction : role of Piezo channels

The exercise pressor reflex reflexively increases sympathetic activity and blood pressure during exercise and thereby adjust the circulation to meet the metabolic demands of the working muscles. Although essential during exercise, recent studies suggest the reflex is exaggerated in early-stage type 1 diabetes mellitus (T1DM), which, in turn, increases the risk of adverse cardiovascular events. These studies have shown that T1DM leads to an exaggerated pressor reflex evoked by static muscle contraction and tendon stretch. Moreover, research suggests that Piezo channels may play a role in this exaggeration. However, it is not known whether the pressor reflex evoked by a simulated dynamic muscle is also exaggerated and whether Piezo channels play a role in eliciting this response. Therefore, the first aim of this project was to test the hypothesis that T1DM leads to an exaggerated exercise pressor reflex evoked by a simulated dynamic muscle contraction. The second aim of this project was to test the hypothesis that Piezo channels play a role in evoking the reflex pressor response to simulated dynamic muscle contraction in T1DM. We used a streptozotocin (50 mg/kg) induced T1DM model and unanaesthetized, decerebrate rats, to measure cardiovascular reflex changes in healthy and T1DM rats. We compared these changes between groups for aim 1 and before and after locally injecting GsMTx-4 (0.25 μM), a Piezo channel blocker, into the vasculature of the hindlimb for aim 2. The major findings from this study are: 1) early-stage T1DM leads to an exaggerated reflex pressor response to simulated dynamic muscle contraction, and 2) Piezo channel blockade attenuates the reflex pressor response by 33%. Notably, the Piezo channel blockade attenuated both the early onset exaggeration and the blood pressure index in T1DM rats, which suggest it effectively reduce cardiovascular strain during a simulated dynamic muscle contraction in T1DM. These findings are significant as they provide novel insight into the effects of T1DM on the reflexive control of the circulation and provide a potential mechanism mediating these adverse effects. Thus, the information gained from this project offers a mechanistic target for the development of effective therapeutic strategies aimed at reducing the heightened cardiovascular risk in this populationKinesiology and Health Educatio

Creating a Type I Diabetic Rat with Streptozotocin

ABSTRACT The American Heart Association (AHA) recognizes diabetes as a risk factor for developing cardiovascular disease. Type 1 diabetes (T1D) is characterized by absolute insulin deficiency due to the destruction of the insulin-producing pancreatic beta cells. A common model for studying T1D is the streptozotocin (STZ) induced rat. STZ targets and destroys beta cells of the pancreas and therefore causes the rats to develop T1D. PURPOSE: The purpose of this project was to describe the procedure for inducing T1D in a in vivo rat model, to study the cardiovascular responses to exercise. METHODS: Male and female Sprague Dawley rats were fasted overnight (12 hours) and brought in to the lab the following day for injection. Rats were anesthetized with 5% isoflurane in 100% oxygen for about four minutes. Once rats were unresponsive to mechanical stimulation, they were weighed and placed on a nosecone and ventilated through a nose cone with 3% isoflurane in 100% oxygen. The tail was pricked, and venous blood sampled for measurement of baseline blood glucose (Nova Biomedical) level. While the rat was on the nose cone, STZ (Sigma Aldrich) dosage (50mg/kg) was calculated, based on recorded body weight, and dissolved in 200 µl of citrate buffer. The drug solution was vortexed for 30 sec before being injected intraperitoneally followed by lactated Ringers (5 ml), which was injected subcutaneously to prevent dehydration. The rat was then taken off isoflurane and returned to its cage in a supine position with its head slightly elevated to prevent choking. RESULTS: Rats typically developed T1D within 24 to 48 hrs following STZ injection. The rats were considered diabetic when non-fasted blood glucose level was \u3e300 mg/dl. STZ induced T1D rats had an exaggerated blood pressure response to both static contraction and passive stretch compared to healthy controls. CONCLUSION: We have shown that inducing rats with T1D by injecting them with STZ resulted in a T1D rat model whose blood pressure response to exercise was exaggerated compared to healthy rats. This model allows for investigations addressing neurovascular control mechanisms during exercise in T1D

Functional Hyperemia is Attenuated in Type 1 Diabetic Rats Following Static Hindlimb Contraction

Functional hyperemia is a responsive, localized increase in blood flow following muscular contraction, which results from the accumulation of metabolites in the contracted region. Type 1 diabetes (T1DM) is associated with reduced vascular function and an abnormal cardiovascular response to exercise; it is not known, however, if the reactive increase in blood flow succeeding exercise is also impaired in T1DM. PURPOSE: The purpose of this study was to determine the effects of T1DM on functional hyperemia following static muscle contraction. METHODS: We injected (i.p.) 50mg/kg of Streptozotocin (STZ) or the vehicle (CTL) in male and female Sprague Dawley rats and waited 1-2 weeks before performing experiments (STZ: BW=300.6±23.39 g, glucose=461.8±43.78 mg/dl, n=5; CTL: BW=403±39.63 g, glucose=187.7±8.22 mg/dl, n=6). During the experiment, in decerebrate, unanaesthetized rats, we statically contracted the left hindlimb muscles for 30s by stimulating the sciatic nerve. Changes in Popliteal blood flow were measured using an ultrasonic flow probe (Transonic). The blood flow response was assessed 5 s before, 30 s during, and 30 s after muscle contraction and compared between STZ and CTL rats. RESULTS: The functional hyperemic response was assessed by calculating both the incremental (AUCi) and total (AUC) area under the curve of the blood flow response during the 30s following the cessation of contraction. We found that STZ rats displayed both significantly lower AUCi (STZ: 0.0681±0.0112 ml/min x 30 s; CTL: 0.5694±0.1239 ml/min x 30 s; p=0.003) and AUC (STZ: 0.210±0.0439 ml/min x 30 s; CTL: 1.439±0.210 ml/min x 30 s; p0.05). CONCLUSION: We conclude that popliteal artery blood flow is significantly attenuated in T1DM rats compared to healthy controls following static muscle contraction. These results suggest that T1DM impairs functional hyperemia in this model

Sex Differences in the Augmented Metaboreflex in Type 1 Diabetic Rats

Studies have shown that blood pressure and heart rate responses to metabolites produced during muscle contraction (i.e., metaboreflex) are exaggerated in type 1 diabetes (T1DM). It is not known, however, if these responses differ between females and males. PURPOSE: The purpose of this study was to determine if the metaboreflex, stimulated by either lactic acid or ATP, differs between sexes and if these differences change with the progression of the disease. METHODS: We compared adult female (F) and male (M) Sprague Dawley rats one week, three weeks, and six weeks after the induction of T1DM (Streptozotocin, 50mg/kg; F: n = 37; M: n = 46) to their corresponding vehicle-injection controls (CTL; citrate buffer; F: n = 31; M: n = 40). On the day of the experiment, the metaboreflex was evoked in decerebrate, unanesthetized rats by injecting α,β meth-ATP (10µg/0.2mLµl) or lactic acid (0.2 mL, 24mM) into the arterial supply of the left hindlimb. Peak pressor and cardioaccelerator responses were recorded within 30 s of injecting either stimuli. Non-fasted blood glucose and body weight were measured before and after induction of T1DM. All data are reported as mean ± SD. RESULTS: All T1DM rats were considered diabetic if blood glucose \u3e 300 mg/dl. Pressor and cardioaccelerator responses to injecting lactic acid into the arterial supply of the hindlimb evoked a significant pressor response in all rats (p0.05). CONCLUSION: Overall, we conclude that changes in cardioaccelerator and pressor responses to lactic acid and α,β meth-ATP may exist in T1DM and should be investigated further in future studies

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 meta-boreflex 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