Exercise pressor reflex in decerebrate and anesthetized rats

I investigated whether muscular contraction evokes cardiorespiratory increases (exercise pressor reflex) in alpha-chloralose- and chloral hydrate-anesthetized and precollicular, midcollicular, and postcollicular decerebrated rats. Mean arterial pressure (MAP), heart rate (HR), and minute ventilation (Ve) were recorded before and during 1-min sciatic nerve stimulation, which induced static contraction of the triceps surae muscles, and during 1-min stretch of the calcaneal tendon, which selectively stimulated mechanosensitive receptors in the muscles. Anesthetized rats showed various patterns of MAP response to both stimuli, i.e., biphasic, depressor, pressor, and no response. Sciatic nerve stimulation to muscle in precollicular decerebrated rats always evoked spontaneous running, so the exercise pressor reflex was not determined from these preparations. None of the postcollicular decerebrated rats showed a MAP response or spontaneous running. Midcollicular decerebrated rats consistently showed biphasic blood pressure response to both stimulations. The increases in MAP, HR, and Ve were related to the tension developed. The static contractions in midcollicular decerebrated rats (381 +/- 65 g developed tension) significantly increased MAP, HR, and Ve from 103 +/- 12 to 119 +/- 24 mmHg, from 386 +/- 30 to 406 +/- 83 beats/min, and from 122 +/- 7 to 133 +/- 25 ml/min, respectively. After paralysis, sciatic nerve stimulation had no effect on MAP, HR, or Ve. These results indicate that the midcollicular decerebrated rat can be a model for the study of the exercise pressor reflex.     

Exercise pressor reflex function in female rats fluctuates with the estrous cycle

In women, sympathoexcitation during static handgrip exercise is reduced during the follicular phase of the ovarian cycle compared with the menstrual phase. Previous animal studies have demonstrated that estrogen modulates the exercise pressor reflex, a sympathoexcitatory mechanism originating in contracting skeletal muscle. The present study was conducted in female rats to determine whether skeletal muscle contraction-evoked reflex sympathoexcitation fluctuates with the estrous cycle. The estrous cycle was judged by vaginal smear. Plasma concentrations of estrogen were significantly (P < 0.05) higher in rats during the proestrus phase of the estrus cycle than those during the diestrus phase. In decerebrate rats, either electrically induced 30-s continuous static contraction of the hindlimb muscle or 30-s passive stretch of Achilles tendon (a maneuver that selectively stimulates mechanically sensitive muscle afferents) evoked less renal sympathoexcitatory and pressor responses in the proestrus animals than in the diestrus animals. Renal sympathoexcitatory response to 1-min intermittent (1- to 4-s stimulation to relaxation) bouts of static contraction was also significantly less in the proestrus rats than that in the diestrus rats. In ovariectomized female rats, 17β-estradiol applied into a well covering the dorsal surface of the lumbar spinal cord significantly reduced skeletal muscle contraction-evoked responses. These observations demonstrate that the exercise pressor reflex function and its mechanical component fluctuate with the estrous cycle in rats. Estrogen may cause these fluctuations through its attenuating effects on the spinal component of the reflex arc.     

Venous plasma potassium is not associated with maintenance of the exercise pressor reflex in humans

Increases in the concentration of interstitial potassium concentration during exercise may play a role in the modulation of the cardiovascular response to exercise. However, it is not known if changes in potassium correlate with indexes of muscle reflex engagement. Eight healthy subjects performed dynamic [rhythmic handgrip (RHG)] and static handgrip (SHG) exercise at 40% of maximal voluntary contraction. Forearm circulatory arrest was performed to assess the metaboreceptor component of the exercise pressor reflex. Mean arterial pressure (MAP) and muscle sympathetic nerve activity (MSNA) were measured during each exercise paradigm. Venous plasma potassium concentrations ([K(+)](V)) were measured and used as a surrogate marker for interstitial potassium. [K(+)](V) were measured at baseline and at 1-min intervals during dynamic handgrip. During SHG, [K(+)](V) were measured at baseline, 30 and 90 s of exercise, and twice during forearm circulatory arrest. Mean [K(+)](V) was 3.6 mmol/l at rest before both paradigms. During RHG, [K(+)](V) rose by approximately 1.0 mmol/l by min 2 and remained constant throughout the rest of handgrip. During SHG, [K(+)](V) rose significantly at 30 s and rose an additional approximately 1.0 mmol/l by peak exercise. MAP and MSNA rose during both exercise paradigms. During posthandgrip circulatory arrest (PHG-CA), MSNA and blood pressure remained above baseline. [K(+)](V) and MSNA did not correlate during either exercise paradigm. Moreover, during PHG-CA, there was clear dissociation of MSNA from [K(+)](V). These data suggest that potassium does not play a direct role in the maintenance of the exercise pressor reflex.     

Plasma ghrelin levels in patients with end-stage renal disease

Ghrelin is an acylated peptide stimulating secretion of the growth hormone (GH). It was originally isolated from the rat stomach as an endogenous ligand for the growth hormone secretagogue receptor. Although being predominantly produced by endocrine cells of the gastric fundus, its secretion has been found in various tissues including the kidney. To study the influence of renal failure on plasma ghrelin levels we examined 16 patients with end-stage renal disease (ESRD) receiving hemodialysis (8 men and 8 women) and 19 controls (10 men and 9 women). Both groups were comparable in age and BMI. In all subjects we assessed plasma levels of ghrelin, leptin, soluble leptin receptor, insulin, IGF-I, IGFBP-1, IGFBP-3 and IGFBP-6. Ghrelin levels were significantly higher in the group of dialyzed patients (4.49+/-0.74 vs. 1.79+/-0.15 ng/ml; p<0.001). These patients had significantly higher levels of GH, IGFBP-1, IGFBP-6, leptin and percentage of body fat (p<0.05). In the group of patients with ESRD plasma ghrelin levels positively correlated with IGFBP-1 (p<0.01). In the control group, ghrelin positively correlated with GH concentrations (p<0.01) and negatively correlated with the levels of insulin and creatinine (p<0.05). In conclusion, patients with ESRD have higher ghrelin concentrations, which might be caused by a decreased excretion/metabolism of ghrelin in the kidney during renal failure.     

Identifying cardiovascular neurocircuitry involved in the exercise pressor reflex in humans using functional neurosurgery

Groups III and IV afferents carry sensory information regarding the muscle exercise pressor reflex, although the central integrating circuits of the reflex in humans are still poorly defined. Emerging evidence reports that the periaqueductal gray (PAG) could be a major site for integrating the "central command" component that initiates the cardiovascular response to exercise, since this area is activated during exercise and direct stimulation of the dorsal PAG causes an increase in arterial blood pressure (ABP) in humans. Here we recorded local field potentials (LFPs) from various "deep" brain nuclei during exercise tasks designed to elicit the muscle pressor reflex. The patients studied had undergone neurosurgery for the treatment of movement or pain disorders, thus had electrodes implanted stereotactically either in the PAG, subthalamic nucleus, globus pallidus interna, thalamus, hypothalamus, or anterior cingulate cortex. Fast Fourier transform analysis was applied to the neurograms to identify the power of fundamental spectral frequencies. Our PAG patients showed significant increases in LFP power at frequencies from 4 to 8 Hz (P < 0.01), 8 to 12 Hz (P < 0.001), and 12 to 25 Hz (P < 0.001). These periods were associated with maintained elevated ABP during muscle occlusion following exercise. Further increases in exercise intensity resulted in corresponding increases in PAG activity and ABP. No significant changes were seen in the activity of other nuclei during occlusion. These electrophysiological data provide direct evidence for a role of the PAG in the integrating neurocircuitry of the exercise pressor reflex in humans.     

Options in uraemia therapy for diabetics with end-stage renal disease

In many countries, diabetic renal disease has become, or will soon become, the single most common cause of end-stage renal disease (ESRD). End-stage renal failure (ESRF) in type-2 diabetic patients is increasing worldwide. Incidence of ESRF caused by diabetic nephropathy (DN) in 1996 in the USA was 41.7% and prevalence was 32.4%. ESRD and ESRF caused by DN was 10%, 5-15% in different haemodialysis centres in adults in the year 2000 in the Republic of Macedonia. In this review article we discuss options in uraemia therapy for diabetics with ESRD. Assessment and treatment of a diabetic with ESRD must be highly individualized. Haemodialysis (HD) has emerged as the most common treatment for all forms of renal failure including diabetic nephropathy. In diabetics patients with ESRD, dialysis is started early at creatinine clearance as high as 15-20 ml/min, at serum creatinin levels as low as 3-5 mg/dl. The first choice of HD access in diabetics is an autologous a-v fistula of the Cimino-Brescia type. The A-V fistula should be created several months before starting HD when creatinine clearance is above 20-25 ml/min. When peritoneal dialysis (PD) is selected, advance planning should ensure that a suitable peritoneal catheter is in situ 2-4 weeks before starting dialysis. HD procedures should be with low ultrafiltration rates and prolonged duration of dialysis sessions. The ultrafiltration in diabetics should not exceed more than 500-600 ml/h on HD. This means dialysis sessions of more than 4h and, in larger patients, of more than 5h HD three times per week. Renal transplantation (RT) is a safe and effective treatment modality for diabetic subjects with ESRD. Cardiovascular disease and serious infections are the major causes of death in haemodialysed and transplanted diabetics. Despite recent improvement, rehabilitation of HD diabetics continues to be inferior to that of non-diabetics. Improvement of survival is a matter of reduction of cardiovascular death and infection.     

Biomarkers and cardiac disease in patients with end-stage renal disease on dialysis

Very soon after troponin was introduced to routine clinical use in the mid-1990s, it was observed that troponin T was often increased in the blood of asymptomatic patients undergoing chronic dialysis for end-stage renal disease. Observation of these patients showed that the presence of troponin T in blood was predictive of a worse outcome for these patients.Cardiac disease is the major cause of death in dialysis patients. This review considers the heterogeneous cardiac disease that is found in these patients and reviews the role of cardiac biomarkers in identifying patients at risk of an adverse outcome.     

Determinants of premature atherosclerosis in children with end-stage renal disease

Cardiovascular disease is a major cause of morbidity and mortality in young adults with end-stage renal disease (ESRD), but its basis is still not well understood. We therefore evaluated the determinants of atherosclerosis in children with ESRD. A total of 37 children with ESRD (with 31 who had undergone transplantation) were examined and compared to a control group comprising 22 healthy children. The common carotid intima-media thickness (CIMT) was measured by ultrasound as a marker of preclinical atherosclerosis. The association of CIMT with anthropometrical data, blood pressure, plasma lipid levels, and other biochemical parameters potentially related to cardiovascular disease was evaluated. Children with ESRD had significantly higher CIMT, blood pressure, and levels of lipoprotein (a), urea, creatinine, ferritin, homocysteine, and serum uric acid as well as significantly lower values of apolipoprotein A. The atherogenic index of plasma (log(triglycerides/HDL cholesterol)) was also higher in patients with ESRD; however, this difference reached only borderline significance. In addition, a negative correlation was found between CIMT and serum albumin and bilirubin in the ESRD group, and this correlation was independent of age and body mass index. In the control group, a significant positive correlation was observed between CIMT and ferritin levels. Factors other than traditional cardiovascular properties, such as the anti-oxidative capacity of circulating blood, may be of importance during the early stages of atherosclerosis in children with end-stage renal disease.     

Paraoxonase-1 and ischemia-modified albumin in patients with end-stage renal disease

End-stage renal disease (ESRD) with and/or without treatment by hemodialysis (HD) is associated with accelerated atherosclerosis, leading to cardiovascular disease (CVD) including acute coronary syndromes. Therefore, the regulation of CVD is a crucial issue for ESRD patients. Given the recent reports that paraoxonase-1 (PON-1) and ischemia-modified albumin (IMA) could predict CVD-related mortality in ESRD, the two recent biomarkers may be useful for preventive strategies for CVD. This review paper presents current data on the relationships between PON-1, IMA, and ESRD. Many studies have shown that circulating PON-1 activity is lower in ESRD patients, and we have shown that its levels increase after HD. Although circulating IMA levels can increase before HD in ESRD patients, there remains to be little data. Our pilot study has shown a significant inverse correlation between PON-1 and IMA in ESRD patients. Although the pathogenic link between PON-1 and IMA remains speculative, considering both biomarkers may provide new insights into the prevention of CVD in ESRD patients.     

Gadolinium attenuates exercise pressor reflex in cats

The exercise pressor reflex, which arises from the contraction-induced stimulation of group III and IV muscle afferents, is widely believed to be evoked by metabolic stimuli signaling a mismatch between blood/oxygen demand and supply in the working muscles. Nevertheless, mechanical stimuli may also play a role in evoking the exercise pressor reflex. To determine this role, we examined the effect of gadolinium, which blocks mechanosensitive channels, on the exercise pressor reflex in both decerebrate and alpha-chloralose-anesthetized cats. We found that gadolinium (10 mM; 1 ml) injected into the femoral artery significantly attenuated the reflex pressor responses to static contraction of the triceps surae muscles and to stretch of the calcaneal (Achilles) tendon. In contrast, gadolinium had no effect on the reflex pressor response to femoral arterial injection of capsaicin (5 microg). In addition, gadolinium significantly attenuated the responses of group III muscle afferents, many of which are mechanically sensitive, to both static contraction and to tendon stretch. Gadolinium, however, had no effect on the responses of group IV muscle afferents, many of which are metabolically sensitive, to either static contraction or to capsaicin injection. We conclude that mechanical stimuli arising in contracting skeletal muscles contribute to the elicitation of the exercise pressor reflex.     

MLR stimulation and exercise pressor reflex activate different renal sympathetic fibers in decerebrate cats.

Although mesencephalic locomotor region (MLR) stimulation and the exercise pressor reflex have been shown to increase whole nerve renal sympathetic activity, it is not known whether these mechanisms converge onto the same population of renal sympathetic postganglionic efferents. In decerebrate cats, we examined the responses of single renal sympathetic postganglionic efferents to stimulation of the MLR and the exercise pressor reflex (i.e., static contraction of the triceps surae muscles). We found that, in most instances (24 of 28 fibers), either MLR stimulation or the muscle reflex, but not both, increased the discharge of renal postganglionic sympathetic efferents. In addition, we found that renal sympathetic efferents that responded to static contraction while the muscles were freely perfused responded more vigorously to static contraction during circulatory arrest. Moreover, stretch of the calcaneal (Achilles) tendon stimulated the same renal sympathetic efferents as did static contraction. These findings suggest that MLR stimulation and the exercise pressor reflex do not converge onto the same renal sympathetic postganglionic efferents.     

Quantifying rates of protein synthesis in humans by use of 2H2O: application to patients with end-stage renal disease

A method is introduced for quantitating protein synthetic rates in humans by use of (2)H(2)O. Its validity was tested in subjects with end-stage renal disease. Six clinically stable subjects, hemodialyzed three times weekly, ingested (2)H(2)O to a body water (2)H enrichment of approximately 0.4%. On dialysis, body water enrichment declined to approximately 0.1%. Enrichment of the alpha-hydrogen of plasma free alanine was also approximately 0.4% before and approximately 0.1% after dialysis. Beta-hydrogen enrichment was approximately 80-100% of alpha-hydrogen enrichment. (2)H(2)O was ingested to replace (2)H(2)O removed after each dialysis for 15-51 days, returning enrichment to approximately 0.4%. Enrichment of alanine from plasma albumin gradually increased, with again approximately 80-100% as much (2)H in beta- as in alpha-hydrogens. With continued dialyses, without (2)H(2)O replacement, alanine from albumin enrichment gradually declined, whereas free alanine and water enrichments were negligible. The fractional albumin synthesis rate, calculated from the increase in enrichment in alanine from albumin, was 4.0 +/- 0.5%/day, and from the decrease, 4.6 +/- 0.2%/day. Thus body water enrichment in a subject given (2)H(2)O can be maintained constant long term. A rapid exchange, essentially complete, occurs between the hydrogens of alanine and body water. An integrated measure over a long period of albumin's synthetic rate can be estimated from both the rise in enrichment of alanine from the protein during (2)H(2)O ingestion and fall on (2)H(2)O withdrawal, while the subject's living routine is uninterrupted. Estimates are in subjects with renal disease, but the method should be applicable to estimates of protein synthetic rates in normal subjects and in other pathological states.     

Large-scale mitochondrial DNA deletions in skeletal muscle of patients with end-stage renal disease

End-stage renal disease (ESRD) is associated with enhanced oxidative stress. This disease state provides a unique system for investigating the deleterious effect of exogenous sources of free radicals and reactive oxygen species (ROS) on mitochondrial DNA (mtDNA). To test the hypothesis that uremic milieu might cause more severe damage to mtDNA, we investigated the prevalence and abundance of mtDNA deletions in the skeletal muscles of ESRD patients. The results showed that the frequencies of occurrence of the 4977 bp and 7436 bp deletions of mtDNA in the muscle tissues of the older ESRD patients were higher than those of the younger patients. The frequency of occurrence of the 4977 bp-deleted mtDNA in the muscle was 33.3% for the patients in the age group of < 40 years, 66.6% in the 41-60-year-old group, 100% in the 61-80-year-old group, and 100% in patients >80 years of age, respectively. Only 22% of the normal aged controls carried the 4977 bp mtDNA deletion, whereas 77% (17/22) of the ESRD patients exhibited the mtDNA deletion. Using a semiquantitative PCR method, we determined the proportion of the 4977 bp-deleted mtDNA from the muscles that had been confirmed to harbor the deletion. We found that the proportions of the 4977 bp-deleted mtDNA in the muscle were significantly higher than those of the aged matched controls. Using long-range PCR techniques, a distinctive array of mtDNA deletions was demonstrated in the muscle of uremic patients. In summary, we found diverse and multiple mtDNA deletions in the skeletal muscles of ESRD patients. These deletions are more prevalent and abundant in ESRD patients than those found in normal populations. Accumulation of uremic toxins and impaired free radical scavenging systems may be responsible for the increased oxidative stress in ESRD patients. Such stress may result in oxidative damage and aging-associated mutation of the mitochondrial genome.     

Mechanisms of endothelial dysfunction in resistance arteries from patients with end-stage renal disease

The study focuses on the mechanisms of endothelial dysfunction in the uremic milieu. Subcutaneous resistance arteries from 35 end-stage renal disease (ESRD) patients and 28 matched controls were studied ex-vivo. Basal and receptor-dependent effects of endothelium-derived factors, expression of endothelial NO synthase (eNOS), prerequisites for myoendothelial gap junctions (MEGJ), and associations between endothelium-dependent responses and plasma levels of endothelial dysfunction markers were assessed. The contribution of endothelium-derived hyperpolarizing factor (EDHF) to endothelium-dependent relaxation was impaired in uremic arteries after stimulation with bradykinin, but not acetylcholine, reflecting the agonist-specific differences. Diminished vasodilator influences of the endothelium on basal tone and enhanced plasma levels of asymmetrical dimethyl L-arginine (ADMA) suggest impairment in NO-mediated regulation of uremic arteries. eNOS expression and contribution of MEGJs to EDHF type responses were unaltered. Plasma levels of ADMA were negatively associated with endothelium-dependent responses in uremic arteries. Preserved responses of smooth muscle to pinacidil and NO-donor indicate alterations within the endothelium and tolerance of vasodilator mechanisms to the uremic retention products at the level of smooth muscle. We conclude that both EDHF and NO pathways that control resistance artery tone are impaired in the uremic milieu. For the first time, we validate the alterations in EDHF type responses linked to kinin receptors in ESRD patients. The association between plasma ADMA concentrations and endothelial function in uremic resistance vasculature may have diagnostic and future therapeutic implications.     

A large regional hospital's experience with treatment of end-stage renal disease.

During the first 10 years of the treatment program for end-stage renal disease at the Saint John (New Brunswick) Regional Hospital 164 adults were treated by hemodialysis (with or without renal transplantation, performed outside of the province) or peritoneal dialysis. The primary causes of renal disease were not significantly different in men and women except for glomerulonephritis, which was twice as common in men as in women. Life-table analysis showed that the younger transplant recipients had the highest survival rate, but that the prognosis was almost as good among the much older patients who received continuous ambulatory peritoneal dialysis. Probably because they tended to be younger and their renal disease was caused by less threatening conditions, men survived longer than women. The survival rates were significantly related to the primary cause of the renal disease; patients with diabetes or systemic disease had the worst prognosis. Overall, these results compare well with those obtained in major university centres.