Early structural effects of oestrogen on pudendal nerve regeneration in the rat

The Expression of Hormone Receptors as a Gateway toward Understanding Endocrine Actions in Female Pelvic Floor Muscles

OBJECTIVE

To provide an overview of the hormone actions and receptors expressed in the female pelvic floor muscles, relevant for understanding the pelvic floor disorders.

METHODS

We performed a literature review focused on the expression of hormone receptors mainly in the pelvic floor muscles of women and female rats and rabbits.

RESULTS

The impairment of the pelvic floor muscles can lead to the onset of pelvic floor dysfunctions, including stress urinary incontinence in women. Hormone milieu is associated with the structure and function alterations of pelvic floor muscles, a notion supported by the fact that these muscles express different hormone receptors. Nuclear receptors, such as steroid receptors, are up till now the most investigated. The present review accounts for the limited studies conducted to elucidate the expression of hormone receptors in pelvic floor muscles in females.

CONCLUSION

Hormone receptor expression is the cornerstone in some hormone-based therapies, which require further detailed studies on the distribution of receptors in particular pelvic floor muscles, as well as their association with muscle effectors, involved in the alterations relevant for understanding pelvic floor disorders.

Long-term outcome of obstetric anal sphincter injury repaired by experienced obstetricians

OBJECTIVE

To examine the long-term outcomes of women with obstetric anal sphincter injury (OASI) repaired by obstetricians without the involvement of colorectal surgeons.

METHODS

A retrospective cohort study was undertaken of women who had been admitted for delivery between 2004 and 2012. All OASIs had been repaired by experienced obstetricians using the end-to-end technique. An unexposed group (no OASI) was matched at a ratio of 1:1. Fecal incontinence was graded using a modified Wexner questionnaire.

RESULTS

Overall, 113 OASIs were recorded. Sixty-seven (59.3%) and 71 (62.8%) women from the exposed and unexposed groups, respectively, agreed to participate in the study (P=0.9). Continence to both stool and gas was reported by 48 (71.6%) and 64 (90.1%) women in the exposed and unexposed groups, respectively (P=0.03). The incidence of urgency, dyspareunia, and use of a pad or constipating agents was the same in both groups.

CONCLUSION

The long-term outcome of OASI repair performed by experienced obstetricians is comparable to that reported in the literature. Some women who did not sustain an OASI reported fecal incontinence, which suggests that only a proportion of fecal incontinence can be attributed to OASI.

Vulnerability of continence structures to injury by simulated childbirth

The goal of this study was to examine acute morphological changes, edema, muscle damage, inflammation, and hypoxia in urethral and vaginal tissues with increasing duration of vaginal distension (VD) in a rat model. Twenty-nine virgin Sprague-Dawley rats underwent VD under anesthesia with the use of a modified Foley catheter inserted into the vagina and filled with saline for 0, 1, 4, or 6 h. Control animals were anesthetized for 4 h without catheter placement. Urogenital organs were harvested after intracardiac perfusion of fixative. Tissues were embedded, sectioned, and stained with Masson's trichrome or hematoxylin and eosin stains. Regions of hypoxia were measured by hypoxyprobe-1 immunohistochemistry. Within 1 h of VD, the urethra became vertically elongated and displaced anteriorly. Edema was most prominent in the external urethral sphincter (EUS) and urethral/vaginal septum within 4 h of VD, while muscle disruption and fragmentation of the EUS occurred after 6 h. Inflammatory damage was characterized by the presence of polymorphonuclear leukocytes in vessels and tissues after 4 h of VD, with the greatest degree of infiltration occurring in the EUS. Hypoxia localized mostly to the vaginal lamina propria, urethral smooth muscle, and EUS within 4 h of VD. Increasing duration of VD caused progressively greater tissue edema, muscle damage, and morphological changes in the urethra and vagina. The EUS underwent the greatest insult, demonstrating its vulnerability to childbirth injury.

Animal models of stress urinary incontinence

Stress urinary incontinence (SUI) is a common health problem significantly affecting the quality of life of women worldwide. Animal models that simulate SUI enable the assessment of the mechanism of risk factors for SUI in a controlled fashion, including childbirth injuries, and enable preclinical testing of new treatments and therapies for SUI. Animal models that simulate childbirth are presently being utilized to determine the mechanisms of the maternal injuries of childbirth that lead to SUI with the goal of developing prophylactic treatments. Methods of assessing SUI in animals that mimic diagnostic methods used clinically have been developed to evaluate the animal models. Use of these animal models to test innovative treatment strategies has the potential to improve clinical management of SUI. This chapter provides a review of the available animal models of SUI, as well as a review of the methods of assessing SUI in animal models, and potential treatments that have been tested on these models.

Local anesthetic sciatic nerve block and nerve fiber damage in diabetic rats

BACKGROUND AND OBJECTIVES

A concern for anesthesiologists is whether local anesthetics are more toxic to peripheral nerves in diabetic patients. A previous study in streptozotocin-induced diabetic rats showed that larger doses of lidocaine produce moderate nerve injury after nerve block in normal rats and worse injury in diabetic rats. However, it is not clear whether a smaller local anesthetic dose that produces negligible nerve fiber damage in normal rats will produce significant nerve damage in diabetic rats and if adding adjuvant drugs modulates this effect.

METHODS

Rats were intravenously injected with 50 mg/kg streptozotocin to induce diabetes (blood glucose levels 9250 mg/dL) and diabetic neuropathy. After waiting 35 days, an injection (0.1 mL) of 1% lidocaine alone, or with 5 kg/mL epinephrine or 7.5 kg/mL clonidine added, or 0.5% ropivacaine alone was performed at the left sciatic notch in both diabetic and nondiabetic rats. The duration of sensory (pin prick) and motor (toe spreading reflex) nerve block in the hind paws was determined.For histologic controls, all rats also received saline vehicle injection into the right sciatic notch. Another group of uninjected rats was used as naive controls. Left and right nerves were removed 2 days after injection and fixed in situ with a 4% glutaraldehyde solution. Myelinated axon profiles suggestive of neuropathy (myelin figures, pale and swollen,or dark-staining axoplasm) were counted and expressed as a percentage of the total number of fibers in each rat sciatic nerve.

RESULTS

All streptozotocin-injected rats became diabetic and had pronounced tactile allodynia. All rats had sensory and motor nerve blocks lasting for at least 50 mins after injection of local anesthetic. The duration of sensory and motor nerve block was longer in diabetic rats than in nondiabetic rats for all drug groups tested. None of the sciatic nerves examined showed greater than 3% nerve fiber degeneration. Although lidocaine in diabetic rats did not produce nerve fiber damage,diabetic rats receiving lidocaine/clonidine or ropivacaine had more abnormal myelinated axon profiles than did nondiabetic rats receiving the same drug.

CONCLUSIONS

The duration of sciatic nerve block with local anesthetics is longer in diabetic compared with nondiabetic rats. A small, but statistically significant, increase in nerve damage occurred in diabetic rats after nerve block with ropivacaine alone or when duration of lidocaine block was extended with clonidine. These findings may have implications for dosing of local anesthetics in diabetic patients undergoing regional analgesia with nerve blocks.

Postpartum stress urinary incontinence: lessons from animal models

Postpartum stress urinary incontinence (SUI) is associated with chronic SUI in later life, which is 240% more likely to occur in women who deliver vaginally than those who did not. The etiology of SUI is multifactoral and has been associated with defects in both neuromuscular and structural components of continence. Specifically, clinical studies have demonstrated that pudendal nerve damage occurs during vaginal delivery, supporting the concept that neuromuscular damage to the continence mechanism can result in postpartum SUI. Urethral hypermobility and the loss of pelvic floor support, such as that involved in pelvic organ prolapse, have also been associated with SUI. Animal models provide an opportunity to investigate these injuries, individually and in combination, enabling researchers to gain further insight into their relative contributions to the development of SUI and the effectiveness of potential therapies for it. This article discusses the use of animal models of postpartum SUI in addition to the broad insights into treatment efficacy they provide.

Natural progression of anal incontinence after childbirth

INTRODUCTION AND HYPOTHESIS

The aim of work is to study the natural progression of anal incontinence (AI) in women 10 years after their first delivery and to identify risk factors associated with persistent AI.

METHODS

A prospective cohort study of 304 primiparous women with singleton, cephalic delivery giving vaginal childbirth in 1995. Questionnaires distributed and collected at delivery, 9 months, 5 years and 10 years after, assessing anorectal symptoms, subsequent treatment, and obstetrical events.

RESULTS

Women, 246 of 304, answered all questionnaires (81%). Thirty-five of 246 (14%) had a sphincter tear at the first delivery. One hundred ninety-six of 246 (80%) women had additional vaginal deliveries and no caesarean sections. The prevalence of AI at 10 years after the first delivery was 57% in women with a sphincter tear and 28% in women, a nonsignificant increase compared to the 5-year follow-up. Women who sustained a sphincter tear at the first delivery had an increased risk of severe AI (RR 3.9, 95% CI 1.3-11.8). Neither age, nor subsequent deliveries added to the risk. Severe AI at baseline and 5 years after delivery were independently strong predictors of severe AI at 10 years (RR 12.6, CI 3.3-48.3, and RR 8.3, CI 3.9-17.8, respectively).

CONCLUSION

Persistent anal incontinence 10 years after the first parturition is frequent and sometimes severe, especially if vaginal delivery was complicated by an anal sphincter disruption.

Estrogen promotes sympathetic nerve regeneration in rat proximal urethra

OBJECTIVES

To assess whether sympathetic reinnervation of the rat proximal urethra is affected by differences in estrogen levels. Sympathetic innervation mediates tonic contraction of proximal urethral smooth muscle, thus contributing to urinary continence. Urethral innervation is particularly susceptible to damage during vaginal delivery, a time characterized by decreasing estrogen levels.

METHODS

Adult female rats were ovariectomized and implanted with pellets containing vehicle or estrogen to achieve serum levels similar to rodent pregnancy. The rats were injected intravenously with vehicle or the selective sympathetic neurotoxin 6-hydroxydopamine, which produces uniform and complete destruction of terminal sympathetic axons. At 1, 4, 12, and 25 days, tyrosine hydroxylase-immunoreactive sympathetic innervation of the proximal urethral smooth muscle was assessed quantitatively.

RESULTS

In rats with intact innervation, the proximal urethra is densely innervated, and nerve density is comparable, irrespective of estrogen status. 6-Hydroxydopamine induced marked sympathetic axon disruption by day 1 and complete denervation by 4 days after injection in the ovariectomized rats receiving vehicle or estrogen. In the vehicle-treated rats, few nerves were present at 12 days after sympathectomy, and innervation remained substantially less than normal levels at 25 days. In estrogen-treated rats, sympathetic reinnervation was twofold greater at 12 days and by 25 days was comparable to that of the controls.

CONCLUSIONS

Estrogen improves sympathetic reinnervation of the proximal urethra. Estrogen titers in individuals with urethral sympathetic nerve damage might, therefore, influence the rate and extent of urethral smooth muscle reinnervation.

Clinical and nerve conduction studies in female patients with diabetic dermopathy

This study aims to assess the clinical and electrophysiological characteristics of diabetic polyneuropathy (PNP) in female patients. We investigated clinical and electrophysiological features in 175 female patients with diabetes mellitus to compare those with PNP only, diabetic dermopathy (DD), or diabetic foot (DF). Among clinical features, the loss of deep tendon reflexes, the presence of negative sensory symptoms, superficial sensory loss, and the loss of vibration sense were more common in DD patients than PNP patients. As compared with DD patients, the presence of skin atrophy, superficial and positive sensory symptoms were more common in DF patients. Neuropathic symptom and disability scores were significantly higher in DD and DF patients than PNP patients. In the electrophysiological studies, the only significant difference was observed in the mean distal latencies for ulnar nerves, which were longer in DD patients as compared with PNP patients, but similar between DD and DF patients. All other parameters failed to show significant difference among patients, though values for DD patients lied in between PNP and DF patients. Carpal tunnel syndrome was present in 45% of PNP patients, 63.8% of DD patients, and 50% of DF patients (P = 0.031). Our results suggest that female patients with diabetic dermopathy might have a more severe sensorial neuropathy than patients without these skin lesions.

Animal models of female stress urinary incontinence

PURPOSE

Urinary incontinence affects 40% of women in the United States and stress urinary incontinence accounts for a large portion of affected patients. As defined by the International Continence Society, stress urinary incontinence is the involuntary leakage of urine upon effort, exertion, sneezing or coughing. Since the ultimate success of long-term management for any condition is based on an understanding of its pathophysiology, and because the pathophysiology of stress urinary incontinence is incompletely defined, animal models have recently been developed to better understand stress urinary incontinence and develop novel treatment alternatives.

MATERIALS AND METHODS

Several animal models for urethral dysfunction have emerged in the last few years, including those based on pathophysiological theories of urethral sphincter dysfunction that were designed to simulate maternal birth trauma. Other models have focused on the creation of a durable model of dysfunction for investigating novel treatments.

RESULTS

Since animals cannot express intent, these animal models have focused on measuring decreased urethral resistance. The most widely used methods are the sneeze test, the tilt table technique and the leak point pressure test. Newer techniques include abdominal leak point pressure, urethral pressure measurement and retrograde urethral perfusion pressure. In addition to the advantages and disadvantages of each technique, all methods measure the composite contribution to urethral resistance from smooth and striated muscle, urethral closure and connective tissue, although none measures intent.

CONCLUSIONS

We critically reviewed the different models of stress urinary incontinence and urethral dysfunction as well as the different methods of measuring urethral resistance.

The neurophysiology of sexual arousal

Our understanding of the process and initiation of sexual arousal is being enhanced by both animal and human studies, inclusive of basic science principles and research on clinical outcomes. Sexual arousal is dependent on neural (sensory and cognitive) factors, hormonal factors, genetic factors and, in the human case, the complex influences of culture and context. Sexual arousal activates the cognitive and physiologic processes that can eventually lead to sexual behavior. Sexual arousal comprises a particular subset of central nervous system arousal functions which depend on primitive, fundamental arousal mechanisms that cause generalized brain activity, but are manifest in a sociosexual context. The neurophysiology of sexual arousal is seen as a bidirectional system universal to all vertebrates. The following review includes known neural and genomic mechanisms of a hormone-dependent circuit for simple sex behavior. New information about hormone effects on causal steps related to sex hormones' nuclear receptor isoforms expressed by hypothalamic neurons continues to enrich our understanding of this neurophysiology.

Time course of neuroanatomical and functional recovery after bilateral pudendal nerve injury in female rats

The pudendal nerve innervates the external urethral sphincter (EUS) and is among the tissues injured during childbirth, which may lead to symptoms of stress urinary incontinence (SUI). To understand the mechanisms of injury and repair, urethral leak-point pressure (LPP) was measured 4 days, 2 wk, or 6 wk after bilateral pudendal nerve crush. Morphometric changes in the distal nerve and EUS were examined by light and electron microscopy. To determine whether recovery resulted from pudendal neuroregeneration, LPP was measured before and after pudendal nerve transection 2 wk after nerve crush. LPP was significantly decreased 4 days after pudendal nerve crush compared with sham-injured animals as well as 2 or 6 wk after nerve crush. LPP was not significantly different 2 or 6 wk after nerve crush compared with sham-injured animals, suggesting that urethral function had returned to normal. Four days after pudendal nerve crush, the EUS branch of the pudendal nerve distal to the injury site showed evidence of nerve degeneration and the EUS appeared disrupted. Two weeks after nerve crush, the distal nerve and EUS both showed evidence of both nerve degeneration and recovery. Two weeks after nerve crush, LPP was significantly decreased after nerve transection. Six weeks after nerve injury, evidence of neuroregeneration was observed in the pudendal nerve and the EUS. This study has demonstrated that functional recovery and neuroregeneration are significant 2 wk after nerve crush, although by anatomical assessment, recovery appears incomplete, suggesting that 2 wk represents an early time point of initial neuroregeneration.

Effects of hysterectomy on bowel function: a three-year, prospective cohort study

PURPOSE

This study was a prospective evaluation of the long-term effects of hysterectomy on bowel function using self-reported outcome measures on symptoms of constipation, rectal emptying difficulties, and anal incontinence.

METHODS

In this prospective cohort study, 120 consecutive patients undergoing hysterectomy for benign conditions answered a questionnaire on bowel habits and anorectal symptoms preoperatively. Forty-four patients underwent vaginal and 76 abdominal hysterectomy. Follow-up was performed one and three years postoperatively. Data were analyzed by using multivariate regression and nonparametric statistics.

RESULTS

The bowel and anorectal survey was answered by 115 of 120 patients (96 percent) after one year and 107 of 120 patients (89 percent) after three years. Abdominal hysterectomy was associated with increased anal incontinence symptoms at one-year (P < 0.01) and three-year follow-up (P < 0.01). Vaginal hysterectomy was not associated with increased anal incontinence symptoms at one year follow-up, although there was a significant increase in incontinence symptoms at the three-year follow-up (P < 0.05). Risk factor analysis indicated that a reported history of obstetric sphincter injury was correlated to an increased risk of developing posthysterectomy anal incontinence (odds ratio, 2.07; 95 percent confidence interval, 1.05-2.87; P < 0.05). There was no significant rise in constipation symptoms or rectal emptying difficulties in either cohort through the follow-up.

CONCLUSIONS

Neither abdominal nor vaginal hysterectomy was associated with constipation, aggravation of constipation, or rectal emptying difficulties three years after surgery. Abdominal and vaginal hysterectomy was, however, associated with an increased risk of mild anal incontinence symptoms, and patients with a reported history of obstetric sphincter injury were at particular risk for posthysterectomy fecal incontinence.

Striated muscle and nerve fascicle distribution in the female rat urethral sphincter

The anatomical basis for urinary continence depends on a thorough understanding of the tissues in the urethra. The objective of this study was to evaluate the morphology and neuroanatomy of urethral striated muscle, called the rhabdosphincter or external urethral sphincter, in normal female rats. Urethras from 12 female rats were dissected from the bladder, fixed, embedded in paraffin or epon, and sectioned every 1 mm. Striated muscle content was taken as the ratio of the striated muscle area to net urethral area. Nerve fascicles containing myelinated axons near the rhabdosphincter were counted and mapped. Both striated muscle content and number of nerve fascicles peak in the proximal third of the urethra, with a secondary peak at the distal end of the urethra. This secondary peak may correspond to an analog of the combined compressor urethrae/urethrovaginal sphincter located in the distal urethra in human. The rhabdosphincter has a variable distribution along the length of the urethra. In the middle and distal thirds of the urethra, the dorsal striated muscle fibers between the urethra and vagina become more sparse. The majority of nerve fascicles are contained in the lateral quadrants of the urethra, similar to the lateral distribution of somatic nerves in humans. In conclusion, this study demonstrates the normal distribution of the striated musculature and neuroanatomy in the urethra, with similarities to the human. It thus supports and extends the usefulness of the rat as an experimental model for studying urinary incontinence.

Effect of estrogen on urethral function and nerve regeneration following pudendal nerve crush in the female rat

PURPOSE

We tested the hypothesis that estrogen promotes improvement in urethral function and nerve regeneration following bilateral pudendal nerve crush in ovariectomized female rats.

MATERIALS AND METHODS

A total of 52 female rats underwent ovariectomy 6 days before bilateral pudendal nerve crush. Estrogen and sham capsules were subcutaneously implanted at the time of nerve crush in 16 and 14 of these rats, respectively, while 22 served as unoperated controls. Seven days following nerve crush urethral LPP testing was performed using urethane anesthesia. Spinal cord sections containing motoneurons of Onufrowicz's nucleus were subjected to in situ hybridization to detect the expression of beta(II) tubulin mRNA, a marker of the neuroregenerative response.

RESULTS

Mean LPP +/- SEM was significantly decreased after pudendal nerve crush in sham treated animals compared to unoperated controls (32.1 +/- 6.8 vs 54.4 +/- 11.6 cm H2O). Rats with an estrogen implant had an LPP of 42.5 +/- 16.8 cm H2O, which was significantly greater than rats given sham implants and significantly less than unoperated controls. Rats that received an estrogen implant had increased beta(II) tubulin mRNA expression compared to those that received a sham implant.

CONCLUSIONS

The results of this research suggest that estrogen given at the time of pudendal nerve crush promotes and facilitates the recovery of urethral function and an increase in the nerve regenerative response. Future studies will include the investigation of molecular pathways activated by estrogen in response to peripheral nerve injury.

Age-related alteration of neurturin receptor GFRa2 and nNOS in pelvic ganglia

Neurturin is a neurotrophic factor that is widely expressed in cavernous tissue and retrogradely transported to penis-projecting neurons via its receptor, glial cell line derived neurotrophic factor family receptor alpha-2 (GFRa2). To investigate the influence of aging on neural function on the penis, we examined the expression of GFRa2 mRNA in the major pelvic ganglion and its relationship to neuronal nitric oxide synthase (nNOS)- and tyrosine hydroxylase (TH)-positive neurons. GFRa2 and nNOS mRNA expression levels in RT-PCR showed age-related decreases in 1-, 3-, 6-, 12-, 18- and 24-month-old rats. In situ hybridization also revealed that the number of GFRa2-positive neurons in pelvic ganglia decreased with aging. A double-labeling study revealed the co-expression of GFRa2 and nNOS, which simultaneously decreased in old adult (24 months) and young castrated rats compared with young adult rats (3 months). These results suggest that aging and castration influence the numbers of nNOS- and GFRa2-positive neurons. Higher age might affect not only cavernous tissue but also the neural plasticity of the cavernous nerve related to erectile function.

Estrogen alters trkA and p75 neurotrophin receptor expression within sympathetic neurons

Survival and growth of sympathetic neurons is regulated by nerve growth factor acting through trkA and p75NTR receptors. Sympathetic neurons are also affected by gonadal steroid hormones, particularly estrogen. To determine if estrogen may influence sympathetic neurons via altered neurotrophin receptor expression, we investigated effects of acute or chronic estrogen administration on levels of trkA and p75NTR proteins, numbers of immunoreactive neurons, and numbers of neurons expressing trkA, p75NTR, and estrogen receptor-alpha transcripts. Superior cervical ganglia from ovariectomized or estradiol-treated rats were processed for in situ hybridization or immunohistochemistry, and percentages of stained neurons quantitated or processed for Western blot analysis. In ovariectomized rats, approximately 50% of sympathetic neurons expressed trkA mRNA and protein. Acute estrogen administration did not affect trkA transcript expression, but reduced trkA protein significantly. Chronic treatment did not alter neuronal trkA expression. Approximately 70% of sympathetic neurons in ovariectomized rats expressed p75NTR transcripts and about 50% showed p75NTR immunoreactivity. Acute estrogen did not affect p75NTR expression. However, chronic estrogen reduced p75NTR mRNA and protein expression significantly. Fifty to sixty percent of sympathetic neurons in ovariectomized rats displayed estrogen receptor-alpha mRNA. After acute estrogen administration, estrogen receptor-alpha transcript expression increased by 35%, although this was not maintained chronically. These findings indicate that estrogen can influence sympathetic neuronal neurotrophin receptor expression as well as estrogen receptor-alpha. Reduced trkA expression after acute estrogen may transiently predispose neurons to degenerative events, while diminished p75NTR expression by chronic estrogen administration may exert long-term effects on survival or axonal outgrowth in sympathetic neurons.

Estrogen selectively increases sensory nociceptor innervation of arterioles in the female rat

Differences exist in vascular function and disease susceptibility in males and females, and estrogen is apparently a primary factor. One mechanism by which estrogen may influence vascular function is by affecting vasomotor innervation. We have shown previously that estrogen increases calcitonin gene-related peptide (CGRP)-immunoreactive sensory innervation of the rat mammary gland, but it is not known if this occurs in other tissues. The objective of this study was to determine if estrogen modulates CGRP-immunoreactive innervation of vascular and non-vascular tissues. Ovariectomized adult virgin female rats were implanted with pellets containing 17beta-estradiol or placebo. After 7 days, innervation was examined in the external ear, jejunal mesenteric arterioles, superficial epigastric, femoral, and uterine arteries, and foot skin. Immunofluorescence microscopy of the external ear pinna revealed increased CGRP-immunoreactive sensory innervation in estrogen-treated rats, and this was attributable specifically to increased innervation of arterioles. Tyrosine hydroxylase-immunoreactive innervation was unchanged. Total nerve density, revealed by the pan-neuronal marker PGP 9.5, was also greater after estrogen treatment, implying structural proliferation of nociceptor vasodilator fibers. Mesenteric arteriolar CGRP-immunoreactive nerve density was also selectively increased by estrogen treatment. However, estrogen did not affect CGRP-immunoreactive nerve density of superficial epigastric, femoral, or uterine arteries, or foot skin. Therefore, estrogen increases sensory innervation of arterioles, but not of large arteries or skin. We conclude that sensory nociceptor vasodilatory innervation of arterioles is selectively enriched by estrogen, which may influence cardiovascular function in health and disease.