Sex-associated differences in hypertension have been observed repeatedly in epidemiological studies; however, the mechanisms conferring vascular protection to females are not totally elucidated. Sex-related differences in intracellular Ca2+ handling or, more specifically, in mechanisms that regulate Ca2+ entry into vascular smooth muscle cells have been identified as players in sex-related differences in hypertension-associated vascular dysfunction. Recently, new signalling components that regulate Ca2+ influx, in conditions of intracellular store depletion, were identified: STIM1 (stromal interaction molecule 1), which works as an intracellular Ca2+ sensor; and Orai1, which is a component of the CRAC (Ca2+ release-activated Ca2+) channels. Together, these proteins reconstitute store-operated Ca2+ channel function. Disturbances in STIM1/Orai1 signalling have been implicated in pathophysiological conditions, including hypertension. In the present article, we analyse evidence for sex-related differences in Ca2+ handling and propose a new hypothesis where sex-related differences in STIM/Orai signalling may contribute to hypertension-associated vascular differences between male and female subjects.
- sex difference
- stromal interaction molecule (STIM)
Defective regulation of intracellular Ca2+ is a hallmark of hypertension-associated vascular dysfunction and plays a key role in the augmented vascular reactivity, characteristic of clinical and experimental hypertension. The recent identification of new signalling components linking intracellular Ca2+ stores to plasma membrane Ca2+ entry has brought a new insight into the understanding of Ca2+ homoeostasis. STIM1 (stromal interaction molecule 1) is the Ca2+ sensor protein that triggers Ca2+ influx in response to Ca2+ store depletion, whereas Orai is an essential component of CRAC (Ca2+ release-activated Ca2+) channels. Although research on STIM1/Orai signalling is entering an exponential phase of growth, the role of these proteins in vascular dysfunction is unknown.
Mechanisms contributing to hypertension and its associated end-organ dysfunction are differentially regulated in males and females. Critical sex differences are observed in the intrinsic vascular mechanisms that regulate total peripheral resistance, namely that gonadally intact females display less vascular dysfunction associated with experimental hypertension [1–6], compared with gonadally intact males. Although the existence of sex differences in vascular Ca2+ handling is well established [7–10], no studies have addressed differences in vascular STIM1/Orai signalling in male and female hypertensive animals.
Given the importance of STIM1/Orai signalling in intracellular Ca2+ homoeostasis, it seems plausible that increased activation of STIM1/Orai contributes to increased vascular contraction in the vasculature of hypertensive animals.
In the present article, we will review evidence supporting these hypotheses. First, we will address the participation of STIM1 and Orai1 on Ca2+-handling mechanisms during physiological conditions, as well as in hypertension. Then, we will focus on how alteration of STIM1/Orai signalling contributes to differences in Ca2+ handling, and how this impairment may contribute to sex-related differences in vascular function in hypertension.
STIM1/ORAI1 PATHWAY REPRESENTS A KEY Ca2+-HANDLING MECHANISM
Nearly all cell types depend on cytoplasmic Ca2+ signals to trigger specific responses . Upon stimulation, most excitable cells display a biphasic increase in cytosolic Ca2+ concentrations. The initial transient increase is due to Ca2+ release from intracellular stores, e.g. Ins(1,4,5)P3-mediated release of ER (endoplasmic reticulum) Ca2+, whereas the subsequent prolonged increase requires extracellular Ca2+ influx through various pathways. Upon depletion of Ca2+ from the ER, Ca2+ channels are activated in the plasma membrane to refill internal Ca2+ stores. This mechanism, by which the ER acts as a capacitor, leads to the term SOCE (store-operated Ca2+ entry) [12,13]. SOCE carries a highly Ca2+-selective, non-voltage-gated, inwardly rectifying current termed the CRAC current or ICRAC [13,14].
The discovery of new signalling components linking intracellular Ca2+ stores to plasma-membrane Ca2+ entry brought a new insight into the understanding of Ca2+ homoeostasis. STIM1 was identified as a Ca2+ sensor essential for Ca2+-store-depletion-triggered Ca2+ influx [15,16]. Roos et al.  showed that knockdown of STIM in Drosophila S2 cells significantly reduced thapsigargin-dependent Ca2+ entry and completely suppressed ICRAC. In addition to being an ER Ca2+ sensor, STIM1 functions within the plasma membrane to control the operation of the Ca2+ entry channel itself , and STIM1 migrates from the Ca2+ store to the plasma membrane in conditions of store depletion .
It was later demonstrated that Orai1 is an essential pore subunit of the CRAC channel . Accordingly, upon depletion of ER Ca2+ stores, STIM1 and Orai move in a co-ordinated fashion to form closely apposed clusters in the ER and plasma membrane , creating the elementary unit of SOCE . In addition, the interaction between STIM1 and Orai1 is greatly enhanced after thapsigargin treatment, which acts as selective inhibitor of the ER Ca2+-ATPase, resulting in depletion of ER Ca2+ stores .
STIM1/Orai1 pathway alterations and hypertension
Since a rise in [Ca2+]i (intracellular free Ca2+ concentration) is the principal process that initiates contraction of VSMCs (vascular smooth muscle cells) , the maintenance of the steady-state Ca2+ is critically important to maintain vascular tone and, consequently, total peripheral resistance . Defective regulation of intracellular Ca2+ plays a major role in the augmented vascular reactivity . Augmented Ca2+ levels in VSMCs from hypertensive animals can be attributed to various mechanisms, including increased Ca2+ release from intracellular stores , reduced Ca2+ uptake by the SR (sarcoplasmic reticulum) , impaired function of Ca2+-binding proteins , decreased Ca2+ extrusion mechanisms in the plasma membrane , and increased Ca2+ influx [29,30].
In the present article, we will focus on how increased Ca2+ influx through the STIM1/Orai1 pathway may contribute to augmented vascular reactivity in hypertension. We have demonstrated that aortas from SHRSP [stroke-prone SHR (spontaneously hypertensive rats)] had increased force development during Ca2+ loading, upon depletion of intracellular Ca2+ stores . The SR Ca2+ store is larger in aortas from SHRSP due to an enhanced influx of Ca2+ across the sarcolemma, rather than an impaired recycling of the cation by the SR Ca2+-ATPase .
It has been shown that depletion of ER Ca2+ stores induces greater SOCE activation in vascular myocytes from SHRSP compared with that in control WKY (Wistar–Kyoto) rats (Figure 1) . This is associated with augmented vascular contractile responses to Ca2+, which is blocked by molecular (neutralizing antibodies against STIM1 and Orai1) and pharmacological (2-APB and Gd3+) inhibition of STIM1/Orai signalling. In addition, vascular expression of STIM1 (Figure 2) and Orai proteins is increased in SHRSP compared with WKY rats. Thus augmented STIM1/Orai signalling may represent a mechanism leading to impaired control of intracellular Ca2+ in hypertension.
The microvasculature network plays an important role in blood pressure control. During hypertension, the microvasculature displays both functional and structural changes that have been implicated in the augmented peripheral resistance. Additionally, the microvasculature displays myogenic activity in response to increases in blood pressure. It seems likely that increased activation of the STIM1/Orai1 pathway can be a contributor of vascular dysfunction during hypertension. Therefore studies addressing the STIM1/Orai1 pathway in the microvasculature represent an exciting topic that should be carefully evaluated.
SEX DIFFERENCES IN HYPERTENSION
Blood pressure is higher in men than in age-matched women, and there is a lower incidence of hypertension in pre-menopausal women than men [33–36]. Although sex-associated differences during hypertension have been repeatedly observed in epidemiological studies, the mechanisms for sex differences in blood pressure control are not totally elucidated.
Additionally, because Ca2+ triggers VSMC function and its regulation is highly controlled, differences in Ca2+-handling mechanisms have been proposed to explain sex-related differences in vascular function during hypertension , as will be discussed.
Sex differences in hypertension and Ca2+ handling by vascular myocytes
Although sex-associated differences in hypertension are well established, with important differences in the neural, renal and vascular mechanisms associated with blood pressure homoeostasis, the mechanisms that determine differences in blood pressure control in males and females are not totally elucidated. Considering that sex-related differences in mechanisms that regulate Ca2+ entry and storage in VSMCs have been identified, differential Ca2+ handling in VSMCs from males and females may explain sex-related differences in vascular function in hypertension . Accordingly, in aorta from both normotensive and female SHR, Ca2+ influx upon contractile stimuli is decreased compared with that in male SHR . In addition, VSMCs from female rats have reduced Ca2+ entry and decreased depolarization-induced Ca2+ levels compared with those in males . Differences in intracellular Ca2+ increases in VSMCs from male and female SHR are abolished in the absence of extracellular Ca2+ [37–39].
Since augmented STIM1/Orai function may represent one mechanism that contributes to abnormal Ca2+ in VSMCs, we hypothesize that vascular protection in hypertensive females reflects an attenuated signalling through STIM1/Orai in vascular myocytes from hypertensive females (Figure 3).
In agreement with this hypothesis, we have shown that, upon store depletion, force development during Ca2+ loading period is augmented in aortas from male SHRSP compared with female SHRSP . Interestingly, pharmacological blockade of the CRAC channel is able to abolish sex differences in spontaneous contractions during the Ca2+ loading period . Additionally, after store depletion, neutralizing antibodies against STIM1 and Orai1 abolish sex-related differences in spontaneous contractions.
Considering that the female sex hormones oestrogen and progesterone have protective effects in the cardiovascular system [41,42], it will be interesting to determine whether sex steroids modulate STIM1/Orai1 activity/expression. In addition, studies showing whether the effects of sex steroids on this pathway are genomic or non-genomic will help to provide a better understanding of the mechanisms associated with female vascular protection. It is possible that augmented STIM1/Orai signalling represents a mechanism for the increased activation of Ca2+-dependent signalling pathways in arteries from hypertensive animals. Identification of the mechanisms leading to sex differences in hypertension may uncover a regulatory mechanism that may be used to confer cardiovascular protection.
This work was supported by the American Heart Association [grant number 2250383].
Abbreviations: CRAC, Ca2+ release-activated Ca2+; ER, endoplasmic reticulum; ICRAC, CRAC current; SHR, spontaneously hypertensive rats; SHRSP, stroke-prone SHR; SOCE, store-operated Ca2+ entry; SR, sarcoplasmic reticulum; STIM, stromal interaction molecule; VSMC, vascular smooth muscle cell; WKY, Wistar–Kyoto
- © The Authors Journal compilation © 2010 Biochemical Society