Gender and vascular smooth muscle cells

Although gender-associated differences in blood pressure have been repeatedly observed in epidemiological studies, the mechanisms for gender differences in blood pressure control are not totally elucidated [1–3]. Blood pressure is higher in men than in age-matched women and there is a lower incidence of hypertension in pre-menopausal women than in men. Similarly, in hypertensive rat models such as the spontaneously hypertensive rat (SHR) or the New Zealand hypertensive rat, males have higher blood pressure than do females. Furthermore, high salt intake increased blood pressure more rapidly in salt-sensitive male than female Dahl rats. As demonstrated by developmental studies in humans and castration studies in rats, androgens exert a prohypertensive effect while oestrogens appears to oppose the rise in blood pressure [2,3]. This is certainly part of the mechanisms involved but observations such as the minor effect of hormone replacement therapy in postmenopausal women indicate a more complex regulatory pattern. Gender-specific differences in the control of vascular tone have been proposed to result primarily from an oestrogen-dependent enhancement of nitric oxide endothelial release [4] in females. The release of cyclooxygenase-derived constricting factors also appeared to be more pronounced in male than female SHR mesenteric arteries [5]. As vascular tone not only reflects the gender-associated neuro-humoral influences but also the mechanical adaptation of vascular cells to blood pressure and shear forces, it is difficult to determine whether gender-associated smooth muscle cells characteristics participate in or result from this gender-dependent control of contractile response. However, observations that the endothelium-independent arterial dilations to sodium nitroprusside were more pronounced in female than in male SHR indicate that the gender differences in vasodilatation are not restricted to the endothelium-dependent oestrogen-induced stimulation of nitric oxide (NO) production, and that the sensitivity of arterial smooth muscle cells to NO was higher in the female than in the male group [6]. This raises the possibility that, in vascular smooth muscle cells, signal transduction pathways or contractile equipment differ with gender. Special attention was paid to the role of cytosolic Ca2+ signalling in the control of numerous cell functions, including contraction, gene expression and cell proliferation. Murphy and Khalil recently reported a direct influence of gender on smooth muscle cell contractility and Ca2+ handling [7]. This study was performed on cells freshly isolated from the thoracic aorta of intact and gonadectomized male and female Wistar–Kyoto and spontaneously hypertensive rats. Compared to cells from male rats, the contraction triggered by Ca2+ entry from the extracellular space as well as the basal cytosolic [Ca2+] and the maintained phenylephrine or depolarization-induced [Ca2+] levels, were reduced in female rats. Ovariectomy suppressed these differences but oestrogen replacement re-established cell contraction and [Ca2+]i to levels similar to those obtained in cells from intact female rats. In contrast, cell contraction and [Ca2+], due to Ca2+ release from the intracellular stores, was not affected by gender or gonadectomy. This study clearly demonstrates gender-specific changes in vascular smooth muscle cells from female rats. However, it is still unclear whether these differences reflect intrinsic features of smooth muscle cells, or only their adaptation to in-vivo gender-dependent mechanical, neuro-humoral or endothelial influences. A further step in the analysis of gender-dependent changes in smooth muscle cells was to investigate Ca2+ handling in cultured cells, in which the above influences should be attenuated. In the present issue of the journal, Loukotova et al. [8] compared the basal cytosolic free Ca2+ concentration and its response to angiotensin II in cultured vascular smooth muscle cells from male and female spontaneously hypertensive rats [8]. This study was performed in confluent primary cell cultures that express structural and functional characteristics representative of those expressed in the original vessel [9,10]. A major difficulty in this type of study is the marked proliferation-dependent changes in the expression of angiotensin AT1 receptor gene [11] and in that of many Ca2+ transport systems, including various Ca2+ channels and SERCA pump isoforms [12–14]. Therefore, it is crucial to control the phenotype of cells to be compared, especially as vascular smooth muscle cells from male SHR were reported to exhibit a faster growth and a shorter cell cycle than female SHR [15]. Proliferation of vascular smooth muscle cells is associated with modified expression of major contractile protein genes, as well as that of voltage-gated Ca2+ currents and Ca2+ transport systems. Compared to confluent cells, rat aortic proliferating smooth muscle cells exhibit a lowered angiotensin II- and thapsigargin-induced Ca2+ rise with an impaired ryanodyne-sensitive Ca2+ release component [9,10], an increased proportion of T-type Ca2+ channels while the expression of L-type decreased [16], and decreased SERCA2a and ryanodine receptor expression [17]. In human arterial myocytes, cell proliferation has been reported to be associated with higher resting cytosolic [Ca2+] and enhanced capacitative Ca2+ entry [14]. Loukotova et al. [8] observed a greater [Ca2+]i response to angiotensin II in SHR male than female cells, a difference that was abolished in the absence of extracellular calcium, and an increase in thapsigargin-stimulated Ca2+ release in cells from female rat. As only confluent cells were studied, these results can hardly be ascribed to a difference in proliferation rate. These results question the possible gender-dependent direct control of Ca2+ handling in smooth muscle cells and their participation in hypertension and atherosclerosis, which is a proposal that obviously requires further investigation.