Effects of Steroids on Calcium-Activated Potassium Channels - Essay Example

Effects of steroids on calcium activated potassium channels and secretion of LH and FSH in Atlantic cod (Gadus morhua) Instructor’s Name Course Date Effects of steroids on calcium activated potassium channels and secretion of LH and FSH in Atlantic cod (Gadus morhua) Luteinizing and follicle-stimulating hormones (LH & FSH) are vital in regulating vital growth and development processes in vertebrates. These two hormones specifically control gametogenesis and by extension development into puberty. LH and FSH are secreted by specialized hormone producing gonadotropic cells in the pituitary gland. The pituitary gland is comprised of two main compartments, that is, the neural (neurohypophysis) and non-neural (adenohypophysis). LH and FSH gonadotropes are located on the adenohypophysis compartment and there is empirical evidence suggesting a higher level of organization of these cells in teleost than in humans (Ball & Baker, 1969). The secretion of these hormones by the gonadotropes is in turn mediated by the activity of gonadotropin releasing hormone (GnRH) produced by the BPG axis (Borg, 1994). Apart from regulating gametogenesis, these hormones also control steroidogenesis, a process that produces sex steroids involved in moderating gametogenesis (Schulz and Goos, 1999). Another important regulating hormone for the gonadotropins is dopamine, which has been reported as a down regulator in several teleost fish (Borg, 1994). The releasing hormone, GnRH, once produced binds and activates its receptors found on the membrane of the hormone producing cells of the pituitary gland (Bliss, Navratil, Xie, & Roberson, 2010). Contrastingly in teleost, there is a direct innervation of the pituitary gland by GnHR neurons, whereas, in humans and other mammals, the action of GnHR is via neurochemostasis (Mousa & Mousa, 2003). The binding of GnHR to its receptors on the gonadotropes triggers an increase in intracellular calcium ion concentration an action that stimulates the secretion of the hormones by the activated cells. The initial and immediate secretion of the hormones is by exocytosis of hormone vesicles, which is later followed by long term gene transcription to sustain prolonged secretions. The increase in calcium ion concentration is mediated by various mechanisms, such as an influx via voltage-gated channels, and release from intracellular stores. For example, action potentials tend to promote the influx of extracellular calcium ions in voltage-gated calcium ion channels expressing cells. The elevated calcium ions concentration, in the cells activates, calcium activated Potassium channels (CAPC), a pathway that plays a significant role in modulating membrane potentials and thus the electrophysiological effect on hypothalamic releasing hormones (Fettiplace & Fuchs, 1999). CAPCs are present in both neuronal and non-neuronal tissues and have other physiological functions such as mediating neurosecretion, epithelial transport, cell proliferation and cell volume regulation (Ouadid-Ahidouch, Roudbaraki, Delcourt, Ahidouch, & Joury, N, et al, 2004; Vergara, Latorre, Marrion, & Adelman, 1998). Calcium activated potassium channels are a large family of channels and can be categorized into three groups according to their conductance properties. The three categories comprise of the BK (big conductance), IK (intermediate conductance), and SK (small conductance) channels (Vergara, Latorre, Marrion, & Adelman, 1998). Other significant differences among these groups are seen in the sequence of amino acids and pharmacological characteristics. Big conductance channels have bigger conductance ability than intermediate and small conductance channels combined (Kaczorowski, Knaus, Leonard, McManus, & Garcia, 1996). In vivo, these channels can either be activated by intracellular calcium ions or plasma membrane depolarization (Calderone, 2002). Although they have a wide anatomical distribution the majority of these channels are abundant in smooth muscles, pancreas and neural membranes. These channels serve many physiological functions such as modulating action potentials in neurons and regulating smooth muscle tone in circulatory vessels (Fettiplace & Fuchs, 1999). The BK channels are comprised of two subunits; the alpha and beta subunits complexes each with distinct functions. The alpha subunit, for example forms the pore-forming unit (a tetrameric arrangement of alpha subunits) of the channel while the beta subunit is involved in regulatory function (Rohman, Deitcher, & Bass, 2009). The BK channels are encoded by a single gene whose splice variants enhance the functional diversity of the channels. The channel has seven transmembrane domains (formed by the alpha subunit) with the amino and carboxyl terminals located on the extracellular and intracellular sides of the membrane respectively (Meera, Wallner, Song, & Toro, 1997). This folding creates calcium ions binding sites and the binding of a few ions provides the energy for activation of the channel (Sweet & Cox, 2008). The beta subunit on the other regulates sensitivity of the channel to calcium ions and there is evidence suggesting that it regulates channel activation (Meera, Wallner, & Toro, 2000). Unlike the alpha subunit, the beta subunit is encoded by more than two genes and forms one transmembrane loop with intracellular located amine and carboxyl termini (Torres, Morera, Carvacho, & Latorre, 2007). Small conductance channels, on the other hand, are subdivided into three SK1 to SK3 subtypes. Unlike the BK channels, the alpha subunit in SK channels is encoded by more than one gene. The conductance ability of the SK channels is twice lesser than that of the BK channel. SK1 and SK2 subtypes of SK channels are predominantly found in CNS neurons, while SK 3 is expressed in both neuronal and non-neuronal cells (Köhler, Hirschberg, Bond, Kinzie, & Marrion, et al., 1996). The activation of SK channels is by intracellular increase in calcium ions and their activation causes hyperpolarization of the membrane (Vergara, Latorre, Marrion, & Adelman, 1998). This hyperpolarization of the membrane potentially regulates the frequency of action potential since none can be transmitted during this state. SK channels are insensitive to transmembrane voltage and thus cannot be activated by voltage-gated channels like BK channels. Lastly, the intermediate conductance channels have many similarities with the SK channels and were previously identified as SK4 (Latorre, Oberhauser, Labarca, & Alvarez, 1989). These channels are encoded by a single gene like the BK channels, and have conductance ability that lies between the other two channels (Latorre, Oberhauser, Labarca, & Alvarez, 1989). The activation of IK channels is similar to that of SK channels. The channels are predominantly expressed in non-neuronal cells and tissues such as blood cells (Pedarzani, & Stocker, 2008). The IK channels also have various physiological functions some of which include; control of potassium ions balance, and volume regulation in red blood cells (Pedarzani, & Stocker, 2008). There is also evidence that suggest the involvement of IK channels in membrane potential regulation; however, more studies are needed to make it conclusive (Devor, Singh, Frizzell, & Bridges, 1996). In the pituitary gland, the secretion of hormones by gonadotropes is mainly controlled by intracellular calcium ions concentration. The sudden increase in calcium ions in turn activates CAPCs which affect the response of gonadotropes to the GnRH releasing hormone. There is sufficient evidence suggesting the localization of CAPCs in several pituitary cells, which provide further support to the theory of CAPC involvement in LH and FSH secretion (Shipston, Kelly, & Antoni, 1996). However, the expression of these channels in some of the pituitary cells is subgroup dependent. For instance, only BK channels and not SK or IK are expressed in lactotropes of rodents (Van Goor, Zivadinovic, & Stojilkovi, 2001). In Atlantic cod, the presence of these channels in cod pituitary cells has been demonstrated by Haug, Hodne, Weltzien, and Sand, (2007) in electrophysiological studies. The Atlantic cod has differential expression of these membrane proteins on its pituitary gland, which act as an important factor in the control the non-steroid hormones. This analysis is based on Atlantic cod as an animal model because of its’ potential in neural and electrophysiological studies. The cod is a teleost classified under the family of Gadidae and order Gadiformes. Unlike other teleost models, the cod has a long pubertal period a characteristic which can be used to have a closer look at the secretion of hormones involved in adulthood development. As a research model, the cod also exhibits some challenges that have hindered its’ application in some studies. A telling example of these challenges is the precocious sexual maturation and highly unpredictable brood-stock ovulation timing in growth and development research. Development in Atlantic cod is influenced by both intrinsic and extrinsic factors. The intrinsic factors include hormonal modulation such as level of circulating LH and FSH at a given time. Extrinsic factors on the other hand, include a multitude of abiotic factors such as temperature, photoperiod, pH, and oxygen availability among others (Brett, & Groves, 1979). Patterns of LH and FSH secretion during pubertal development show inter-species variation and may not regular pattern (Schulz, & Goos, 1999). For instance LH has a stronger stimulus for the secretion of Testosterone and estradiol in some species of the cod than FSH. In other species, however, FSH is secreted during the early developmental stage while LH is secreted toward the end of the pubertal development (Schulz, & Goos, 1999). A significant advantage of this model over other animals is the ability of differential secretion of the two hormones (LH and FSH) by specialized gonadotropes of the pituitary gland. This implies that the effect of GnRH on the gonadotropes depends on the targeted hormone to be secreted meaning GnRH may only influence the secretion of one of the two hormones at one given time. It is also important to note that the two hormones are not equally secreted in terms of magnitude and there may be time variation depending on the developmental stage. It is the differential secretion of the two hormones that make the Atlantic cod an ideal model for investigating and studying LH and FSH hormonal regulations over humans (tissue cultures) or other animal models. In mammals CAPCs play the essential role in regulating membrane excitability of pituitary cells, however, the role was not supported in gonadotropes of other teleost such as goldfish (Van Goor, Zivadinovic, & Stojilkovi, 2001). Interestingly, contrasting the above observation, a study by Huag, Hodne, Weltzien, and Sand (2007) demonstrated the presence of CAPCs in cod pituitary cells though the exact class of CAPCs is not reported. In follow up studies, Huag, Hodne, Weltzien, and Sand (2007) also demonstrated hyperpolarization of gonadotropes by GnRH, which highly suggestive of the involvement of CAPCs in the secretion of LH and FSH. Corticosteroids are essential mediators of homeostasis and ensure normal physiologic functions of body organs and tissues. Steroids have numerous effects on body system functions such as control of carbohydrate and protein metabolism. In cod, there is differential expression of CAPCs channels, depending on the maturational stage. For example, during pubertal development there is up-regulation of the gene encoding for BK and a down-regulation of the gene encoding SK1 channels. The elevated expression of BK channels enhances membrane excitability due to the probability of activation of BK by voltage gated calcium channel. Consequently membrane excitability increases hormone secretion. Importantly during the pubertal growth stage there is increased synthesis and secretion of gonadal steroids a phenomenon that suggest synergetic effect between the two (steroidal and LH & FSH) hormones. In support of this effect of steroids on CAPCs, mouse gonadotropes pretreated with steroid 17beta-estradiol showed a significant increase in the expression of BK channels when stimulated with GnRH compared to control cells (Waring, & Turgeon, 2006). There is also a correction between the increased excitability of plasma membrane when SK channels are down-regulated with increased GnRH stimulated LH secretion. A possible explanation for this observation in Atlantic cod model is that the increased expression of BK channels may be cell specific. However, it was immediately clear the mechanism by which estradiol influenced the differential expression of the calcium activated potassium channels. In the cod, the seasonal variation in expression of CAPCs is also seen in other hormone secreting cells such as somatotropes suggesting the involvement of other regulatory mechanisms. Differences in expression of CAPCs channels has also been reported between SK and IK subtypes. The effect of steroids (estradiol) on LH secretion is, however, limited to a narrow dose concentration and extremely high levels of steroids have been shown to inhibit the secretion of LH hormone in vertebrates (Sakakura, Takebe & Nakagawa 1975). However, this only act as a negative feedback mechanism and excess steroid may directly regulate the secretion of GnRH and not the CAPC channels. Another possibility of the inhibitory effect of steroids on LH secretion may be mediated through down regulating the stimulatory effect of GnRH on pituitary gonadotropes (Sakakura, Takebe & Nakagawa 1975). There is no evidence suggesting direct stimulatory or inhibitory effect of steroids on CAPCs of either the gonadotropes or other hormone secreting cells. However, that does not rule out that theory and more studies may be needed to shed more light on the matter. In addition sex steroids such as estradiol, dihydrotestosteone and testosterone act as a negative feedback to the brain pituitary axis (BPG) axis (Schulz, & Goos, 1999). In this axis GnRH (produced by the hypothalamus) simulates the pituitary to produce LH and FSH, which stimulates the gonads to produce the sex steroid hormones. In turn these hormones (testosterone and estradiol) have a direct inhibitory effect to both the pituitary and hypothalamus (Schulz, & Goos, 1999). Equally these hormones can also stimulate the some organs to enhance hormonal secretion depending on the developmental stage. Cortisol on the other hand, enhances the secretion of LH and FSH by enhancing the sensitivity of GnRH receptors to GnRH (Schulz, & Goos, 1999). The secretion of Cortisol is increased during pubertal development as an effect to the increased metabolic activity. Progesterone has been reported to induce enhanced pituitary responsiveness to GnRH following a previous exposure to estrogen. This effect has been proposed to be associated direct interactions between GnRH and progesterone receptors that induce a transactivation of membrane receptors (Sakakura, Takebe & Nakagawa 1975). In another mechanism, progesterone moderates the pattern and magnitude of calcium ion signals mediated by GnRH responses in pituitary gonadotrophs (Waring, & Turgeon, 2006). In turn this initial modulation of calcium ion signal alters the subsequent activation of CAPCs, which may impact on LH and FSH secretion. For example, pretreatment of gonadotropes with progesterone shifts the response to biphasic, which would stimulate secretion of LH more than FSH. In the light to the above effects, there is a high likelihood that an introduction to excessive steroids at pre-puberty may potential delay growth into adulthood due to the above effect. However, the above effect of steroids on LH has not been reported on FSH even though it is produced by different hormone secreting cells of the pituitary gland in Atlantic cod. There is a compelling theory that FSH secreting gonadotropes express mainly BK channels while LH secreting cells express SK channels. The difference in composition of CAPC channels provides a partial explanation to the observed effect to steroids. It is also postulated the two gonadotropes (LH and FSH secreting) in the cod differ in their sensitivity to GnRH. For instance, LH cells have shown a typical biphasic response after stimulation with GnRH while FSH cells tend to show a monophasic response. In biphasic response the cells undergo hyperpolarization which is then followed by increased firing frequency, whereas, in a monophasic response, the cells undergo only increased firing frequency. In line with this theory, it is hypothesized that secretion of FSH may require higher doses of the stimulant compared to the stimulus required to secrete LH. Furthermore, inhibition of GnRH activity by immunoneutralization or other method potentially arrests the secretion of LH while it exerts minimal effect on FSH (Fraser, & McNeilly, 1983). This observation suggests different mechanisms of control between the two hormones. However, there is limited literature on FSH and its regulatory mechanisms in mammals as well as teleost and this partly explains the gap in known mechanisms. Apart from steroids FSH is also regulated by a number of other proteins such as activin, inhibin and follistatin (Schwall Nikolics, Szonyi, Gorman, & Mason, 1988). The presence of activin in GnRH blocked tissue culture assays stimulates the secretion of FSH in gonadotropes (Carroll, Corrigan, Gharib, Vale, & Chin, 1989). Follistatin binds to activin and inactivate it thus acting as a negative regulator. In conclusion, development to puberty is controlled and regulated by steroid and non-steroid hormones. Understanding the physiological processes involved would provide essential information that can be applied in several intervention measures. Gonadotropins produced by the pituitary gland play a pivotal role in this developmental process; however, the exact mechanisms are still a grey area. The synthesis and secretion of LH and FSH is mediated by a Gonadotropin releasing hormone and several steroid hormones. GnRH on the gonadotropes stimulates and activates secondary calcium ion messenger pathway, which in turn activates CAPCs. There are three subgroups of CAPCs that exhibit differential expression gonadotropes. These CAPCs directly regulate the secretion of the gonadotropic hormones in Atlantic cod. The cod has different LH and FSH secreting cells and thus is an ideal model organism for these studies. In the cod, the effect of steroids on secretion of the two hormones is dependent on the composition of CAPCs subtypes. There is compelling evidence suggesting a direct and indirect influence of steroids on LH secretion, however, little is known about FSH. It has also been demonstrated that FSH is co-influenced by other proteins such as activin apart from the pituitary GnRH. An ideal research model is, therefore, necessary to elucidate the exact mechanism and effect of steroids on LH and FSH. 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