Evidence that signal transduction for osmoreception is mediated by stretch-activated ion channels in tilapia

Temperature modulates the osmosensitivity of tilapia prolactin cells

In euryhaline fish, prolactin (Prl) plays an essential role in freshwater (FW) acclimation. In the euryhaline and eurythermal Mozambique tilapia, Oreochromis mossambicus, Prl cells are model osmoreceptors, recently described to be thermosensitive. To investigate the effects of temperature on osmoreception, we incubated Prl cells of tilapia acclimated to either FW or seawater (SW) in different combinations of temperatures (20, 26 and 32 °C) and osmolalities (280, 330 and 420 mOsm/kg) for 6 h. Release of both Prl isoforms, Prl188 and Prl177, increased in hyposmotic media and were further augmented with a rise in temperature. Hyposmotically-induced release of Prl188, but not Prl177, was suppressed at 20 °C. In SW fish, mRNA expression of prl188 increased with rising temperatures at lower osmolalities, while and prl177 decreased at 32 °C and higher osmolalities. In Prl cells of SW-acclimated tilapia incubated in hyperosmotic media, the expressions of Prl receptors, prlr1 and prlr2, and the stretch-activated Ca2+ channel, trpv4,decreased at 32 °C, suggesting the presence of a cellular mechanism to compensate for elevated Prl release. Transcription factors, pou1f1, pou2f1b, creb3l1, cebpb, stat3, stat1a and nfat1c, known to regulate prl188 and prl177, were also downregulated at 32 °C. Our findings provide evidence that osmoreception is modulated by temperature, and that both thermal and osmotic responses vary with acclimation salinity.

Temperature modulates the osmosensitivity of tilapia prolactin cells

In euryhaline fish, prolactin (Prl) plays an essential role in freshwater (FW) acclimation. In the euryhaline and eurythermal Mozambique tilapia, Oreochromis mossambicus, Prl cells are model osmoreceptors, recently described to be thermosensitive. To investigate the effects of temperature on osmoreception, we incubated Prl cells of tilapia acclimated to either FW or seawater (SW) in different temperature (20, 26 and 32°C) and osmolality (280, 330 and 420 mOsm/kg) combinations for 6 h. Release of both Prl isoforms, Prl188 and Prl177, increased in hyposmotic media and were further augmented with a rise in temperature. Hyposmotically-induced release of Prl188 was inhibited at 20°C. In SW fish, mRNA expression of prl188 and prl177 showed direct and inverse relationships with temperature, respectively. In SW-acclimated tilapia Prl cells incubated in hyperosmotic media, Prl receptors, prlr1 and prlr2, and the stretch-activated Ca2+ channel, trpv4, were inhibited at 32°C, suggesting the presence of a cellular mechanism to compensate for elevated Prl release. Transcription factors, pou1f1, pou2f1b, creb3l1, cebpb, stat3, stat1a and nfat1c, known to regulate prl188 and prl177, were also downregulated at 32°C. Our findings provide evidence that osmoreception is modulated by temperature, and that both thermal and osmotic responses vary with acclimation salinity.

Tilapia prolactin cells are thermosensitive osmoreceptors

Prolactin (PRL) cells within the rostral pars distalis (RPD) of euryhaline and eurythermal Mozambique tilapia, Oreochromis mossambicus, rapidly respond to a hyposmotic stimulus by releasing two distinct PRL isoforms, PRL₁₈₈ and PRL₁₇₇. Here, we describe how environmentally relevant temperature changes affected mRNA levels of PRL₁₈₈ and PRL₁₇₇ and the release of immunoreactive prolactins from RPDs and dispersed PRL cells. When applied under isosmotic conditions (330 mOsm/kg), a 6 °C rise in temperature stimulated the release of PRL₁₈₈ and PRL₁₇₇ from both RPDs and dispersed PRL cells under perifusion. When exposed to this same change in temperature, ~50% of dispersed PRL cells gradually increased in volume by ~8%, a response partially inhibited by the water channel blocker, mercuric chloride. Following their response to increased temperature, PRL cells remained responsive to a hyposmotic stimulus (280 mOsm/kg). The mRNA expression of transient potential vanilloid 4, a Ca²⁺-channel involved in hyposomotically-induced PRL release, was elevated in response to a rise in temperature in dispersed PRL cells and RPDs at 6 and 24 h, respectively; prl₁₈₈ and prl₁₇₇ mRNAs were unaffected. Our findings indicate that thermosensitive PRL release is mediated, at least partially, through a cell-volume dependent pathway similar to how osmoreceptive PRL release is achieved.

Generation of GCaMP6s-Expressing Zebrafish to Monitor Spatiotemporal Dynamics of Calcium Signaling Elicited by Heat Stress

The ability of organisms to quickly sense and transduce signals of environmental stresses is critical for their survival. Ca²⁺ is a versatile intracellular messenger involved in sensing a wide variety of stresses and regulating the subsequent cellular responses. So far, our understanding for calcium signaling was mostly obtained from ex vivo tissues and cultured cell lines, and the in vivo spatiotemporal dynamics of stress-triggered calcium signaling in a vertebrate remains to be characterized. Here, we describe the generation and characterization of a transgenic zebrafish line with ubiquitous expression of GCaMP6s, a genetically encoded calcium indicator (GECI). We developed a method to investigate the spatiotemporal patterns of Ca²⁺ events induced by heat stress. Exposure to heat stress elicited immediate and transient calcium signaling in developing zebrafish. Cells extensively distributed in the integument of the head and body trunk were the first batch of responders and different cell populations demonstrated distinct response patterns upon heat stress. Activity of the heat stress-induced calcium signaling peaked at 30 s and swiftly decreased to near the basal level at 120 s after the beginning of exposure. Inhibition of the heat-induced calcium signaling by LaCl₃ and capsazepine and treatment with the inhibitors for CaMKII (Ca²²/calmodulin-dependent protein kinase II) and HSF1 (Heat shock factor 1) all significantly depressed the enhanced heat shock response (HSR). Together, we delineated the spatiotemporal dynamics of heat-induced calcium signaling and confirmed functions of the Ca²⁺-CaMKII-HSF1 pathway in regulating the HSR in zebrafish.

Transcriptional regulation of prolactin in a euryhaline teleost: Characterisation of gene promoters through in silico and transcriptome analyses

The sensitivity of prolactin (Prl) cells of the Mozambique tilapia (Oreochromis mossambicus) pituitary to variations in extracellular osmolality enables investigations into how osmoreception underlies patterns of hormone secretion. Through the actions of their main secretory products, Prl cells play a key role in supporting hydromineral balance of fishes by controlling the major osmoregulatory organs (ie, gill, intestine and kidney). The release of Prl from isolated cells of the rostral pars distalis (RPD) occurs in direct response to physiologically relevant reductions in extracellular osmolality. Although the particular signal transduction pathways that link osmotic conditions to Prl secretion have been identified, the processes that underlie hyposmotic induction of prl gene expression remain unknown. In this short review, we describe two distinct tilapia gene loci that encode Prl177 and Prl188 . From our in silico analyses of prl177 and prl188 promoter regions (approximately 1000 bp) and a transcriptome analysis of RPDs from fresh water (FW)- and seawater (SW)-acclimated tilapia, we propose a working model for how multiple transcription factors link osmoreceptive processes with adaptive patterns of prl177 and prl188 gene expression. We confirmed via RNA-sequencing and a quantitative polymerase chain reaction that multiple transcription factors emerging as predicted regulators of prl gene expression are expressed in the RPD of tilapia. In particular, gene transcripts encoding pou1f1, stat3, creb3l1, pbxip1a and stat1a were highly expressed; creb3l1, pbxip1a and stat1a were elevated in fish acclimated to SW vs FW. Combined, our in silico and transcriptome analyses set a path for resolving how adaptive patterns of Prl secretion are achieved via the integration of osmoreceptive processes with the control of prl gene transcription.

Acute salinity tolerance and the control of two prolactins and their receptors in the Nile tilapia (Oreochromis niloticus) and Mozambique tilapia (O. mossambicus): A comparative study

Osmoregulation in vertebrates is largely controlled by the neuroendocrine system. Prolactin (PRL) is critical for the survival of euryhaline teleosts in fresh water by promoting ion retention. In the euryhaline Mozambique tilapia (Oreochromis mossambicus), pituitary PRL cells release two PRL isoforms, PRL₁₈₈ and PRL₁₇₇, in response to a fall in extracellular osmolality. Both PRLs function via two PRL receptors (PRLRs) denoted PRLR1 and PRLR2. We conducted a comparative study using the Nile tilapia (O. niloticus), a close relative of Mozambique tilapia that is less tolerant to increases in environmental salinity, to investigate the regulation of PRLs and PRLRs upon acute hyperosmotic challenges in vivo and in vitro. We hypothesized that differences in the regulation of PRLs and PRLRs underlie the variation in salinity tolerance of tilapias within the genus Oreochromis. When transferred from fresh water to brackish water (20‰), Nile tilapia increased plasma osmolality and decreased circulating PRLs, especially PRL₁₇₇, to a greater extent than Mozambique tilapia. In dispersed PRL cell incubations, the release of both PRLs was less sensitive to variations in medium osmolality in Nile tilapia than in Mozambique tilapia. By contrast, increases in pituitary and branchial prlr2 gene expression in response to a rise in extracellular osmolality were more pronounced in Nile tilapia relative to its congener, both in vitro and in vivo. Together, these results support the conclusion that inter-specific differences in salinity tolerance between the two tilapia congeners are tied, at least in part, to the distinct responses of both PRLs and their receptors to osmotic stimuli.

Autocrine Positive Feedback Regulation of Prolactin Release From Tilapia Prolactin Cells and Its Modulation by Extracellular Osmolality

Prolactin (PRL) is a vertebrate hormone with diverse actions in osmoregulation, metabolism, reproduction, and in growth and development. Osmoregulation is fundamental to maintaining the functional structure of the macromolecules that conduct the business of life. In teleost fish, PRL plays a critical role in osmoregulation in fresh water. Appropriately, PRL cells of the tilapia are directly osmosensitive, with PRL secretion increasing as extracellular osmolality falls. Using a model system that employs dispersed PRL cells from the euryhaline teleost fish, Oreochromis mossambicus, we investigated the autocrine regulation of PRL cell function. Unknown was whether these PRL cells might also be sensitive to autocrine feedback and whether possible autocrine regulation might interact with the well-established regulation by physiologically relevant changes in extracellular osmolality. In the cell-perfusion system, ovine PRL and two isoforms of tilapia PRL (tPRL), tPRL177 and tPRL188, stimulated the release of tPRLs from the dispersed PRL cells. These effects were significant within 5-10 minutes and lasted the entire course of exposure, ceasing within 5-10 minutes of removal of tested PRLs from the perifusion medium. The magnitude of response varied between tPRL177 and tPRL188 and was modulated by extracellular osmolality. On the other hand, the gene expression of tPRLs was mainly unchanged or suppressed by static incubations of PRL cells with added PRLs. By demonstrating the regulatory complexity driven by positive autocrine feedback and its interaction with osmotic stimuli, these findings expand upon the knowledge that pituitary PRL cells are regulated complexly through multiple factors and interactions.

Hormonal regulation of aquaporin 3: opposing actions of prolactin and cortisol in tilapia gill

Aquaporins (Aqps) are expressed within key osmoregulatory tissues where they mediate the movement of water and selected solutes across cell membranes. We leveraged the functional plasticity of Mozambique tilapia (Oreochromis mossambicus) gill epithelium to examine how Aqp3, an aquaglyceroporin, is regulated in response to osmoregulatory demands. Particular attention was paid to the actions of critical osmoregulatory hormones, namely, prolactin (Prl), growth hormone and cortisol. Branchial aqp3 mRNA levels were modulated following changes in environmental salinity, with enhanced aqp3 mRNA expression upon transfer from seawater to freshwater (FW). Accordingly, extensive Aqp3 immunoreactivity was localized to cell membranes of branchial epithelium in FW-acclimated animals. Upon transferring hypophysectomized tilapia to FW, we identified that a pituitary factor(s) is required for Aqp3 expression in FW. Replacement with ovine Prl (oPrl) was sufficient to stimulate Aqp3 expression in hypophysectomized animals held in FW, an effect blocked by coinjection with cortisol. Both oPrl and native tilapia Prls (tPrl177 and tPrl188) stimulated aqp3 in incubated gill filaments in a concentration-related manner. Consistent with in vivo responses, coincubation with cortisol blocked oPrl-stimulated aqp3 expression in vitro Our data indicate that Prl and cortisol act directly upon branchial epithelium to regulate Aqp3 in tilapia. Thus, within the context of the diverse actions of Prl on hydromineral balance in vertebrates, we define a new role for Prl as a regulator of Aqp expression.

Genome-wide association study reveals novel variants for growth and egg traits in Dongxiang blue-shelled and White Leghorn chickens

This study was designed to investigate the genetic basis of growth and egg traits in Dongxiang blue-shelled chickens and White Leghorn chickens. In this study, we employed a reduced representation sequencing approach called genotyping by genome reducing and sequencing to detect genome-wide SNPs in 252 Dongxiang blue-shelled chickens and 252 White Leghorn chickens. The Dongxiang blue-shelled chicken breed has many specific traits and is characterized by blue-shelled eggs, black plumage, black skin, black bone and black organs. The White Leghorn chicken is an egg-type breed with high productivity. As multibreed genome-wide association studies (GWASs) can improve precision due to less linkage disequilibrium across breeds, a multibreed GWAS was performed with 156 575 SNPs to identify the associated variants underlying growth and egg traits within the two chicken breeds. The analysis revealed 32 SNPs exhibiting a significant genome-wide association with growth and egg traits. Some of the significant SNPs are located in genes that are known to impact growth and egg traits, but nearly half of the significant SNPs are located in genes with unclear functions in chickens. To our knowledge, this is the first multibreed genome-wide report for the genetics of growth and egg traits in the Dongxiang blue-shelled and White Leghorn chickens.

Prolactin 177, prolactin 188, and extracellular osmolality independently regulate the gene expression of ion transport effectors in gill of Mozambique tilapia

This study characterized the local effects of extracellular osmolality and prolactin (PRL) on branchial ionoregulatory function of a euryhaline teleost, Mozambique tilapia (Oreochromis mossambicus). First, gill filaments were dissected from freshwater (FW)-acclimated tilapia and incubated in four different osmolalities, 280, 330, 380, and 450 mosmol/kg H2O. The mRNA expression of Na(+)/K(+)-ATPase α1a (NKA α1a) and Na(+)/Cl(-) cotransporter (NCC) showed higher expression with decreasing media osmolalities, while Na(+)/K(+)/2Cl(-) cotransporter 1a (NKCC1a) and PRL receptor 2 (PRLR2) mRNA levels were upregulated by increases in media osmolality. We then incubated gill filaments in media containing ovine PRL (oPRL) and native tilapia PRLs (tPRL177 and tPRL188). oPRL and the two native tPRLs showed concentration-dependent effects on NCC, NKAα1a, and PRLR1 expression; Na(+)/H(+) exchanger 3 (NHE3) expression was increased by 24 h of incubation with tPRLs. Immunohistochemical observation showed that oPRL and both tPRLs maintained a high density of NCC- and NKA-immunoreactive ionocytes in cultured filaments. Furthermore, we found that tPRL177 and tPRL188 differentially induce expression of these ion transporters, according to incubation time. Together, these results provide evidence that ionocytes of Mozambique tilapia may function as osmoreceptors, as well as directly respond to PRL to modulate branchial ionoregulatory functions.

Endocrine regulation of prolactin cell function and modulation of osmoreception in the Mozambique tilapia

Prolactin (PRL) cells of the Mozambique tilapia, Oreochromis mossambicus, are osmoreceptors by virtue of their intrinsic osmosensitivity coupled with their ability to directly regulate hydromineral homeostasis through the actions of PRL. Layered upon this fundamental osmotic reflex is an array of endocrine control of PRL synthesis and secretion. Consistent with its role in fresh water (FW) osmoregulation, PRL release in tilapia increases as extracellular osmolality decreases. The hyposmotically-induced release of PRL can be enhanced or attenuated by a variety of hormones. Prolactin release has been shown to be stimulated by gonadotropin-releasing hormone (GnRH), 17-β-estradiol (E2), testosterone (T), thyrotropin-releasing hormone (TRH), atrial natriuretic peptide (ANP), brain-natriuretic peptide (BNP), C-type natriuretic peptide (CNP), ventricular natriuretic peptide (VNP), PRL-releasing peptide (PrRP), angiotensin II (ANG II), leptin, insulin-like growth factors (IGFs), ghrelin, and inhibited by somatostatin (SS), urotensin-II (U-II), dopamine, cortisol, ouabain and vasoactive intestinal peptide (VIP). This review is aimed at providing an overview of the hypothalamic and extra-hypothalamic hormones that regulate PRL release in euryhaline Mozambique tilapia, particularly in the context on how they may modulate osmoreception, and mediate the multifunctional actions of PRL. Also considered are the signal transduction pathways through which these secretagogues regulate PRL cell function.

Regulation of epithelial calcium transport by prolactin: from fish to mammals

Among the reported ∼300 biological actions, the established role of prolactin (PRL) is to act as a vertebrate hypercalcemic hormone that regulates epithelial calcium transport in several organs, such as the gills, intestine, and kidney. In fish, PRL stimulates the branchial calcium transport by increasing the activity of Ca(2+)-ATPase. Although this calciotropic hormone also induces hypercalcemia in amphibians, reptiles and birds, little has been known regarding the underlying mechanism. In contrast, the effects of PRL on the epithelial calcium transport in mammals are well documented. In rodents, PRL has been shown to stimulate the renal tubular calcium reabsorption and intestinal calcium absorption, the latter of which is mediated by the PRL-induced upregulation of calcium transporter gene expression and activities. Recently, we demonstrated that the duodenal calcium absorption in lactating rats was markedly enhanced by the suckling-induced PRL surge, presumably to provide calcium for milk production. The cellular and molecular mechanisms of the PRL-stimulated calcium transport in mammals have been elaborated in this review.

The Combinatorial Nature of Osmosensing in Fishes

Organisms exposed to altered salinity must be able to perceive osmolality change because metabolism has evolved to function optimally at specific intracellular ionic strength and composition. Such osmosensing comprises a complex physiological process involving many elements at organismal and cellular levels of organization. Input from numerous osmosensors is integrated to encode magnitude, direction, and ionic basis of osmolality change. This combinatorial nature of osmosensing is discussed with emphasis on fishes.

Differential regulation of TRPV4 mRNA levels by acclimation salinity and extracellular osmolality in euryhaline tilapia

Prolactin (PRL) cells of the euryhaline Mozambique tilapia, Oreochromis mossambicus, are osmoreceptors. Hyposmotically-induced PRL release is mediated by the inward movement of extracellular Ca(2+) through a stretch-activated Ca(2+) channel, which has been recently identified as the transient receptor potential vanilloid 4 (TRPV4). In the present study, changes in plasma PRL, as well as PRL and TRPV4 mRNA expression from the rostral pars distalis (RPD), were measured in fish transferred from seawater (SW) to fresh water (FW) and in fish transferred from FW to SW. The in vitro effects of osmolality on PRL release and on PRL and TRPV4 mRNA expression in dispersed PRL cells were compared between fish adapted to SW and FW. Both the release and expression of PRL fell when fish were transferred to SW and rose when fish were transferred to FW. By contrast, TRPV4 expression increased by 48h after fish were transferred from FW to SW and declined as early as 6h after transfer from SW to FW. A similar pattern was observed in vitro where TRPV4 expression responded positively to an increase in medium osmolality while PRL expression declined. Incubation with the Ca(2+) ionophore, A23187, and the phosphodiesterase inhibitor, IBMX, stimulated PRL release. While both IBMX and A23187 inhibited TRPV4 expression, only A23187 reduced PRL expression. Together, these findings indicate that the expression of TRPV4 mRNA is osmosensitive, increasing as extracellular osmolality rises. Furthermore, these data suggest that TRPV4 expression may be regulated through the same second messenger pathways involved in hyposmotically-induced PRL release.

Osmoreception: perspectives on signal transduction and environmental modulation

Osmoregulation is essential to life in vertebrates and osmoreception is a fundamental element in osmoregulation. Progress in characterizing the mechanisms that mediate osmoreception has been made possible by using a uniquely accessible cell model, the prolactin (PRL) cell of the euryhaline tilapia, Oreochromis mossambicus. In addition to a brief historical overview, we offer a summary of our recent progress on signal transduction and osmosensitivity in the tilapia PRL cell model. Prolactin is a central regulator of hydromineral balance in teleosts in freshwater (FW). Consistent with its essential role in FW osmoregulation, PRL release in tilapia is inversely related to extracellular osmolality, both in vivo and in vitro. Osmotically-driven changes in PRL cell volume control PRL release. A decrease in extracellular osmolality increases cell volume, leading to a rapid influx of Ca(2+) through stretch-activated channels followed by a sharp rise in PRL release. Our recent studies also suggest that cAMP is involved in the osmotic signal transduction, and that acclimation salinity can modulate PRL cell osmosensitivity. Prolactin cells from FW tilapia show a larger rise in PRL release after a reduction in medium osmolality than those from SW fish. Paradoxically, hyposmotically-induced increase in PRL mRNA was observed only in cells from SW fish. Our studies have revealed differences in the abundance of the water channel, aquaporin 3 (AQP3), and the stretch activated Ca(2+) channel, transient receptor potential vanilloid 4 (TRPV4) in PRL cells of FW and SW fish that may explain their differing osmosensitivity and osmoreceptive output in differing acclimation salinities.

Stretch-activated cation channel TRPV4 mediates hyposmotically induced prolactin release from prolactin cells of mozambique tilapia Oreochromis mossambicus

In teleost fish, prolactin (PRL) is an important hormone for hyperosmoregulation. The release of PRL from the pituitary of Mozambique tilapia is stimulated by a decrease in extracellular osmolality. Previous studies have shown that hyposmotically induced PRL release is linked with cell volume changes, and that stretch-activated Ca2+ channels are likely responsible for the initiation of the signal transduction for PRL release. In this study, we identified the stretch-activated Ca2+ channel transient receptor potential vanilloid 4 (TRPV4) from the rostral pars distalis (RPD) of tilapia acclimated to freshwater (FW). TRPV4 transcripts were ubiquitously expressed in tilapia; the level of expression in RPDs of FW-acclimated fish was lower than that found in RPDs of seawater (SW)-acclimated fish. Immunohistochemical analysis of the pituitary revealed that TRPV4 is localized in the cell membrane of PRL cells of both FW and SW tilapia. A functional assay with CHO-K1 cells showed that tilapia TRPV4 responded to a decrease in extracellular osmolality, and that its function was suppressed by ruthenium red (RR) and activated by 4α-phorbol 12,13-didecanoate (4aPDD). Exposure of dissociated PRL cells from FW-acclimated tilapia to RR blocked hyposmolality induced PRL release. PRL release, on the other hand, was stimulated by 4aPDD. These results indicate that PRL release in response to physiologically relevant changes in extracellular osmolality is mediated by the osmotically sensitive TRPV4 cation channel.

Capacitance increases of dissociated tilapia prolactin cells in response to hyposmotic and depolarizing stimuli

Prolactin (PRL) is the major hormonal mediator of adaptation to hyposmotic conditions. In tilapia (Oreochromis mossambicus), PRL cells are segregated to the rostral pars distalis of the anterior pituitary facilitating the nearly pure culture of dissociated PRL cells. Membrane capacitance (C(m)) was recorded at 1Hz or higher for tens of minutes as a surrogate monitor of PRL secretion by exocytosis from cells under perforated patch clamp. The study compares secretory responses to trains of depolarizing clamps (100 at 2.5 Hz, from -70 to +10 mV for 100 ms) to the physiological stimulus, exposure to hyposmotic medium, here a switch from 350 to 300 mOsm saline ([Ca²⁺] 15 mM). Two-thirds of cells tested with each stimulus responded. In response to depolarizing clamps, C(m) increased linearly at an average rate of 7.2 fF/s. The increase was also linear in response to hyposmotic perfusion, but the average rate was 0.68 fF/s. Response to depolarization was reversibly blocked in Ca²⁺-omitted saline, or in saline with 30 μM Cd²⁺. It was unaffected by 0.1 μM tetrodotoxin. By contrast, responses were reduced but not absent during perfusion of hyposmotic saline with Ca²⁺-omitted; 30 μM Cd²⁺ appeared to enhance the hyposmotic response. BAPTA-AM eliminated responses to both stimuli, confirming that secretion was dependent on increases of intracellular [Ca²⁺]. Together with previous observations from this laboratory of [Ca²⁺](i) with simultaneous collection and immunoassay of perfusate for PRL, we conclude that depolarization and hyposmotic stimuli initiate secretion by independent mechanisms.

Involvement of the cAMP messenger system and extracellular Ca(2+) during hyposmotically-induced prolactin release in the Mozambique tilapia

In accord with its role in freshwater osmoregulation, prolactin (PRL) release from the tilapia pituitary is stimulated by small, physiologically relevant reductions in plasma osmolality, a response that is mediated by an acute influx of intracellular Ca(2+) through stretch-activated Ca(2+)channels. In the present study, the role of the calcium and cyclic AMP (cAMP) messenger system in the transduction of a response to a hyposmotic stimulus was examined using dispersed PRL cells and PRL cell membrane preparations from freshwater-acclimated tilapia. When PRL cells were treated with the phosphodiesterase (PDE) inhibitor, 3-isobutyl-1-methylxanthine (IBMX) (100μM), significant increases in cAMP levels and PRL release were observed at 1h. Exposure to reduced medium osmolality (300 mOsmolal) in the presence of IBMX further augmented PRL release. Depletion of Ca(2+) from the incubation medium blocked PRL release even in the presence of IBMX. By contrast, exposure of PRL cells to cholera toxin (CTX), an activator of adenylyl cyclase (AC), stimulated PRL release and cAMP accumulation in both the presence and absence of extracellular Ca(2+). On the other hand, treatment with the Ca(2+) ionophore A23187, which elicits a large rise in intracellular free Ca(2+), reduced cAMP accumulation. Likewise, the AC activity of a PRL cell membrane preparation was reduced as extracellular Ca(2+) concentration increased from 0.1 to 1 μM. These results indicate that: (1) the stimulation of PRL release and cAMP formation by a fall in extracellular osmolality are Ca(2+)-dependent; (2) large increases in intracellular Ca(2+) attenuate cAMP formation; (3) direct agonists of cAMP messenger system, such as cholera toxin, however, stimulate PRL release independently of the extracellular Ca(2+). These findings add to the evidence that the osmosensitive response of the tilapia PRL cell is mediated through a Ca(2+)-dependent mechanism. Nevertheless, the present findings also suggest that tilapia PRL cells have the ability to rapidly augment release PRL both via a Ca(2+)-dependent manner and via a cAMP-dependent pathway in the absence of extracellular Ca(2+).

Characterization of ionic currents and electrophysiological properties of goldfish somatotropes in primary culture

Growth hormone release in goldfish is partly dependent on voltage-sensitive Ca(2+) channels but somatotrope electrophysiological events affecting such channel activities have not been elucidated in this system. The electrophysiological properties of goldfish somatotropes in primary culture were studied using the whole-cell and amphotericin B-perforated patch-clamp techniques. Intracellular Ca(2+) concentration ([Ca(2+)]i) of identified somatotropes was measured using Fura-2/AM dye. Goldfish somatotropes had an average resting membrane potential of -78.4 ± 4.6 mV and membrane input resistance of 6.2 ± 0.2 GΩ. Voltage steps from a holding potential of -90 mV elicited a non-inactivating outward current and transient inward currents at potentials more positive than 0 and -30 mV, respectively. Isolated current recordings indicate the presence of 4-aminopyridine- and tetraethylammonium (TEA)-sensitive K(+), tetrodotoxin (TTX)-sensitive Na(+), and nifedipine (L-type)- and ω-conotoxin GVIA (N-type)-sensitive Ca(2+) channels. Goldfish somatotropes rarely fire action potentials (APs) spontaneously, but single APs can be induced at the start of a depolarizing current step; this single AP was abolished by TTX and significantly reduced by nifedipine and ω-conotoxin GVIA. TEA increased AP duration and triggered repetitive AP firing resulting in an increase in [Ca(2+)]i, whereas TTX, nifedipine and ω-conotoxin GVIA inhibited TEA-induced [Ca(2+)]i pulses. These results indicate that in goldfish somatotropes, TEA-sensitive K(+) channels regulate excitability while TTX-sensitive Na(+) channels together with N- and L-type Ca channels mediates the depolarization phase of APs. Opening of voltage-sensitive Ca(2+) channels during AP firing leads to increases in [Ca(2+)]i.

PRL and GH synthesis and release from the sea bream (Sparus auratus L.) pituitary gland in vitro in response to osmotic challenge

The endocrine factors prolactin (PRL) and growth hormone (GH) are believed to have counteracting effects in the adaption of fish to changes in environmental salinity. In order to further investigate this interaction sea bream were challenged with full seawater (SW) or freshwater (FW) for 7 days and the response of pituitary glands cultured in vitro to an osmotic challenge (230, 275 and 320 mOsm/kg) was assessed. In vitro PRL secretion from pituitaries of SW-adapted fish was unaltered in response to an osmotic challenge, while GH secretion increased in the lowest osmolality (230 mOsm/kg). In contrast, both GH and PRL secretion by pituitaries from FW challenged fish was significantly increased (p<0.01) over that of pituitaries from SW fish at the highest osmolality (320 mOsm/kg). After FW challenge pituitary PRL content and de novo synthesised and released PRL were significantly increased (p<0.01), while total PRL secretion was not different from SW animals. GH pituitary content decreased in FW animals while total secretion and secretion of de novo synthesised protein were significantly increased (p<0.01). In addition, after transfer of fish to FW expression of PRL and GH increased 3- and 2-fold, respectively. Despite the increase in PRL expression, no increase in total PRL secretion occurred and although in gills a 2-fold increase in the osmoregulatory marker, Na(+)/K(+)-ATPase activity was detected, profound haemodilution and a cumulative mortality of 40% occurred in sea bream placed in FW. Taken together the results suggest that the sea bream pituitary gland fails to respond appropriately to the osmotic challenge caused by low salinity and the physiological response evoked in vivo is not enough to allow this species to withstand and adapt to FW.

Extracellular calcium-sensing receptor distribution in osmoregulatory and endocrine tissues of the tilapia

The extracellular calcium-sensing receptor (CaSR) serves an important detector function in vertebrate Ca(2+) homeostasis. In this study, we surveyed using immunohistochemistry the tissue and cellular distribution of the CaSR protein in the Mozambique tilapia (Oreochromis mossambicus) and the Japanese eel (Anguilla japonica). Specifically, we examined receptor expression in ion-transporting barrier tissues that may be directly responsive to extracellular Ca(2+) levels, and in tissues that are implicated in endocrine signaling to homeostatic effectors such as Ca(2+)-transporting epithelia. In tilapia osmoregulatory tissues, CaSR protein is strongly expressed in proximal segments of renal tubule, but not in distal segments (where Na(+),K(+)-ATPase is prominently expressed) or in glomeruli. The receptor was also localized in the ion-transporting mitochondria-rich cells of gill and in ion- and nutrient-transporting epithelia of middle and posterior intestine. Consistent with our earlier RT-PCR assessment of mRNA expression in tilapia, CaSR protein expression was salinity dependent in some osmoregulatory tissues. In tilapia pituitary gland, CaSR expression was observed in the rostral pars distalis (containing prolactin-secreting cells, and in the pars intermedia (containing somatolactin-secreting and melanocyte-stimulating hormone-secreting cells), with notably greater expression in the latter. In the eel, weak immunostaining was seen in the stanniocalcin-secreting cells of the corpuscles of Stannius. Olfactory lobe CaSR expression suggests an environment-sensing role for the receptor. Altogether, these findings support the involvement of CaSR in piscine Ca(2+) homeostasis at the levels of environmental sensing, of integrative endocrine signaling through both hypercalcemic (prolactin, and perhaps somatolactin) and hypocalcemic (stanniocalcin) hormones, and of direct local regulation of Ca(2+)-transporting tissues.

Osmosensitivity of prolactin cells is enhanced by the water channel aquaporin-3 in a euryhaline Mozambique tilapia (Oreochromis mossambicus)

In teleost fish, prolactin (PRL) has important actions in the regulation of salt and water balances in freshwater (FW) fish. Consistent with this role, the release of PRL from the pituitary of the Mozambique tilapia is stimulated as extracellular osmolality is reduced. Stretch-activated calcium-permeant ion channels appear to be responsible for the initiation of the signal transduction that leads to increased PRL release when PRL cells are exposed to reductions in extracellular osmolality. In this study, we examined a possible involvement of the aquaporin-3 (AQP3) water channel in this osmoreceptive mechanism in PRL cells of the tilapia. AQP3 expression levels in the rostral pars distalis of the pituitary, consisting predominantly of PRL cells, were higher in fish adapted to FW than in seawater (SW)-adapted fish. Immunohistochemical studies revealed that AQP3 is located in the cell membrane and perinuclear region of PRL cells, with more intense immunosignals in PRL cells of FW-adapted fish than in those of SW fish. In FW PRL cells, the magnitude of hyposmoticity-induced cell volume increase was greater than that seen in SW PRL cells. Mercury, a potent inhibitor of AQP3, inhibited hyposmoticity-induced cell volume increase and PRL release from FW PRL cells. The inhibitory effect of mercury was partially restored by β-mercaptoethanol, whereas no effect of mercury was observed on PRL release stimulated by a depolarizing concentration of KCl, which induces Ca2+influx and stimulates the subsequent Ca2+-signaling pathway. These results indicate significant contribution of AQP3 to osmoreception in PRL cells in FW-adapted tilapia.

Effect of extracellular osmolality and ionic levels on pituitary prolactin release in euryhaline silver sea bream (Sparus sarba)

In many euryhaline fish, prolactin (PRL) plays a key role in freshwater adaptation. Consistent with this function, the present study showed a remarkable reduction in pituitary PRL content of silver sea bream abruptly transferred to low salinity (6ppt). This reduction in pituitary PRL content followed closely the temporal changes in serum osmolality and ion levels. Serum osmolality, Na(+) and Cl(-) levels of silver sea bream abruptly transferred to hyposmotic salinity (6ppt) were markedly reduced 2h after the transfer. The decline in pituitary PRL content lagged behind the serum changes implying that reduction in pituitary PRL content is a response to the drop in serum ion levels and osmotic pressure. Silver sea bream pituitary cells were dispersed and exposed to a medium with reduced ion levels and osmolality in vitro, and PRL released from pituitary cells was significantly elevated. In hyposmotic exposed anterior pituitary cells, cell volume exhibited a 20% increase when exposed to a medium with a 20% decrease in osmolality. The enlarged pituitary cells did not shrink until the surrounding hyposmotic medium was replaced, a phenomenon suggesting an osmosensing ability of silver sea bream PRL cells for PRL secretion in response to a change in extracellular osmotic pressure. The decrease in pituitary PRL content in vivo and stimulated pituitary PRL release in vitro under reduced osmolality together suggest hyposmotic exposure triggers PRL release from the pituitary.

Osmotic stress sensing and signaling in fishes

In their aqueous habitats, fish are exposed to a wide range of osmotic conditions and differ in their abilities to respond adaptively to these variations in salinity. Fish species that inhabit environments characterized by significant salinity fluctuation (intertidal zone, estuaries, salt lakes, etc.) are euryhaline and able to adapt to osmotic stress. Adaptive and acclimatory responses of fish to salinity stress are based on efficient mechanisms of osmosensing and osmotic stress signaling. Multiple osmosensors, including calcium sensing receptor likely act in concert to convey information about osmolality changes to downstream signaling and effector mechanisms. The osmosensory signal transduction network in fishes is complex and includes calcium, mitogen-activated protein kinase, 14-3-3 and macromolecular damage activated signaling pathways. This network controls, among other targets, osmosensitive transcription factors such as tonicity response element binding protein and osmotic stress transcription factor 1, which, in turn, regulate the expression of genes involved in osmotic stress acclimation. In addition to intracellular signaling mechanisms, the systemic response to osmotic stress in euryhaline fish is coordinated via hormone- and paracrine factor-mediated extracellular signaling. Overall, current insight into osmosensing and osmotic stress-induced signal transduction in fishes is limited. However, euryhaline fish species represent excellent models for answering critical emerging questions in this field and for elucidating the underlying molecular mechanisms of osmosensory signal transduction.

Voltage-gated currents of tilapia prolactin cells

The first recordings of neuron-like electrical activity from endocrine cells were made from fish pituitary cells. However, patch-clamping studies have predominantly utilized mammalian preparations. This study used whole-cell patch-clamping to characterize voltage-gated ionic currents of anterior pituitary cells of Oreochromis mossambicus in primary culture. Due to their importance for control of hormone secretion we emphasize analysis of calcium currents (I(Ca)), including using peptide toxins diagnostic for mammalian neuronal Ca(2+) channel types. These appear not to have been previously tested on fish endocrine cells. In balanced salines, inward currents consisted of a rapid TTX-sensitive sodium current and a smaller, slower I(Ca); there followed outward potassium currents dominated by delayed, sustained TEA-sensitive K(+) current. About half of cells tested from a holding potential (V(h)) of -90 mV showed early transient K(+) current; most cells showed a small Ca(2+)-mediated outward current. I-V plots of isolated I(Ca) with 15 mM [Ca(2+)](o) showed peak currents (up to 20 pA/pF from V(h) -90 mV) at approximately +10 mV, with approximately 60% I(Ca) for V(h) -50 mV and approximately 30% remaining at V(h) -30 mV. Plots of normalized conductance vs. voltage at several V(h)s were nearly superimposable. Well-sustained I(Ca) with predominantly Ca(2+)-dependent inactivation and inhibition of approximately 30% of total I(Ca) by nifedipine or nimodipine suggests participation of L-type channels. Each of the peptide toxins (omega-conotoxin GVIA, omega-agatoxin IVA, SNX482) alone blocked 36-54% of I(Ca). Inhibition by any of these toxins was additive to inhibition by nifedipine. Combinations of the toxins failed to produce additive effects. I(Ca) of up to 30% of total remained with any combination of inhibitors, but 0.1mM cadmium blocked all I(Ca) rapidly and reversibly. We did not find differences among cells of differing size and hormone content. Thus, I(Ca) is carried by high voltage-activated Ca(2+) channels of at least three types, but the molecular types may differ from those characterized from mammalian neurons.

Ionotropic glutamate receptor activation increases intracellular calcium in prolactin-releasing cells of the adenohypophysis

Endocrine cells of the anterior pituitary are controlled by the central nervous system through hormonal interactions and are not believed to receive direct synaptic connections from the brain. Studies suggest that some pituitary cells may be modulated by the neurotransmitter glutamate. We investigated prolactin (PRL)-releasing cells of the anterior pituitary of a euryhaline fish, the tilapia (Oreochromis mossambicus), for the presence of possible glutamate receptors (GluRs). Fura-2 imaging addressed the ability of glutamate to increase intracellular calcium. We observed a dose-dependent increase in intracellular calcium with transient perfusion (1-2 min) of glutamate (10 nM to 1 mM) in two-thirds of imaged cells. This increase was attenuated by the ionotropic GluR antagonist kynurenic acid (0.5-1.0 mM). The increase was also blocked or attenuated by antagonists of L-type voltage-gated calcium channels. The GluR agonist alpha-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA; 100 microM) produced intracellular calcium increases that were reversibly blocked by the selective AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In contrast, the selective agonist N-methyl-D-aspartate (NMDA; 100 microM to 1 mM in magnesium-free solution with 10 microM glycine) had no effect on intracellular calcium. Radioimmunoassays demonstrated that glutamate stimulated PRL release. CNQX but not the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid blocked this release. Antibodies for mammalian AMPA- and NMDA-type GluR produced a similar punctate immunoreactivity in the periphery of PRL cells. However, the NMDA antibody recognized a protein of a different molecular mass in PRL cells compared with brain cells. These results clearly indicate the presence of GluRs on tilapia PRL cells that can stimulate PRL release.

Prolactin Receptor Signaling Mediates the Osmotic Response of Embryonic Zebrafish Lactotrophs

The pituitary hormone prolactin (PRL) regulates salt and water homeostasis by altering ion retention and water uptake through peripheral osmoregulatory organs. To understand the role of osmotic homeostasis in the development of PRL-secreting lactotrophs, we generated germline transgenic zebrafish coexpressing red fluorescent protein directed by Prolactin regulatory elements (PRL-RFP) and green fluorescent protein by the Pro-opiomelanocortin promoter (POMC-GFP). Transparent embryos expressing fluorescent markers specifically targeted to lactotrophs and corticotrophs, the two pituitary lineages involved in teleost osmotic adaptation, allowed in vivo dynamic tracing of pituitary ontogeny during altered environmental salinity. Physiological osmotic changes selectively regulate lactotroph but not corticotroph proliferation during early ontogeny. These changes are not suppressed by pharmacological dopamine receptor blockade but are completely abrogated by morpholino knockdown of the PRL receptor. PRL receptor signaling exerts robust effects on lactotroph development and plays a permissive role in lactotroph osmo-responsiveness, reflecting the dual peripheral and central interactions required for early pituitary development and embryonic homeostasis.

Disparate release of prolactin and growth hormone from the tilapia pituitary in response to osmotic stimulation

In most teleost fishes, prolactin (PRL) plays a key role in freshwater (FW) adaptation, whereas growth hormone (GH) is involved in seawater (SW) adaptation in salmonids and certain euryhaline species including the tilapia, Oreochromis mossambicus. Consistent with its osmoregulatory activity, PRL release increases in response to physiologically relevant reductions in extracellular osmolality. When dispersed PRL and GH cells from FW-acclimatized fish were incubated in media of varying osmolalities, PRL release increased significantly in response to a 12% reduction in medium osmolality during 1 and 4h of exposure. By contrast, cells from SW-acclimatized fish responded only to a 24% reduction in osmolality. Growth hormone release on the other hand increased whether medium osmolality was reduced or raised. Cell volume increased together with PRL release during the perifusion of dispersed PRL cells in direct proportion to the reduction in medium osmolality. Growth hormone release increased whether GH cell volume increased or decreased. In in vivo studies, circulating PRL levels increased as early as 1h after the transfer of fish from SW to FW, whereas GH levels remained unchanged during 24h of acclimatization. These results indicate that while PRL and GH cells are osmosensitive, the PRL cells respond to reductions in extracellular osmolality in a manner that is consistent with PRL's physiological role in the tilapia. While the rise in GH release following the reduction in osmolality is of uncertain physiological significance, the rise in GH release with the elevation of medium osmolality may be connected to its role in SW adaptation.

Gene expression after freshwater transfer in gills and opercular epithelia of killifish: insight into divergent mechanisms of ion transport

We have explored the molecular basis for differences in physiological function between the gills and opercular epithelium of the euryhaline killifish Fundulus heteroclitus. These tissues are functionally similar in seawater, but in freshwater the gills actively absorb Na+ but not Cl-, whereas the opercular epithelium actively absorbs Cl- but not Na+. These differences in freshwater physiology are likely due to differences in absolute levels of gene expression (measured using real-time PCR), as several proteins important for Na+ transport, namely Na+,H+-exchanger 2 (NHE2), carbonic anhydrase 2 (CA2), Na+,HCO3- cotransporter 1, and V-type H+-ATPase, were expressed at 3- to over 30-fold higher absolute levels in the gills. In gills, transfer from 10% seawater to freshwater increased the activity of Na+,K+-ATPase by twofold (from 12 h to 7 days), increased the expression of NHE2 (at 12 h) and CA2 (from 12 h to 7 days), and decreased the expression of NHE3 (from 12 h to 3 days). In opercular epithelium, NHE2 was not expressed; furthermore, Na+,K+-ATPase activity was unchanged after transfer to freshwater, CA2 mRNA levels decreased, and NHE3 levels increased. Consistent with their functional similarities in seawater, killifish gills and opercular epithelium expressed Na+,K+-ATPase alpha 1a, Na+,K+,2Cl- cotransporter 1 (NKCC1), cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel and the signalling protein 14-3-3a at similar absolute levels. Furthermore, NKCC1 and CFTR were suppressed equally in each tissue after freshwater transfer, and 14-3-3a mRNA increased in both. These results provide insight into the mechanisms of ion transport by killifish gills and opercular epithelia, and demonstrate a potential molecular basis for the differences in physiological function between these two organs.

Physiological concentrations of ouabain rapidly inhibit prolactin release from the tilapia pituitary

Ouabain, a cardiac glycoside and inhibitor of Na(+), K(+)-ATPase, is now believed to be a steroid hormone in mammals. We have recently identified ouabain immunoreactivity in the plasma of the tilapia, a euryhaline teleost. Changes in plasma concentrations of immunoreactive ouabain (20-40 pM) in response to salinity change were well correlated with the changes in plasma osmolality and cortisol. Our previous studies have shown that cortisol rapidly inhibits prolactin (PRL) release from the tilapia pituitary by suppressing intracellular Ca(2+) ([Ca(2+)]i) and cAMP. In the present study, low doses of ouabain (10-1000 pM) inhibited PRL release dose-dependently during 2-24 h of incubation. There was no effect on growth hormone (GH) release, except for a significant increase at 1000 pM during 8-24 h of incubation. Significant dose-related increases in PRL release were observed at higher doses of ouabain (100-1000 nM), whereas significant inhibition was seen in GH release at 1000 nM during 2-24h of incubation. Ouabain at 1-100 pM had no effect on Na(+), K(+)-ATPase activity of the pituitary homogenate. The enzyme activity was inhibited by higher concentrations of ouabain, 10% at 1 nM, 15% at 10 nM, 28% at 100 nM, and 45% at 1000 nM. Ouabain also attenuated stimulation of PRL release by the Ca(2+) ionophore, A23187, and by a combination of dibutyryl cAMP and a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthin. Intracellular Ca(2+) concentrations were monitored in the dispersed PRL cells with the Ca(2+)-sensitive dye, fura-2. Ouabain at 1 nM reversibly reduced [Ca(2+)]i within seconds, whereas 1 microM ouabain increased [Ca(2+)]i. A rapid reduction in [Ca(2+)]i was also observed when PRL cells were exposed to 1 microM cortisol, whereas there was no consistent effect at 1 nM. These results suggest that ouabain at physiological concentrations rapidly inhibits PRL release from the tilapia pituitary by suppressing intracellular Ca(2+) and cAMP metabolism. The stimulation of PRL release by high concentrations of ouabain (100-1000 nM) may result from an increase in [Ca(2+)]i, and subsequent depolarization due to the inhibition of Na(+), K(+)-ATPase activity.

Hormone release is tied to changes in cell size in the osmoreceptive prolactin cell of a euryhaline teleost fish, the tilapia, Oreochromis mossambicus

Prolactin (PRL) cells from a teleost fish, the tilapia, Oreochromis mossambicus, facilitate the direct study of osmoreception. The release of two prolactins, PRL(188) and PRL(177), which act in freshwater osmoregulation in teleost fish, rises in vitro within 5 min after extracellular osmolality falls. An increase in cell size accompanied this rise. Cell size and PRL release also increased, albeit more slowly, following the partial replacement of medium NaCl (55 mOsmolal) with an equivalent concentration of urea, a membrane-permeant molecule. Similar replacement using mannitol, which is membrane-impermeant, elicits no response. These findings suggest that osmoreception is linked to changes in cell volume rather than to extracellular osmolality per se.

In vitro effects of cortisol on the release and gene expression of prolactin and growth hormone in the tilapia, Oreochromis mossambicus

Exposure to cortisol inhibits prolactin (PRL) release from the tilapia pituitary within 10-20min through a plasma membrane-associated, non-genomic pathway. In the present study, in vitro effects of cortisol on the release and mRNA levels of two PRLs (PRL(188) and PRL(177)) and growth hormone (GH) were examined in the organ-cultured pituitary of the Mozambique tilapia, Oreochromis mossambicus. The PRL release was significantly greater in hyposmotic (300mOsmolal) than in hyperosmotic (350mOsmolal) medium during the 2-8h of incubation. The mRNA levels of two PRLs, as estimated by RNase protection assay, were increased after 8h in hyposmotic medium. Cortisol (200nM) inhibited the release of two PRLs under hyposmotic conditions within 1h, and the inhibitory effects lasted for 24h. Cortisol also reduced the gene transcription of both PRLs during 2-8h of incubation but not after 24h. No effect of cortisol was observed on PRL release or on its mRNA levels under hyperosmotic condition. There was no significant effect of medium osmolality on the release or mRNA levels of GH during 8h of incubation. However, GH release was significantly stimulated by cortisol after 4h, and the effect lasted for 24h under both hyposmotic and hyperosmotic conditions. Cortisol also caused a significant increase in GH mRNA levels at 8 and 24h. These results suggest that cortisol inhibits PRL release from the tilapia pituitary through non-genomic and also through transcriptional pathways, while stimulating GH release through classical genomically mediated glucocorticoid actions.

Cell volume increase and extracellular Ca2+ are needed for hyposmotically induced prolactin release in tilapia

In the tilapia (Oreochromis mossambicus), as in many euryhaline teleost fish, prolactin (PRL) plays a central role in freshwater adaptation, acting on osmoregulatory surfaces to reduce ion and water permeability and increase solute retention. Consistent with these actions, PRL release is stimulated as extracellular osmolality is reduced both in vivo and in vitro. In the current experiments, a perfusion system utilizing dispersed PRL cells was developed for permitting the simultaneous measurement of cell volume and PRL release. Intracellular Ca(2+) was monitored using fura 2-loaded cells under the same conditions. When PRL cells were exposed to hyposmotic medium, an increase in PRL cell volume preceded the increase in PRL release. Cell volume increased in proportion to decreases of 15 and 30% in osmolality. However, regulatory volume decrease was clearly seen only after a 30% reduction. The hyposmotically induced PRL release was sharply reduced in Ca(2+)-deleted hyposmotic medium, although cell volume changes were identical to those observed in normal hyposmotic medium. In most cells, a rise in intracellular Ca(2+) concentration ([Ca(2+)](i)) during hyposmotic stimulation was dependent on the availability of extracellular Ca(2+), although small transient increases in [Ca(2+)](i) were sometimes observed upon introduction of Ca(2+)-deleted media of the same or reduced osmolality. These results indicate that an increase in cell size is a critical step in the transduction of an osmotic signal into PRL release and that the hyposmotically induced increase in PRL release is greatly dependent on extracellular Ca(2+).