Urotensin II: lessons from comparative studies for general endocrinology

The importance of combining studies across vertebrates to provide insights into the functionality of hormone systems is considered, using recent advances in Urotensin II (UII) biology to illustrate this. The impact of genome analyses on understanding ligand and UII receptor (UT) structures is reviewed, noting their high conservation from fish to mammals. The early linkage of UII with fish osmoregulatory physiology drove our investigation of possible renal actions of UII in mammals. The kidney is a potential major source of UII in mammals and endogenous peptide appears to have tonal influence over renal excretion of water and electrolytes. Blockade of UII actions by administration of UT receptor antagonist, urantide, in anaesthetised rats, indicates that endogenous UII lowers renal filtration rates and excretion of water and ions. These effects are considered in relation to apparent association of UII with a number of human cardiovascular and renal disorders. Following up the sequencing of UT in mammals here we contrast the first fish UT sequences with those in other species. It is now evident that UT expression in fish osmoregulatory tissues, such as the gill and kidney, exhibits considerable plasticity in response to physiological challenge, providing an important component of the adaptive organismal responses. A number of areas of UII research, which will continue to benefit from moving questions between appropriate vertebrate groups, have been highlighted. These comparative approaches will yield improved understanding and further novel actions of this intriguing endocrine and paracrine system, so highly conserved across the vertebrate series.

Effect of acute restraint on hypothalamic pro-vasotocin mRNA expression in flounder, Platichthys flesus

Arginine vasotocin (AVT) stimulates release of adenocorticotrophin hormone (ACTH) in trout. However, AVT's role in fish hypothalamic-pituitary-interrenal-axis (HPIA) is not fully understood. Here, we examined distribution of AVT and glucocorticoid receptor (GR) in the magnocellular preoptic nucleus (PM) and the AVT/cortisol response to acute restraint in flounder. The GR/AVT distribution in the PM was determined using double immunohistochemistry (IHC). Flounder were confined in nets, immersed in water for 30m, with plasma and tissue samples taken prior to, 3, 24 and 48h post-confinement. Plasma osmolality, Na(+), Cl(-) and cortisol were taken as indicators of HPIA activation. Plasma AVT was measured proVT mRNA expression in the PM was detected using in situ hybridisation (ISH) with a S35 labelled oligoprobe for homologous flounder proVT. Double IHC showed the presence of GR in AVT synthesising neurones of the PM. Plasma Na(+), Cl(-), osmolality and cortisol (1.0+/-0.9 to 183.6+/-3.1mM; p<0.001) increased significantly 3h post-restraint: recovering to control levels after 48h. Plasma AVT levels did not change. However, a concomitant increase in proVT mRNA expression in the magnocellular (PMm) and gigantocellular (PMg) neurones of the PM was observed (11.1+/-1.8 to 55.2+/-9.1% 24h post-restraint; p<0.001) and levels still remained significantly elevated at 48h (p<0.01). This suggests that PMm and PMg AVT neurones are associated with HPIA activation following acute restraint, including potential cortisol negative feedback. The extended elevation of hypothalamus proAVT mRNA expression following a single acute stressor affords a possible mechanism to moderate sensitivity of the HPIA to subsequent challenges.

Salinity adaptation and gene profiling analysis in the European eel (Anguilla anguilla) using microarray technology

The life cycle of the European eel (Anguilla anguilla) includes two long migratory periods, when the newly hatched leptocephali larvae drift on ocean currents from the Sargasso Sea to the shores of Western Europe and then again up to 30 years later when adult eels swim back to their place of birth for reproductive purposes. Prior to the migration from fresh water (FW) to sea water (SW) adult yellow eels undergo various anatomical and physiological adaptations (silvering) which promote sexual development and aid the transition to increased environmental salinities. The aim of this study was to identify and characterise changes in gene expression within the major osmoregulatory tissues of the eel which enable these fish to make the physiological adaptations required for transfer to SW environments. In particular, changes in the expression of the FW-adapting hormone prolactin were correlated with differential expression of known osmoregulatory important genes within the gill, intestine and kidney following the acclimation of eels to SW. Various tissues were sampled from individual fish at selected intervals over a 5-month period following FW/SW transfer and RNA was isolated. Suppressive subtractive hybridization (SSH) was used for enrichment of differentially expressed genes. Microarrays comprising 6144 cDNAs spotted in triplicate, from brain, gill, intestine and kidney libraries (1536 randomly selected clones per tissue library), were hybridized with appropriate targets and analysed. Microarray results were validated using known genes implicated in osmoregulation, such as prolactin, growth hormone, Na, K-ATPase and some unknown genes, the role of which in osmoregulation needs to be elucidated.

Measurement of PTHrP, PTHR1, and CaSR expression levels in tissues of sea bream (Sparus aurata) using quantitative PCR

A quantitative PCR (Q-PCR) method has been established to measure the mRNA expression levels of parathyroid hormone-related protein (PTHrP), parathyroid hormone receptor type 1 (PTHR1), and calcium-sensing receptor (CaSR) in sea bream (Sparus aurata), using the housekeeping gene, beta-actin, as endogenous control. TaqMan primers and probes were designed using the Primer Express program, according to the published/unpublished sequences of the three target genes and beta-actin of sea bream. Different tissues including gill, kidney, duodenum, hindgut, rectum, liver, heart, brain, pituitary, skin, muscle, and gonad were removed and immediately snap-frozen from three juvenile sea bream (100-150 g) cultured in sea water. The mRNAs were extracted and reverse-transcribed into cDNAs, which were subsequently examined by the ABI 5700 system using an optimized Q-PCR method. Triplicate measures of each sample indicated consistency of the technique. However, the mRNA expression levels for each transcript in these tissues were variable between fish and also relatively low. Nevertheless, this methodology can be used in the future studies of factors that may alter gene expression in these tissues.

Arginine vasotocin a key hormone in fish physiology and behaviour: a review with insights from mammalian models

The arginine vasotocin (AVT) neuroendocrine system clearly provides integrative regulation of many aspects of fish physiology and behaviour, including circadian and seasonal biology, responses to stress, metabolism, reproduction, cardiovascular function, and osmoregulation. These are all considered here providing an important context for the design of experiments and interpretation of results for investigations of specific aspects of AVT function. Salt and water balance is a consistent function from fish to mammals and is examined in more detail. Both AVT and AVP secretion is sensitive to hyperosmotic stimuli and associated cellular dehydration, while hypovolaemia would appear less important. AVT and AVP both mediate renal water conservation, though actions involve different receptors and precise targets in fish (V1) and mammals (V2). The actions of AVT to promote gill NaCl extrusion in fish are conserved in the AVP-induced natriuresis in mammalian kidney to support restoration of plasma osmolality. The AVT/AVP regulatory mechanisms involve both altered neurohypophysial peptide secretion and changes in target-tissue receptor expression/modulation of action. Both mechanisms importantly afford integration with the actions of other related hormone systems.

Urotensin II and renal function in the rat

Urotensin II (UII) is a potent vasoactive hormone in mammals. However, despite its well-known effects on epithelial sodium transport in fish, little is known about its actions on the mammalian kidney. The aim of this study was to determine the effects of UII on renal function in the rat. Using standard clearance methods, the effects of rUII and the rat UII receptor (UT) antagonist, urantide, were studied. UII was measured in plasma and urine by radioimmunoassay. UII and UT were localized in the kidney by immunohistochemistry and mRNA expression quantified. Rat urinary [UII] was 1,650-fold higher than that in plasma. Immunoreactive-UII was localized to the proximal tubules, outer and inner medullary collecting ducts (IMCD); UT receptor was identified in glomerular arterioles, thin ascending limbs, and IMCD. UII and UT mRNA expression was greater in the medulla; expression was higher still in spontaneously hypertensive rats (SHRs) associated with raised plasma (UII). Injection of rUII induced reductions in glomerular filtration rate (GFR), urine flow, and sodium excretion. Urantide infusion resulted in increases in these variables. Endogenous UII appears to contribute to the regulation of GFR and renal sodium and water handling in the rat. While hemodynamic changes predominate, we cannot rule out the possibility of a direct tubular action of UII. Increased expression of UII and UT in the SHR suggests that UII plays a role in the pathophysiology of cardiovascular disease.

Urotensin II: Ancient Hormone with New Functions in Vertebrate Body Fluid Regulation

Urotensin II (UII), described in many fish species, is secreted by the caudal neurosecretory system, a unique fish neuroendocrine structure. We have examined UII secretion and its control in euryhaline fish, supporting a proposed role in osmoregulation. However, it is now apparent that UII is present in other vertebrates, including mammals. The 12-amino-acid peptide has been highly conserved and the key cyclic region is common from fish to humans. Our UII radioimmunoassay for flounder, directed to this cyclic region, has shown circulating UII levels in humans and rats comparable with those in fish. In mammals, UII cardiovascular effects vary between species, with vasoconstriction only evident in specific vascular beds. The kidney expresses UII receptors and responds to UII administration by a reduction in glomerular filtration rate, urine flow, and excretion of the major ions. Interestingly, plasma levels of UII are chronically elevated in rat models of hypertension. These observations imply an unforeseen role for this ancient fish hormone in the physiological and perhaps pathophysiological regulation of body fluids in higher vertebrates, including humans.

Altered plasma and pituitary arginine vasotocin and hypothalamic provasotocin expression in flounder (Platichthys flesus) following hypertonic challenge and distribution of vasotocin receptors within the kidney

Plasma AVT concentration, pituitary AVT content, hypothalamic provasotocin mRNA expression and other osmoregulatory parameters were measured in euryhaline flounder 4, 8, and 24 h after the hypertonic challenge of transfer from fresh water (FW) to seawater (SW). Osmolality and the concentration of major plasma ions, sodium and chloride, were significantly higher in fish transferred to SW by comparison with time matched controls, an effect evident within 4 h. By comparison with time matched controls, pituitary store of AVT was lower while plasma AVT concentration was higher 8 and 24 h after transfer to SW. Higher provasotocin mRNA expression in the hypothalamus was also seen at 4 and 8 h in flounder transferred from FW to SW compared with time matched controls. The lower pituitary store and higher circulating levels imply substantial AVT secretion occurs in the early phase response to this hypertonic challenge. Changes in the regulation of AVT synthesis and secretion appeared quickly following movement to SW, consistent with the rapid osmoregulatory response, including reduced urine production that fish require to accommodate the dehydrative water losses and salt loading on exposure to the new hyperosmotic environment. qPCR measures of whole kidney vasotocin receptor mRNA expression indicated similar levels in SW and FW. Immunohistochemistry for the vasotocin receptor in flounder kidney showed localisation on the afferent and efferent arterioles of the glomerulus and on the capillary bed that extends from the efferent arteriole to the smooth muscle surrounding the collecting duct. Localisation of the vasotocin receptor was comparable in SW and FW fish.

Diabetes in rat pregnancy alters renal calcium and magnesium reabsorption and bone formation in adult offspring

AIMS/HYPOTHESIS

We tested the hypothesis that diabetes in pregnancy can result in the in-utero reprogramming of renal calcium and magnesium handling and of bone formation in the offspring, which persists into adulthood.

METHODS

Male offspring of streptozotocin-treated diabetic rats (OD rats) and of control non-diabetic animals (OC rats) were investigated as neonates and at 8, 12 and 16 weeks of age.

RESULTS

Compared with OC rats, urinary calcium and magnesium output was significantly reduced in OD rats at every age studied; Na+ and K+ outputs were unaffected. The renal expression of proteins involved in the tubular reabsorption of calcium (calcium ATPase, calbindin-D28k and epithelial calcium channel) was increased in OD animals compared with that in OC animals. Additionally, we observed that adult OD rats had lower trabecular and higher cortical femoral bone volumes, explained by deposition of bone on the endosteal surface.

CONCLUSIONS/INTERPRETATION

These data show that diabetes in pregnancy has profound effects on male offspring in terms of renal tubular calcium and magnesium reabsorption and the normal pattern of bone formation. These effects persist into adulthood. Such long-lasting effects of diabetes on kidney and the skeleton were not suspected and could have important implications for the health of children born to diabetic women.

Renal morphology of the euryhaline flounder (Platichthys flesus): distribution of arginine vasotocin receptor

The current study characterized tubular segmentation of the European flounder nephron and localized the vasotocin receptor expression by immunohistochemistry. Flounder nephron was shown to comprise a prominent renal corpuscle, short neck segment, proximal tubule I, proximal tubule II, collecting tubule, and collecting duct. Using specific antibodies raised against flounder vasotocin receptor, specific V(1) receptor staining was detected within the glomeruli, the endothelial surface of the afferent and efferent arterioles, and the capillaries surrounding the collecting duct system. Immunostaining for the receptor was exclusively vascular and there did not appear to be a tubular component.

Electrical activity of caudal neurosecretory neurons in seawater and freshwater-adapted Platichthys flesus, in vivo

This study examined the electrical firing activity of neuroendocrine Dahlgren cells in the caudal neurosecretory system (CNSS) of the euryhaline flounder in vivo. Intracellular recordings revealed generally similar activity patterns and membrane properties to those previously reported in vitro. To investigate the potential role of the CNSS in osmoregulatory adaptation, extracellular, multiunit, recordings compared the activity patterns of Dahlgren cells in fully seawater- and freshwater-adapted fish. The proportion of cells showing bursting (as opposed to phasic or tonic) activity was greater in seawater-than in freshwater-adapted fish, as was the Correlation Index, a measure of the degree of correlation between firing activities of cells recorded simultaneously from the same preparation. Acute transfer of fish from seawater to freshwater gill perfusion led to recruitment of previously silent Dahlgren cells and a reduction in Correlation Index;freshwater to seawater transfer increased the Correlation Index. Severing the spinal cord anterior to the CNSS led to an increase in overall Dahlgren cell activity. Electrical stimulation of branchial nerve branches providing input to the brainstem, or tactile (pinch) stimulation of lips or fins, led to a reduction in CNSS activity lasting up to 500 s, indicating the presence of descending modulatory pathways from the brain. These results are consistent with a role for CNSS neuropeptides, urotensins, in supporting survival in a hypertonic, seawater, environment.

Voltage- and Ca2+-dependent burst generation in neuroendocrine Dahlgren cells in the teleost Platichthys flesus

The neuroendocrine Type 1 Dahlgren cells of the caudal neurosecretory system of the flounder display characteristic bursting activity, which may increase secretion efficiency. The firing activity pattern in these cells was voltage-dependent; when progressively depolarized, cells moved from silent (approximately -70 mV), through bursting and phasic to tonic firing (< -65 mV). Brief (10 s) evoked bursts of spikes were followed by a slow after-depolarization (ADP; amplitude up to 10 mV, duration 10-200 s), which was also voltage-dependent and could trigger a prolonged burst. The ADP was significantly reduced in the absence of external Ca(2+) ions or the presence of the L-type Ca(2+) channel blocker, nifedipine. BayK 8644 (which increases L-type channel open times) significantly increased ADP duration, whereas the Ca(2+)-activated nonselective cation channel blocker, flufenamic acid, had no effect. Pharmacological blockade of Ca(2+)-activated K(+) channels, using apamin and charybdotoxin, increased the duration of both ADP and evoked bursts. However, action potential waveform was unaffected by either apamin/charybdotoxin, nifedipine, BayK 8644 or removal of external Ca(2+). The short duration (approximately 100 ms), hyperpolarization-activated, postspike depolarizing afterpotentials (DAP), were significantly reduced by nifedipine. We propose that long duration ADPs underlie bursts and that short duration DAPs play a role in modulation of spike frequency.

Electrical activity of caudal neurosecretory neurons in seawater- and freshwater-adapted flounder: responses to cholinergic agonists

The caudal neurosecretory system (CNSS) of the euryhaline flounder is involved in osmoregulatory responses underlying adaptation to seawater and freshwater. This study compared electrophysiological activity and responses to cholinergic agonists in the neuroendocrine Dahlgren cells in an in vitro preparation taken from fully seawater- (SWA) or freshwater-adapted(FWA) fish. Resting membrane and action potential parameters showed few differences between SWA and FWA cells. The hyperpolarisation-activated sag potential and depolarising afterpotential were present under both conditions;however, amplitude of the latter was significantly greater in SWA cells. The proportions of cells within the population exhibiting different firing patterns were similar in both adaptation states. However, bursting parameters were more variable in FWA cells, suggesting that bursting activity was less robust. The muscarinic agonist, oxotremorine, was largely inhibitory in Dahlgren cells, but increased activity in a non-Dahlgren cell population,α neurons. Nicotine promoted bursting activity in SWA Dahlgren cells,whereas it inhibited over half of FWA cells.

Determination of tissue and plasma concentrations of PTHrP in fish: development and validation of a radioimmunoassay using a teleost 1-34 N-terminal peptide

A specific and sensitive radioimmunoassay (RIA) for the N-terminus of sea bream (Sparus auratus) and flounder (Platichthys flesus) parathyroid hormone-related protein (PTHrP) was developed. A (1-34) amino-terminal sequence of flounder PTHrP was synthesized commercially and used as the antigen to generate specific antiserum. The same sequence with an added tyrosine (1-35(Tyr)) was used for iodination. Human (1-34) parathyroid hormone (PTH), human (1-34) PTHrP, and rat (1-34) PTHrP did not cross-react with the antiserum or displace the teleost peptide. Measurement of PTHrP in fish plasma was only possible after denaturing by heat treatment due to endogenous plasma binding activity. The minimum detectable concentration of (1-34) PTHrP in the assay was 2.5 pg/tube. The level of immunoreactive (1-34) PTHrP in plasma was 5.2+/-0.44 ng/ml (mean+/-SEM, n=20) for flounder and 2.5+/-0.29 ng/ml (n=64) for sea bream. Dilution curves of denatured fish plasma were parallel to the assay standard curve, indicating that the activity in the samples was indistinguishable immunologically from (1-34) PTHrP. Immunoreactivity was present, in order of abundance, in extracts of pituitary, oesophagus, kidney, head kidney, gills, intestine, skin, muscle, and liver. The pituitary gland and oesophagus contained the most abundant levels of PTHrP, 37.7+/-6.1 ng/g wet tissue and 2.3+/-0.7 ng/g wet tissue, respectively. The results suggest that in fish PTHrP may act in a paracrine and/or autocrine manner but may also be a classical hormone with the pituitary gland as a potential major source of the protein.

Rapid stimulation of cyclic AMP production by aldosterone in rat inner medullary collecting ducts

Aldosterone stimulates sodium transport in the inner medullary collecting duct (IMCD) via the classic genomic pathway, but it is not known whether it also acts via a rapid, non-conventional pathway in this part of the nephron. The IMCD regulates the final sodium content of urine and expresses vasopressin receptors coupled to adenylate cyclase. The recently reported rapid, non-genomic actions of aldosterone have been associated mainly with an increase in intracellular Ca(2+); however, it has also been shown to stimulate camp generation. Thus the aim of this study was to determine whether aldosterone stimulates rapid generation of cAMP in isolated IMCD segments. IMCD segments were microdissected from Sprague-Dawley rat kidneys and incubated at 37 degrees C for 4 min with aldosterone (10(-12) to 10(-6) M), vasopressin (10(-12) to 10(-6) M), or a combination of hormones in the presence of a phosphodiesterase inhibitor. cAMP was measured by radioimmunoassay. While corticosterone and dexamethasone were ineffective, aldosterone stimulated a dose-dependent increase in cAMP within 4 min (P<0.05). This action of aldosterone was not inhibited by the MR antagonist spironolactone. Co-incubation of aldosterone with vasopressin resulted in a further increase in cAMP generation above that induced by the neurohypophysial hormone alone. Aldosterone-mediated cAMP generation was not inhibited by a vasopressin V(1) or V(2) receptor antagonist. These data support a novel and rapid, non-genomic effect of aldosterone in IMCD. Aldosterone does not apparently interact with the vasopressin receptor to stimulate cAMP generation.

Neurohypophysial hormones and renal function in fish and mammals

The two major basic neurohypophysial peptides, arginine vasopressin (AVP) of mammals and arginine vasotocin (AVT) of all non-mammalian vertebrates, share common structure and major roles in regulating renal function. In this review the complexity of AVP actions within the mammalian kidney is discussed and comparisons are made with the emerging picture of AVT's renal effects in fish. It has become apparent that the antidiuretic action of the neurohypophysial hormones is an ancient phylogenetic phenomenon, although this is based upon reduced glomerular filtration in fish by comparison with predominant tubular effects in mammals. Nonetheless, there appears to be retention of AVP effects upon the functional heterogeneity of nephron populations in mammals. Preliminary evidence for the possible existence of V(2)-type (tubular) neurohypophysial hormone receptors in fish, implies possible AVT actions which parallel those in mammals on tubular ion transport. Further insight from recent mammalian tubule microperfusion studies suggests that in teleost fish both apical (tubular lumen) and basolateral (blood borne) AVT have the potential to modulate renal function, though this remains to be examined.

Calcium-sensing receptors and parathyroid hormone-related protein in the caudal neurosecretory system of the flounder (Platichthys flesus)

The caudal neurosecretory system of the flounder (Platichthys flesus) has been examined by immunocytochemistry and in situ hybridization for the expression of parathyroid hormone-related protein (PTHrP) and calcium-sensing receptors (CaSR). The N-terminus nucleotide and deduced amino acid sequences of flounder PTHrP were determined and used to prepare oligonucleotide probes and homologous antiserum. The Dahlgren cells of the posterior spinal cord and their axons contained PTHrP protein which was also detected around the capillaries of the urophysis. PTHrP gene expression was abundant in the Dahlgren perikarya and axons in the spinal cord, but it was absent from nerve endings in the urophysis. Calcium-sensing receptor protein was present in the Dahlgren perikarya and axons, also with abundant gene expression, but there was neither protein nor mRNA in the urophysis. There were no apparent differences between freshwater- and seawater-adapted fish in either CaSR or PTHrP expression in the caudal neurosecretory system. These observations suggest that Dahlgren cells produce PTHrP which may be released from axons abutting capillaries in the urophysis. However, the sensing of ionic calcium appears to be confined to the perikarya of the Dahlgren cells in the spinal cord neuropil, suggesting that they are responsive to calcium in the central nervous system rather than the general circulation.

Plasma concentrations of arginine vasotocin and urotensin II are reduced following transfer of the euryhaline flounder (Platichthys flesus) from seawater to fresh water

Plasma concentrations and stored levels of the neuroendocrine peptides arginine vasotocin (AVT) and urotensin II (UII) were measured in the euryhaline flounder (Platichthys flesus) following the acute hypo-osmotic challenge of direct seawater (SW) to fresh water (FW) transfer. Hormone measures, plasma osmolality, and ion concentrations and tissue water content were determined 1, 4, 8, 24, 72, and 144 h after transfer. Plasma AVT concentration fell initially following FW transfer but then returned toward pretransfer levels by day 6. Plasma UII concentration decreased while urophysial UII content was increased following hypo-osmotic challenge relative to SW time-matched controls, suggesting down regulation of the UII system during the initial stages after FW transfer. These changes in neuroendocrine activity were associated with a significant fall in plasma osmolality and major plasma ions. Positive correlations were observed between plasma AVT and osmolality and Cl- and Mg2+ concentrations, suggesting functional association of these plasma parameters with AVT action and/or control of AVT secretion. The initial response to hypotonic challenge involves reduced plasma AVT and UII levels consistent with the proposed role for these hormones, supporting flounder osmoregulation in hypertonic media.

Day-night variations in plasma melatonin and arginine vasotocin concentrations in chronically cannulated flounder (Platichthys flesus)

Chronically catheterised, free swimming flounder (Platichthys flesus) have been used in experiments examining the day-night variations in circulating levels of melatonin (Mel) and arginine vasotocin (AVT). Under normal photoperiod (16 h light/8 h dark) serial blood samples taken from individual fish demonstrated a Mel rhythm with daytime levels at 09.00 and 15.00 h (238+/-14 and 179+/-12 fmol x ml(-1), respectively) lower than those at 23.00 h (1920+/-128 fmol x ml(-1)). Maintenance of fish in 24-h light abolished the light/dark Mel rhythm and circulating levels were comparable to those measured during the day in fish under normal photoperiod illumination. In fish maintained under 24 h dark, although a daily rhythm was still apparent, at the time when it would be normally dark, plasma Mel concentration was reduced and at times when it would be normally light, levels were higher than in fish maintained under normal light/dark illumination. Plasma AVT concentrations were higher in fish during the day (4.4+/-0.8 fmol x ml(-1)) than those at night (1.5+/-0.4 fmol x ml(-1)), the opposite to that seen with Mel. During acute study infusion of AVT resulted in reduced levels of plasma Mel, although this did not achieve statistical significance. Infusion of Mel did not alter circulating AVT concentration.

Bursting properties of caudal neurosecretory cells in the flounder Platichthys flesus, in vitro

Bursting activity in type 1 Dahlgren cells was studied using intra- and extracellular recording from an in vitro preparation of the caudal neurosecretory system of the euryhaline flounder. 45% of cells showed spontaneous bursts of approximately 120s duration and 380s cycle period. Similar bursts were triggered by short duration (&lt;5s) depolarising or hyperpolarising pulses. Cells displayed a characteristic depolarising after potential, following either an action potential with associated afterhyperpolarisation, or a hyperpolarising current pulse. This depolarising after potential was related to a ‘sag’ potential, which developed during the hyperpolarising pulse. Both the depolarising after potential and the sag potential occurred only in cells at more depolarised (&lt;60mV) holding potentials. In addition, the amplitude of the depolarising after potential was dependent on the amplitude and the duration of the hyperpolarising pulse. The depolarising after potential following action potentials may provide a mechanism for facilitating repetitive firing during a burst. Extracellular recording revealed similar bursting in individual units which was not, however, synchronised between units. Spontaneous bursting activity recorded both intra- and extracellularly was inhibited by application of a known neuromodulator of the system, 5-hydroxytryptamine. This study provides a basis for investigating the relationship between physiological status, Dahlgren cell activity and neuropeptide secretion.

Do circulating plasma AVT and/or cortisol levels control pulsatile urea excretion in the gulf toadfish (Opsanus beta)?

Previous work has shown that pulsatile urea excretion at the gills of the gulf toadfish is due to periodic activation of a facilitated diffusion transport system with molecular and pharmacological similarity to the UT-A transport system of the mammalian kidney. In mammals, AVP and glucocorticoids are two important endocrine regulators of this system. The present study focused on the potential role of circulating AVT (the teleost homologue of AVP) and cortisol levels as possible triggers for urea pulses. Long-term (34-84 h) monitoring of plasma levels by repetitive sampling at 2-h intervals from chronic cannulae in individual toadfish demonstrated that circulating AVT concentrations are low (10(-12)-10(-11) M), and show no relationship to the occurrence of natural urea pulses. In contrast, plasma cortisol levels decline greatly prior to natural pulses and rise rapidly thereafter. AVT injections into the caudal artery or ventral aorta elicited pulse events, but these were extremely small (1-10%) relative to natural pulses, and occurred only at unphysiological dose levels (10(-9) M in the plasma). AVP was a partial agonist, but isotocin, insulin-like growth factor-1, and atrial natriuretic peptide were without effect at the same concentration. Artificially raising plasma cortisol levels by cortisol injection tended to reduce responsiveness to AVT. Pharmacological reduction of plasma cortisol levels by metyrapone injection elicited small pulses similar to those caused by AVT. Following such pulse events, AVT was ineffective in inducing pulses. We conclude that decreases in circulating cortisol play an important permissive role in urea pulsing, but that circulating AVT levels are not involved.

The caudal neurosecretory system: control and function of a novel neuroendocrine system in fish

The caudal neurosecretory system (CNSS) of fish was first defined over 70 years ago yet despite much investigation, a clear physiological role has yet to be elucidated. Although the CNSS structure is as yet thought to be confined to piscine species, the secreted peptides, urotensins I and II (UI and UII), have been detected in a number of vertebrate species, most recently illustrated by the isolation of UII in humans. The apparent importance of these peptides, suggested by their relative phylogenetic conservation, is further supported by the complex control mechanisms associated with their secretion. The CNSS in teleosts is known to receive extensive and diverse innervation from the higher central nervous system, with evidence for the presence of cholinergic, noradrenergic, serotonergic, and peptidergic descending inputs. Recent observations also suggest the presence of glucocorticoid receptors in the flounder CNSS, supporting previous evidence for a possible role as a pituitary-independent mechanism controlling cortisol secretion. The most convincing evidence as to a physiological role for the CNSS in fish has stemmed from the direct and indirect influence of the urotensins on osmoregulatory function. Recent advances allowing the measurement of circulating levels of UII in the flounder have supported this. In addition, there is evidence to suggest some seasonal variation in peptide levels supporting the notion that the CNSS may have an integrative role in the control of coordinated changes in the reproductive, osmoregulatory and nutritional systems of migratory euryhaline species.

Effects of ethanol on the changes in renal fluid and electrolyte handling and kidney morphology induced by long-term chloroquine administration to rats

This study investigated the effects of long-term chloroquine and ethanol administration on renal fluid and electrolyte handling and kidney structure. Male Sprague-Dawley rats were orally administered with chloroquine diphosphate (20 microg kg(-1) bw) and/or ethanol (1.6 g kg(-1) bw) every third consecutive day for 4 weeks. Urine volume and total urinary outputs of Na+ and K+ were determined from 24-h samples. For detailed renal studies, rats were subsequently anaesthetised and challenged with a continuous jugular infusion of 0.077 M NaCl at 150 microl min(-1) 24 h after the last treatment. After a 3-h equilibration period, urine flow, Na+ and K+ excretion rates were determined over a 4-h period. Plasma concentrations of AVP and aldosterone were measured in unanaesthetised rats and in anaesthetised rats after hypotonic saline infusion. In separate groups, the rats were anaesthetised with an overdose of ether after 4 weeks of treatment and part of the right kidney was quickly collected and routinely processed for light microscopy. Chloroquine decreased Na+ excretion and increased plasma aldosterone concentrations in anaesthetised rats. Ethanol alone did not alter urinary Na+ outputs or aldosterone levels. Combined chloroquine and ethanol increased renal Na+ excretion, but did not affect plasma aldosterone levels. In unanaesthetised animals all treatments increased aldosterone levels by comparison with control rats. Urinary Na+ excretion was decreased by separate administration of either chloroquine or ethanol, but increased by combined treatment. Microscopic studies showed that concurrent chloroquine and ethanol administration induced extensive damage of the proximal tubule and collecting ducts cells. The results of this study suggest that alcohol consumption and chloroquine administration could result in diminished renal function possibly due to alteration of renally active hormones or kidney morphology.

Cloning of pro-vasotocin and pro-isotocin cDNAs from the flounder Platichthys flesus; levels of hypothalamic mRNA following acute osmotic challenge

Sequences coding for pro-vasotocin and pro-isotocin have been identified by screening a flounder (Platichthys flesus) hypothalamic cDNA library. The 1074-bp proVT and 727-bp proIT sequences contain a signal peptide and hormone, connected to a neurophysin by a Gly-Lys-Arg sequence. Both sequences also have an elongated carboxyl-terminal with a leucine-rich core resembling copeptin but lacking the amino terminal Arg residue. The levels of pro-vasotocin and pro-isotocin mRNA in the hypothalamus were measured concomitantly with pituitary AVT content and plasma AVT concentration following acute transfer of fish between freshwater and seawater. Three days after transfer from seawater to freshwater there appears to be a down regulation of the AVT hormone system with a fall in hypothalamic pro-vasotocin mRNA levels, an increase in pituitary AVT content, and a fall in plasma levels, but these changes did not achieve statistical significance compared to controls. No change in the AVT system was detected 3 days following the transfer of fish from freshwater to seawater. Hypothalamic isotocin mRNA levels did not change following hypo- or hyperosmotic challenge.

Chloroquine inhibits arginine vasopressin production in isolated rat inner medullary segments induced cAMP collecting duct

Previous studies showed that acute chloroquine administration increases plasma arginine vasopressin (AVP) concentration in the rat without influencing urine flow rate. The present study was designed to investigate whether chloroquine inhibits the AVP-induced cAMP production that mediates the antidiuretic effects of vasopressin. Single inner medullary collecting duct (IMCD) segments were pre-incubated at 35 degrees C for 10 min followed by 4 min at 37 degrees C with combinations of AVP and/or chloroquine with 1 mM 3-isobutyl-I-methylxanthine (IBMX) and cAMP concentrations were measured by radioimmunoassay. To establish the possible site of interference in cAMP production IMCD segments were incubated in the presence of chloroquine and forskolin. Chloroquine at concentrations ranging from 10(-9) M to 10(-6) M did not affect cAMP production by comparison with control. However, AVP (10(-8) M) and forskolin (10(-6) M) significantly (p < 0.01) increased cAMP accumulation. Chloroquine at all concentrations significantly suppressed the AVP stimulated cAMP production (e.g., chloroquine (10(-8) M) + AVP (10(-8) M) 41 +/- 12 fmol/4 mm (n = 9 tubules) vs. AVP (10(-8) M) alone 82 +/- 9 fmol/4 min/mm (n = 37 tubules). Chloroquine at all concentrations tested did not have any effect an forskolin-induced cAMP production. The data suggest that chloroquine inhibits the AVP induced cAMP production at the level of hormone/receptor complex. This possibly explains the previously reported lack of the normal antidiuretic responses of AVP in rats following chloroquine administration.

Renal electrolyte and fluid handling in the rat following chloroquine and/or ethanol administration

We postulated that chloroquine and/or ethanol affect plasma arginine vasopressin (AVP) concentrations to alter renal function. Therefore, we studied the effects of chloroquine and/or ethanol on plasma AVP concentrations and fluid, urinary Na(+) and K(+) outputs in separate groups of anaesthetized Sprague-Dawley (SD) rats challenged with a continuous jugular infusion of 0.077 M NaCl at 150 microl.min(-1). After a 3-h equilibration period, vehicle, chloroquine (0.06 microg. min(-1)), ethanol (2.4 or 24 microg.min(-1)) or both chloroquine and ethanol were added to the infusate after 1 h (control) for 1 h 20 min (treatment). The animals were switched back to the infusate alone for the final 1 h 40 min recovery periods. Urine flow Na(+) and K(+) excretion rates were determined at 20-min intervals over the subsequent 4-h postequilibration period. Blood was collected from separate groups of animals at the end of treatment period or equivalent time for control animals for measurement of plasma aldosterone and AVP concentrations by radioimmunoassay. Simultaneous chloroquine and ethanol infusion significantly (p < 0.01) increased plasma chloroquine concentrations in an ethanol dose-dependent manner by comparison with animals administered chloroquine alone. Chloroquine infusion alone (0.06 microg.min(-1)) and/or ethanol (2.4 or 24 microg.min(-1)) elevated plasma AVP concentrations from 9.73 +/- 1.64 fmol.l(-1) in control rats to 15.65 +/- 2.49 fmol.l(-1), 17. 39 +/- 4.21 fmol.l(-1), and 33.87 +/- 6.18 fmol.l(-1), respectively. Separate administration of chloroquine or ethanol at low dose rates increased urinary Na(+) excretion rates. We conclude that the impairment of renal electrolyte handling associated with chloroquine administration may be exacerbated by ethanol.

Altered calbindin mRNA expression and calcium regulating hormones in rat diabetic pregnancy

Offspring of rats with diabetes mellitus are at risk of reduced calcium and bone mineral content. Altered expression of the maternal calcium binding proteins, calbindin-D(9K) and calbindin-D(28K), which are involved in renal and placental calcium transport, may underlie these problems.We have investigated the effect of diabetes on circulating concentrations of regulatory hormones with respect to calbindin-D mRNA concentrations. Three rat groups were studied; control (CP), streptozotocin-induced diabetic (DP), and insulin-treated diabetic (DPI) pregnant rats. Calbindin-D(9K) and calbindin-D(28K) mRNA abundance in placenta and maternal kidney were measured at days 7, 15, 18 and 21 of gestation, together with serum or plasma concentrations of 1,25 dihydroxyvitamin D(3) (1, 25(OH)(2)D(3)), parathyroid hormone (PTH), PTH-related protein (PTHrP), calcitonin, oestradiol and IGF-I. An increase in placental calbindin-D(9K) mRNA abundance between days 18 and 21 in CP and DPI rats was severely blunted in the DP rats. In contrast, renal calbindin-D(28K) mRNA abundance was greater at days 7, 15 and 18 in DP compared with CP rats, as was calbindin-D(9K) at day 18. Calcitonin concentrations showed no differences between the groups, and both PTH and IGF-I were reduced over the first half of gestation, unlike the calbindins. In contrast, the concentrations of PTHrP and 1,25(OH)(2)D(3) were reduced at term in the DP group compared with the other two groups. Plasma oestradiol concentrations were lower in DP than in CP rats at days 7, 15 and 18, and most striking was the absence in DP rats of the peak of oestradiol seen at day 18 in CP rats. Despite the similarity between changes in placental calbindin mRNA and 1,25(OH)(2)D(3), previous work has shown placental calbindin-D(9K) regulation to be vitamin-D-independent. These studies produce suggestive evidence, therefore, that PTHrP and oestradiol may be involved in the altered calbindin-D expression by kidney and placenta in rat diabetic pregnancy.

Renal function in chronically catheterized conscious diabetic rats using constant and servo-controlled infusion

1. Infusion experiments were performed on chronically catheterized conscious rats to assess kidney function before and after the induction of diabetes mellitus with streptozotocin. 2. Two infusion regimens were used, a conventional constant-infusion protocol and a novel computer-driven, servo-controlled fluid replacement technique. The latter enables body fluid status to be maintained throughout a study occasion by servo-controlled replacement of spontaneous urinary fluid losses. 3. The chronically catheterized conscious rat infused using a servo-controlled system appears to be the optimum model for a study of diabetic renal function. The conscious preparation circumvents problems associated with anaesthesia and acute surgery. The servo-controlled infusion protocol maintains the altered fluid status of the diabetic condition. Both hyperfiltration and polyuria, characteristics of human diabetes often absent in anaesthetized and/or constantly infused diabetic rats, were seen in all conscious servo-controlled diabetic animals. 4. The new regimen enables a more accurate assessment of renal function in experimental diabetes than with previous protocols. It should prove useful in future studies, particularly those assessing the role of anti-diabetic drugs on the kidney.

Evolution and physiology of the corticotropin-releasing factor (CRF) family of neuropeptides in vertebrates

Corticotropin-releasing factor (CRF), urotensin-I, urocortin and sauvagine belong to a family of related neuropeptides found throughout chordate taxa and likely stem from an ancestral peptide precursor early in metazoan ancestry. In vertebrates, current evidence suggests that CRF on one hand, and urotensin-I, urocortin and sauvagine, on the other, form paralogous lineages. Urocortin and sauvagine appear to represent tetrapod orthologues of fish urotensin-I. Sauvagine's unique structure may reflect the distinctly derived evolutionary history of the anura and the amphibia in general. The physiological actions of these peptides are mediated by at least two receptor subtypes and a soluble binding protein. Although the earliest functions of these peptides may have been associated with osmoregulation and diuresis, a constellation of physiological effects associated with stress and anxiety, vasoregulation, thermoregulation, growth and metabolism, metamorphosis and reproduction have been identified in various vertebrate species. The elaboration of neural circuitry for each of the two paralogous neuropeptide systems appears to have followed distinct pathways in the actinopterygian and sarcopterygian lineages of vertebrates. A comparision of the functional differences between these two lineages predicts additional functions of these peptides.

A homologous radioimmunoassay for the measurement of urotensin II in the euryhaline flounder, Platichthys flesus

A sensitive and specific homologous radioimmunoassay (RIA) has been developed to measure tissue and circulating levels of the fish caudal neurosecretory system neuropeptide, urotensin II (UII), in the euryhaline flounder Platichthys flesus. A polyclonal antiserum was raised against flounder UII in rabbit; UII-125I was produced by the iodogen method and purified by HPLC. Antiserum specificity to flounder UII was demonstrated through lack of cross-reactivity with several small peptides and parallelism with standard curves for serial dilutions of UII in plasma and urophysial extracts. Biological activity of the peptide measured by UII RIA was confirmed by bioassay. Plasma intra- and interassay coefficients of variation were 9 and 18% (n = 5 and n = 3), respectively, nonspecific binding constituted 4.6% (+/-1.42%, n = 8) of total counts, and the limit of RIA detectability was estimated as 1.5 x 10(-16) M UII/assay tube. Plasma samples were subject to a reversed-phase liquid chromatography purification protocol which had an extraction efficiency of 63% (+/-10%, n = 6) and showed consistent recovery of UII over a range of plasma volumes and peptide concentrations. Plasma UII concentrations in seawater (SW)-adapted flounder (3.80 +/- 0.77 x 10(-11) M, n = 7) were significantly higher than those in freshwater (FW)-adapted fish (1.10 +/- 0.15 x 10(-11) M, n = 7). This variation coincided with differences in plasma osmolality and Na+ levels. No differences were found, however, between urophysial UII concentrations in SW-adapted (3.71 +/- 1.78 x 10(-10) M UII/gland, n = 7) and FW-adapted (2.53 +/- 1.33 x 10(-10) M UII/gland, n = 7) flounder.

Vascular, renal, and endocrine responses to low-dose atrial natriuretic peptide in the fluid-balanced New Zealand genetically hypertensive rats with and without endogenous arginine vasopressin

In hypertension, the relationship between atrial natriuretic peptide (ANP) and vasopressin (AVP) is not yet clear, although their renal actions are effectively autoregulation. To examine the possible interaction further, the responses to ANP infusion (75 ng x min (-1), i.v.) have been investigated in both hypertensive and normotensive AVP-replete (HT and NT) and AVP-deficient (HTDI and NTDI) rats. This study aimed to assess the renal function and the plasma hormone concentrations of AVP, angiotensin II (AII), ANP, aldosterone, and corticosterone in the conscious, chronically catheterized, fluid-balanced rats, and to examine the cardiovascular, renal, and endocrine responses to a constant infusion of a low-dose ANP. Data gained from the present study showed, for the first time, the hormone profile, plasma electrolyte composition, and detailed renal function of the servo-controlled, fluid-balanced rats. The similarities of plasma electrolyte composition between servo-controlled and untreated rats indicated that the servo-controlled fluid replacement technique maintained the differences between the strains and maintained body fluid balance during the experimental periods. Following ANP administration, there were no changes in glomerular filtration rate (GFR) in all groups, but an enduring diuresis and natriuresis were observed in HT and NT, which were milder in HTDI rats. However, the hypotensive effect of ANP was of a similar magnitude in all rat strains. HTDI rats exhibited an inhibition of the renin-angiotensin system (RAS), which may have participated in the reduced mean arterial blood pressure (MAP) and natriuresis observed in these rats. The renal actions of ANP appear to rely upon renal tubular events, as indicated by increased fractional electrolyte excretions in the AVP-replete rats. This study highlights the importance of AVP to the profile of the renal actions of ANP in normal rats.

Separate receptors mediate oxytocin and vasopressin stimulation of cAMP in rat inner medullary collecting duct cells

The two neurohypophysial hormones arginine vasopressin (AVP) and oxytocin have actions in the inner medullary collecting duct (IMCD) where both peptides induce an increase in cAMP accumulation. The present study has employed a novel IMCD cell line to determine whether these two hormones induce cAMP accumulation via common or separate receptors, and to characterize the potential receptors responsible. Equal volumes of vehicle (150 mM NaCl) or hormone/antagonist solutions were added to aliquots of 10(4) IMCD cells in the presence of 10(-3) M 3-isobutylmethylxanthine (IBMX) and incubated at 37 degrees C for 4 min. cAMP levels were determined by radioimmunoassay and protein concentration by Bradford assay. Both AVP and oxytocin elicited dose-dependent increases in cAMP generation, though oxytocin was less potent than AVP (EC50 = 1.6 x 10(-8) M vs. 7.4 x 10(-10) M). AVP at 10(-8) M and oxytocin at 10(-8) M, concentrations sufficient to elicit near-maximal cAMP accumulation, resulted in cAMP levels of 73.4 +/- 1.7 and 69.0 +/- 3.3 pmol (mg protein)-1 (4 min)-1, respectively (n = 10), compared with the vehicle-treated basal value of 37.7 +/- 2.2 pmol (mg protein)-1 (4 min)-1 (P < 0.001, n = 10). Combined AVP (10(-8) M) and oxytocin 10(-6) M) resulted in cAMP accumulation of 63.8 +/- 3.1 pmol (mg protein)-1 (4 min)-1 (n = 10), which was not significantly different from the effect of oxytocin alone, but slightly less than that for AVP alone (P < 0.05). A submaximal concentration of AVP (10(-10) M) induced cAMP accumulation of 48.6 +/- 2.5 pmol (mg protein)-1 (4 min)-1 (P < 0.01 compared with basal level of 34.9 +/- 2.4 pmol (mg protein)-1 (4 min)-1, n = 10), which was blocked in the presence of a vasopressin V2 receptor antagonist (10(-7) M OPC-31260) but not by the oxytocin receptor antagonist (10(-6) M [Pen1,pMePhe2, Thr4,Orn8]oxytocin) (36.3 +/- 6.1 and 45.1 +/- 1.3 pmol (mg protein)-1 (4 min)-1 respectively, P < 0.05, n = 10). A submaximal concentration of oxytocin (10(-7) M) induced a cAMP accumulation of 45.8 +/- 1.8 pmol (mg protein)-1 (4 min)-1 (n = 10), which was reduced by addition of 10(-6) M oxytocin antagonist (36.3 +/- 2.1 pmol (mg protein)-1 (4 min)-1, P < 0.05, n = 10), whereas co-incubation with 10(-6) M of the V2 receptor antagonist had no effect (43.2 +/- 1.3 pmol (mg protein)-1 (4 min)-1, n = 10). These results indicate that AVP and oxytocin induce cAMP accumulation from a common ATP pool in IMCD cells, and that separate vasopressin V2 and oxytocin receptor systems are involved, perhaps coupled to a common adenylate cyclase system.

Indices of carbohydrate and lipid metabolism in vasopressin-replete and -deficient New Zealand genetically hypertensive rats

Indices of carbohydrate and lipid metabolism were investigated in male New Zealand genetically hypertensive and normotensive rats. Cross-breeding of male rats of these strains with female Brattleboro diabetes insipidus rats also provided the opportunity to examine the metabolic impact of vasopressin and its deficiency in hypertensive and normotensive rats. Hypertensive and normotensive rats, with or without diabetes insipidus, were fasted for 24 h, exsanguinated and their blood/plasma analysed for various indices of carbohydrate and lipid metabolism. Whilst each group of rats maintained fasted normoglycemia, hypertensive rats, with or without vasopressin-deficiency, were hypoinsulinaemic relative to normotensive counterparts. Moreover, hypertensive or normotensive vasopressin-deficient rats were hypoinsulinaemic relative to vasopressin-replete counterparts. In vasopressin-replete rats, the apparently improved insulin sensitivity in hypertension was associated with significant falls in plasma glucagon, triglycerides and total cholesterol. Finally, normotensive vasopressin-deficient rats were hypoglucagonaemic relative to the vasopressin-replete group. These data demonstrate that independent of vasopressin status, hypertension in the New Zealand strain and the diabetes insipidus hybrid was associated with improved insulin sensitivity. However, endogenous vasopressin exercises an influential role in carbohydrate and lipid metabolism in normotensive rats.

Native urotensins influence cortisol secretion and plasma cortisol concentration in the euryhaline flounder, platichthys flesus

Adrenocorticotrophic hormone (ACTH) and flounder urotensins I and II (UI and UII) stimulate cortisol secretion of isolated interrenal/head kidney preparations of seawater (SW)-adapted flounder. UI and UII at concentrations of 10(-6) and 10(-7) M, respectively, increased cortisol secretion when acting on SW-derived interrenal but did not affect cortisol secretion in tissue derived from freshwater (FW) fish. Combined UI and UII had no synergistic or additive steroidogenic action, but either 10(-7) M UI or 10(-7) M UII in combination with ACTH produced a very marked, additive, or synergistic steroidogenic response, most apparent on interrenal derived from FW fish. These results suggest that urotensins enhance the steroidogenic action of ACTH in flounder. In all cases, significant steroidogenesis was apparent within 1 h postperifusion of ligands. In SW-adapted flounder intraarterial infusion of UII in vivo caused a concentration-dependent increase in plasma cortisol concentration within 1 h after infusion, while after 5 x 10(-6) M UI infusion a similar trend was evident but this did not achieve statistical significance. The data suggest that the caudal neurosecretory system may control interrenal cortisol secretion, to modulate cortisol secretion independently of the hypophysial axis, perhaps in response to specific stress-induced or osmoregulatory challenge.

Bone mineral density and composition in rat pregnancy: effects of streptozotocin-induced diabetes mellitus and insulin replacement

A disturbed calcium homeostasis characterizes diabetic pregnancy. This study documents changes in bone mineral composition in diabetic pregnant rats and examines the effect of insulin replacement. Control pregnant (CP), diabetic pregnant (DP) and insulin-treated DP (DPi) rats were assessed for femoral calcium and magnesium content, bone mineral density (BMD) and the ratio of hypertrophic to maturing and proliferative cells in the femoral growth plate. DP rats showed a significantly (P < 0.01) lower body weight, femoral weight and length than CP rats. Femoral calcium and magnesium content was also significantly (P < 0.05) lower in DP rats, as was ash weight. When calcium and magnesium were normalized for ash weight no significant differences were apparent. A significantly (P < 0.05) lower total BMD at the distal femur was seen in DP rats. This comprised a significantly (P < 0.01) lower trabecular BMD with no significant change in cortical BMD. A significantly (P < 0.05) higher ratio of hypertrophic to maturing and proliferative cells of the femoral growth plate was evident in DP animals. DPi rats showed normal blood glucose concentrations and femoral growth plate histology. DPi rats also showed normal femoral weight and length but only partially restored femoral ash weight and mineral content. Insulin failed to normalize total or trabecular BMD. Diabetes mellitus clearly has a marked effect on bone growth and mineral content in pregnancy which may be relevant to overall calcium homeostasis. The lower bone growth, bone calcium content and trabecular BMD may be unfortunate consequences of the marked hypercalciuria reported elsewhere in diabetes and may serve to maintain normocalcaemia in the disease.

Distribution and expression of glucocorticoid receptor mRNA in the forebrain of the rainbow trout

The expression and distribution of glucocorticoid receptor mRNA was studied in the forebrain of mature female and immature undifferentiated rainbow trout (Oncorhynchus mykiss) by means of Northern blotting and in situ hybridization. A single mRNA species of 7.5 kb was detected in mRNA polyA+ prepared from the anterior brain. In situ hybridization was carried out using a 35S-labelled riboprobe corresponding to the A/B-domain (between nucleotides 1224 and 1763) of the recently cloned rainbow trout glucocorticoid receptor cDNA. Comparison of adjacent sections hybridized with the sense and antisense probes allowed detection of a specific signal with a similar distribution pattern in all animals studied. In the telencephalon, a specific hybridization was detected in scattered cells of the dorsal telencephalic hemisphere, but the stronger signal was consistently observed in the dorsal nucleus, and to a lesser degree in the ventral nucleus of the ventral telencephalon. Heavy hybridization staining was consistently observed in all subdivisions of the preoptic nucleus and the nucleus lateralis tuberis, which are the main hypophysiotrophic regions in fish. A weaker signal was detected in the nucleus anterioris periventricularis, nucleus suprachiasmaticus and thalamic region. The presence of a strong signal in virtually all magnocellular neurons of the nucleus preopticus, known for producing vasotocin, isotocin and corticotropin-releasing factor favors a direct function of cortisol in regulating its own secretion.

Inhibitory effect of 5-hydroxytryptamine receptor activation on caudal neurosecretory cells of the flounder, Platichthys flesus

Immunocytochemical evidence suggests that the neuroendocrine Dahlgren cells of the teleost caudal neurosecretory system (CNSS) are innervated by descending serotonergic fibres. However, the modulatory effect(s) of 5-hydroxytryptamine (5-HT) on the activity of the CNSS are not known. The present study investigates the effect of superfusion of 5-HT and the selective 5-HT1 receptor agonist 5-carboxamidotryptamine (5-CT) on the electrophysiological properties of Dahlgren cells recorded intracellularly in an isolated CNSS preparation from the flounder. Superfusion of 5-HT (10(-7)-10(-3) M) caused a concentration-dependent, reversible hyperpolarization of the resting membrane potential (Em) of cells previously identified as 'Type 1' (putative urotensin I-secreting) cells (control = -63.5 +/- 1.5 mV; 10(-4) M 5-HT = -95.0 +/- 0.9 mV, n = 6, P<0.01). The EC50 was 7.6 +/- 4.1 microM (n = 6). Hyperpolarization resulted in a reduction or cessation of firing of these cells, suggesting an inhibitory role for the serotonergic input to the CNSS. Hyperpolarization was accompanied by a concomitant decrease in the membrane input resistance (control = 16.6 +/- 2.8 Momega; 10(-4) M 5-HT = 6.4 +/- 1.3 MD; n = 6, P<0.05) and time constant (control = 60.3 +/- 13.1 ms; 10(-4) M 5-HT = 16.0 +/- 4.4 ms, n = 6, P < 0.05). These effects were mimicked by the superfusion of much lower concentrations of 5-CT (EC50 = 47.1 +/- 7.1 nM, n = 4) suggesting that they are possibly mediated by a 5-HT1 receptor subtype, if the teleost 5-HT1 receptor has a markedly higher affinity for 5-CT than 5-HT, in common with mammalian 5-HT1 receptors. In contrast to the findings in Type 1 cells, cells identified as 'Type 2' (putative urotensin II-secreting) did not respond to either 5-HT or 5-CT, suggesting that the serotonergic input into the CNSS plays no role in the modulation of activity of this sub-population of neuroendocrine cells. Accordingly, these data suggest a functional difference between Type 1 and Type 2 Dahlgren cells, previously differentiated only on electrophysiological criteria and spatial distribution within the CNSS.

Changes in plasma arginine vasotocin (AVT) concentration and dorsal aortic blood pressure following AVT injection in the teleost Platichthys flesus

Dorsal aortic blood pressure and plasma arginine vasotocin (AVT) concentrations have been assessed in free swimming, chronically cannulated flounder following AVT injection. Intraarterial AVT at doses of 4.76 x 10(-12) mol.kg-1 and greater caused a biphasic change in blood pressure (an initial fall followed by a sustained pressor response). Doses above 4.76 x 10(-12) mol.kg-1 were associated with plasma AVT concentrations (20 min after injection) 2-3 orders of magnitude greater than the physiological range and must be considered pharmacological. Injection of the lowest pressor AVT dose (4.76 x 10(-12) mol.kg-1), 20 min after injection, increased plasma AVT concentrations to 23.4 +/- 6.1 fmol x ml-1. This increase is close to plasma AVT concentrations recently reported in untreated fish; however, in the initial period after injection plasma levels were calculated to be considerably higher than the physiological range. These results confirm that AVT is pressor in flounder but suggest that the pressor response may occur only at circulating AVT levels above the normal physiological range. The biphasic response to AVT and the differing responses to mammalian V1 and V2 type receptor agonists in the current work suggests that AVT may contribute to regional blood flow distribution in teleosts rather than blood pressure regulation.

Arginine vasopressin increases renal sodium excretion in the anesthetized rat through V1 receptors

We have previously suggested that the increase in renal Na+ excretion in response to physiological doses of arginine vasopressin (AVP) is not directly linked to the V2-mediated antidiuretic effect. In the present study we investigated the possible involvement of AVP V1 receptors in this natriuresis using a specific AVP V1 antagonist [1-(beta-mercapto-beta, beta-cyclopentamethylenepropionic acid), 2-O-methyltyrosine arginine vasopressin, d(CH2)5[Tyr(Me)2]AVP, infused at a rate of 15 ng.min-1. Male anesthetized Sprague-Dawley rats were placed on a continuous jugular infusion of 0.077 M NaCl at 150 microL.min-1. After a 3-h equilibration period, samples were collected at 20-min intervals for 4 h for the determination of urine flow, and Na+ and K+ excretion rates. In those animals in which the effects of AVP were studied, a 1-h control period was allowed following which AVP was infused at 0.02-0.08 pmol.min-1 for 1 h 20 min in separate groups of animals and then returned to the infusate alone for the last part of the experiment. In other groups the AVP V1 antagonist d(CH2)5[Tyr(Me)2]AVP (15 ng.min-1) alone or in combination with AVP (at various dose rates) was also administered for 1 h 20 min. All dose rates of AVP produced an antidiuresis which was associated significantly to increased Na+ excretion rate. However, AVP administration at the median dose rate (0.04 pmol.min-1) significantly (p < 0.01) decreased the amount of urine voided by comparison with control animals (6.34 +/- 1.05 ml vs. 11.892 +/- 0.03 mL, n = 7) although the urinary Na+ was elevated (967 +/- 18 mumol, vs. 742 +/- 81 mumol, n = 7). This AVP-induced increase in urinary Na+ loss was abolished in animals receiving combined AVP (0.04 pmol.min-1) and AVP V1 antagonist (674 +/- 47 mumol, n = 7) although the antidiuretic effect persisted. Urine flow and Na+ excretion rates remained unchanged in groups of animals administered AVP V1 antagonist alone. In all groups, the K+ excretion rates did not significantly differ. It is concluded that the V1 receptor mediates the natriuretic effect of AVP.

Arginine vasopressin mediates the chloroquine induced increase in renal sodium excretion

We postulated that chloroquine increases plasma arginine vasopressin (AVP) concentrations thus altering renal Na+ clearance. Therefore, we studied a relationship between plasma AVP concentrations and urinary Na+ output in separate groups of Sprague-Dawley (SD) rats administered chloroquine (3 micrograms/min) for 1 h 20 min. We also monitored Na+ excretion rates in Brattleboro AVP-deficient Di rats challenged with hypotonic saline load and administered chloroquine for 1 h 20 min. To establish whether chloroquine-induced changes in renal Na+ excretion were mediated via AVP V1 receptors, we studied Na+ excretion rates in groups of SD rats administered chloroquine or AVP in the presence of AVP V1 receptor antagonist (1-(beta-mercapto-beta, beta-cyclopentamethylenepropionic acid)-2-O-methyltyrosine arginine vasopressin (d(CH2)5(Tyr(Me)2) AVP) at 11 pmol/min for 1 h 20 min. The Na+ excretion rate rose significantly (P < 0.01) from a pretreatment level of 9.8 +/- 1.0 mumol/min to a peak of 14.1 +/- 0.9 mumol/min in SD rats (n = 7) administered chloroquine. The Na+ excretion rate remained unaltered around 8.5 mumol/min in rats simultaneously administered chloroquine and the AVP V1 receptor antagonist. This compared with control rats (8.1 +/- 0.5 mumol/min, n = 7) and animals administered AVP V1 receptor antagonist alone (8.7 +/- 0.6 mumol/min, n = 7). Chloroquine did not affect urine flow, Na+ or K+ excretion rates in Brattleboro AVP-deficient Di rats. Administration of AVP alone was associated with significant increases in renal Na+ excretion rate. Blockade of AVP V1 receptors abolished the AVP-dependent increase in urinary Na+ loss. We conclude that at least part of the chloroquine-induced increase in Na+ excretion is mediated by chloroquine stimulating an increase in plasma AVP concentration.

Synthesis and release of acetylcholine by the isolated perifused trout caudal neurosecretory system

The neurons of the caudal neurosecretory system of teleosts contain, in addition to urotensin I and urotensin II, a high concentration of acetylcholine (T. Ichikawa, 1978, Gen. Comp. Endocrinol. 35, 226-233). The isolated urophysis (and attached terminal spinal cord region) of the rainbow trout Oncorhynchus mykiss was incubated with [3H]choline (0.2 MBq/ml) for 45 min at 22 degrees in the presence of the cholinesterase inhibitor, physostigmine. Unreacted choline was removed by perifusion with fish Ringer solution. Incorporation of radioactivity into newly synthesized [3H]acetylcholine was 4.9 +/- 2.1 x 10(5) Bq/g wet tissue wt. When incubations were carried out in the presence of hemicholinium-3, an inhibitor of high-affinity choline uptake, or when physostigmine was omitted from the incubation buffer and/or when [3H]inulin was substituted for [3H]choline, the incorporation of radioactivity was greatly reduced (< 0.5 x 10(5) Bq/g). The release of [3H]acetylcholine from the preparation increased to 338 +/- 59% of basal (P < 0.05) when the concentration of K+ in the perifusion buffer was raised to 41 mM, but neither urotensin I (10(-7) M) nor urotensin II (10(-6) M) had a significant effect on release. The data indicate that the trout caudal neurosecretory system possesses a high-affinity uptake system for choline and that newly synthesized acetylcholine is released in response to a depolarizing stimulus.

Adrenergic receptor activation hyperpolarizes the caudal neurosecretory cells of the flounder, Platichthys flesus

The physiological factors that govern activity of the caudal neurosecretory system in teleost fish are poorly understood. Immunocytochemical evidence indicates that the neurosecretory Dahlgren cells are innervated by descending monoaminergic fibres. Using intracellular recording techniques in an isolated preparation of the posterior spinal cord of the flounder (Platichthys flesus) we have demonstrated that superfusion of adrenaline or noradrenaline (10(-7) - 10(-3) M) causes hyperpolarization of Dahlgren cells (up to -30 mV). This hyperpolarization is likely to reflect an inhibitory effect of noradrenergic nerves on the neurosecretory system in vivo, reducing the rate of hormone release. Fluctuations in the input resistance and membrane time constant suggest involvement of a multiplicity of cellular mechanisms, including the opening and closing of populations of ion-selective channels. Superfusion with dopamine (10(-7) - 10(-3) M) had no effect. Superfusion with the beta-adrenoreceptor agonist, isoprenaline, caused hyperpolarization but to a markedly lesser extent than the maximum effect of adrenaline or noradrenaline, suggesting that their effects are mediated, only in part, by a beta-adrenoreceptor subtype. Superfusion of the preparation with a membrane permeable, non-hydrolysable cyclic AMP analogue (8-[4-chlorophenylthio]-cAMP) resulted in a slight hyperpolarization which was accompanied by a small, but significant, increase in input resistance. These data are consistent with at least part of the beta-adrenoreceptor mediated effect involving closure of cAMP-sensitive ion channels. Superfusion with the alpha 1-adrenoreceptor agonist, phenylephrine, had no effect on any electrophysiological parameter studied. However, the alpha 2-adrenoreceptor agonist, clonidine, caused hyperpolarization which again failed to reach the maximum level produced by adrenaline or noradrenaline. Together, these data suggest that the adrenergic inhibition of Dahlgren cell activity is mediated by both alpha 2- and beta-adrenoreceptor subtypes.

Effect of acute manipulation of blood volume and osmolality on plasma [AVT] in seawater flounder

Chronically cannulated seawater (SW)-adapted flounder (Platichthys flesus) were used unanesthetized and unrestrained in an experimental series that acutely manipulated blood volume and plasma osmolality to determine their influence on plasma arginine vasotocin (AVT) concentrations. Immunoreactive AVT was measured using a radioimmunoassay validated for flounder and other teleosts. After hemorrhage-induced hypovolemia or hypervolemia produced by saline infusion, no major changes in plasma AVT concentrations were detected. Raising plasma osmolality by intraperitoneal injection of 1 M NaCl compared with control 150 mM NaCl-injected fish (329.4 +/- 3.4 vs. 320.4 +/- 3.0 mosmol/kgH2O, P < 0.05) produced an increase in plasma AVT concentration (6.7 +/- 1.2 vs. 4.2 +/- 0.2 fmol/ml, P < 0.05). In a separate study, plasma composition in a large number of uncannulated SW-adapted flounder was determined. This demonstrated a positive linear relationship between the natural variation in plasma AVT concentrations and plasma osmolality and Na+ and Cl- concentrations observed between fish. These data indicate that AVT secretion in SW-adapted flounder is closely related and perhaps directly sensitive to changes in plasma tonicity.

Interaction between adrenergic agonists and forskolin on adenylate cyclase activity in the rabbit proximal tubule

Using a micro-radioimmunoassay, cAMP was measured in single, isolated S2 proximal straight tubules dissected from rabbit kidneys to investigate the effects of adrenergic agonists on adenylate cyclase activity. The baseline activity of adenylate cyclase was low and unaffected by either the alpha 2 agonist clonidine, the beta agonist isoprenaline (in the absence or presence of 1 microM forskolin) or 1 microM forskolin. Adenylate cyclase activity was markedly stimulated by 20 microM forskolin, an effect which was inhibited by 1 microM clonidine. The inhibition by clonidine was not apparent in the presence of the alpha 2 antagonist yohimbine. These results confirm the inability of isoprenaline to stimulate adenylate cyclase in the rabbit proximal tubule and demonstrate the coupling of alpha 2 receptors, in an inhibitory fashion, to adenylate cyclase. The inhibitory action of the alpha 2-receptor agonist was independent of other hormone activity in the renal proximal tubule.

A peptide from the caudal neurosecretory system of the dogfish Scyliorhinus canicula that is structurally related to urotensin I

Using reversed-phase HPLC in combination with a radioimmunoassay for ovine corticotropin-releasing hormone (CRH), a peptide with CRH-like immunoreactivity was isolated in pure form from an extract of the caudal spinal cord region of the spotted dogfish, Scyliorhinus canicula. The primary structure of the peptide was established as Pro-Ala-Glu-Thr-Pro-Asn-Ser-Leu10-Asp-Leu-Thr-Phe-His-Leu-Leu-Arg- Glu-Met-Ile- Glu20-Ile-Ala-Lys-His-Glu-Asn-Gln-Gln-Met-Gln30-Ala-Asp-Ser- Asn-Arg-Arg-Ile-Met - Asp-Thr40-Ile.NH2. This amino acid sequence shows moderate structural similarity to Catostomus urotensin I (51%) and to human CRH (56%). The data provide, therefore, chemical evidence to support the conclusions of earlier immunohistochemical studies that the diffuse caudal neurosecretory system of elasmobranchs produces a peptide that is immunochemically related to teleost urotensin I peptides. However, the primary structure of urotensin I has been poorly conserved during evolution.

Plasma sialic acid in animal models of diabetes mellitus: evidence for modulation of sialic acid concentrations by insulin deficiency

An elevated circulating sialic acid concentration is a risk factor for cardiovascular disease. Serum sialic acid levels are elevated in NIDDM but not in uncomplicated IDDM. To study why sialic acid is increased in some types of diabetes, we assayed plasma sialic acid in various animal models of diabetes: obese (ob/ob) mice, before and after streptozotocin treatment, neonatal streptozotocin-treated (nSTZ) rats, and diabetic BB rats during and after insulin treatment. In obese mice, which exhibit moderate hyperglycemia and marked hyperinsulinemia, plasma sialic acid was decreased by 45% (fed) and 42% (fasted), compared to lean controls. Fasting reduced plasma glucose and insulin but increased sialic acid in the obese and lean mice. There was a negative correlation (r = -0.84, P < 0.001) between log plasma insulin and sialic acid in the lean and obese mice. The plasma sialic acid:globulin ratio was reduced by 35% in obese mice vs. lean controls, indicating that there may be altered sialylation of glycoproteins in obese mice. Streptozotocin treatment of obese and lean mice reduced plasma insulin but increased sialic acid. In nSTZ rats, hyperglycemia was associated with mild hypoinsulinemia, but not significantly different from control animals, and sialic acid was not altered. In diabetic BB rats, plasma glucose rose from a mean of 4.9 to 23.5 mM 48 hr after insulin withdrawal but sialic acid did not change. We conclude that an elevated plasma sialic acid level is associated with marked insulin deficiency, rather than hyperglycemia per se. The magnitude and speed of this change in sialic acid varies between species.

Renal, endocrine and vascular effects of atrial natriuretic peptide in a novel vasopressin-deficient genetically hypertensive strain of rat

In the absence of the potentially confounding influence of vasopressin in hypertension, the effects of atrial natriuretic peptide (ANP) on arterial blood pressure and renal handling of water and sodium were assessed from comparison of responses to intravenous ANP infusion in anaesthetized vasopressin-deficient New Zealand genetically hypertensive (DI/H) rats and their normotensive substrain (DI/N). After 320 min of hypotonic saline infusion, plasma ANP concentration was significantly higher in DI/H compared with DI/N rats. ANP administration increased circulating ANP concentration in both groups. Plasma angiotensin II concentration was higher in DI/H than in DI/N rats; infusion of ANP raised circulating angiotensin II in both groups though this achieved statistical significance only in DI/N rats. Plasma aldosterone concentrations were initially similar in normotensive and hypertensive animals and, in both, were reduced markedly by I.V. ANP infusion. Administration of ANP produced sustained hypotension in both groups. However, the hypotensive effect of ANP was more pronounced in DI/H compared with DI/N rats. Heart rate was initially similar in the two groups, and infusion of ANP produced no detectable change. By comparison with animals maintained on hormone-free infusate, urine flow increased markedly over the 80 min period of ANP infusion in both groups, by 142% in DI/H rats and 127% in DI/N rats. ANP administration produced a natriuresis in both groups but the increase in Na+ excretion was much greater in DI/H (342%) than in DI/N (139%) rats. It appears from the current study that vasopressin-deficient hypertensive rats are more sensitive to ANP with regard to effects on blood pressure and renal excretion than their vasopressin-deficient normotensive substrain. These differences may, in part, reflect adjustments to long-term elevation in blood pressure and in plasma ANP concentration in hypertension and, in part, rely on the associated disturbances in related endocrine systems.