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J N Forrest

Publications and source records attributed to J N Forrest.

At least 37 records · Page 2Linked to original sources
PubMed1993

C-type natriuretic peptides are potent dilators of shark vascular smooth muscle.

Previous studies of the effects of C-type natriuretic peptides (CNP) in intact mammals have demonstrated limited hypotensive responses, in contrast to other natriuretic peptides. Our previous studies, on isolated vascular smooth muscle (VSM) from various fish species, utilizing either mammalian or non-homologous fish atrial natriuretic peptides (ANP), have demonstrated vasodilation with a relatively high sensitivity (EC50 approximately 5 nM). The recent sequencing of a C-type natriuretic peptide from the heart of the dogfish shark, Squalus acanthias, has enabled us to compare the efficacy of this peptide on aortic VSM from that species with two other CNPs (from killifish and pig), as well as rat ANP. The EC50 of dilation for sCNP, as well as kCNP and pCNP, was 0.5 nM, over 15 times lower than the EC50 of the response to rANP. These data suggest that CNP is released from the dogfish shark heart and is a circulating hormone with potent vasodilatory effects, in sharp contrast to the apparent role of CNP predominantly as a brain neuropeptide in mammals.

Animals↗
PubMed1993

Neuropeptide Y inhibits chloride secretion in the shark rectal gland.

We studied the effects of the 36-amino acid peptide, neuropeptide Y (NPY), on salt secretion by the rectal gland of Squalus acanthias. We used three preparations: whole isolated perfused glands, freshly prepared separated rectal gland tubules, and confluent monolayers of cultured rectal gland cells. In perfused glands NPY inhibited secretion stimulated by vasoactive intestinal peptide (VIP), forskolin, or adenosine 3',5'-cyclic monophosphate (cAMP) and theophylline. Maximal inhibition of 63 +/- 3.4% was seen at 3 x 10(-8) M NPY, with half-maximal effect at 3 x 10(-9) M. NPY did not inhibit the basal activity of rectal gland adenylate cyclase or that stimulated by VIP. The inhibitory action of NPY was not prevented by procaine, nifedipine, or diltiazem, suggesting that it was not secondary to the release of somatostatin or other unknown neurotransmitters from rectal gland nerves. In confirmation, somatostatin was not detected in the venous effluent after administration of NPY. NPY also inhibited transport-related oxygen consumption in separated rectal gland tubules and inhibited short-circuit current generated by confluent monolayers of primary cultures of rectal gland cells. The results indicate that NPY inhibits chloride secretion by a direct action on cells of the shark rectal gland at a site distal to the generation of cAMP.

Adenylyl Cyclases↗
PubMed1991

Cl- secretion by cultured shark rectal gland cells. I. Transepithelial transport.

To facilitate analysis of the regulation of epithelial Cl- transport by hormones, neurotransmitters, and autocrine mediators, we have developed a primary monolayer culture system for shark rectal gland (SRG) epithelial cells. Cultures exhibit vigorous transcellular Cl- secretion which can be measured using short-circuit current or 36Cl flux methods. Transport is markedly reduced by bumetanide or barium, inhibitors of Na(+)-K(+)-2Cl- cotransport and K+ channels, respectively. This indicates that Cl- secretion by SRG monolayers occurs by a mechanism similar to that described in numerous native Cl- secretory epithelia. Forskolin, 10 microM 2-chloroadenosine, or vasoactive intestinal peptide, potent secretagogues in the isolated perfused SRG, stimulate Cl- secretion by SRG cultures. Submicromolar concentrations of 2-chloroadenosine, which inhibit Cl- secretion in the native SRG, reduce forskolin-stimulated short-circuit current in SRG cultures. Somatostatin, another inhibitor of Cl- secretion by the native SRG, reduces forskolin-stimulated adenosine 3',5'-cyclic monophosphate accumulation in SRG cultures. These results demonstrate that SRG cultures are fully responsive to mediators which activate or inhibit secretion by the native epithelium.

2-Chloroadenosine↗
PubMed1991

Identification of C-type natriuretic peptide in heart of spiny dogfish shark (Squalus acanthias).

An unidentified atrial natriuretic peptide (ANP)-like substance is the principal hormone regulating NaCl secretion in the shark rectal gland, an epithelial model tissue for hormone-sensitive secondary active chloride transport. Antibodies to mammalian ANP do not recognize the prohormone of marine species. The polymerase chain reaction (PCR) was used to isolate a partial cDNA encoding the shark heart natriuretic peptide. Using this partial sequence as a probe, the full-length clone [882 base pairs (bp)] was obtained from a shark heart cDNA library. Amino acids 119-135 are similar to the recently identified peptide sequences of porcine C-type natriuretic peptide (CNP) and killifish brain natriuretic peptide isolated from the brain of these species. Mature shark heart CNP terminates at the second cysteine residue and lacks a COOH-terminal extension, in contrast to cardiac ANP-like peptides of all other species. The primary amino acid sequence of the shark heart prepro-CNP is distinctly different from all other cardiac natriuretic peptides. Amplification of genomic DNA spanning the coding region produced a 1.5-kb product, indicating the presence of at least one intron. Sequencing confirmed the presence of two exons of 90 and 315 bp, separated by a 1.1-kb intron. This is the first report of a cDNA encoding in nonneuronal tissue. Elasmobranch CNP may represent a primordial form of ANP-like peptides that evolved as an adaptation to environmental osmoregulatory stress.

Amino Acid Sequence↗
PubMed1991

Endogenous adenosine is an autacoid feedback inhibitor of chloride transport in the shark rectal gland.

The present studies define the physiologic role of endogenous adenosine in the perfused shark rectal gland, a model epithelia for hormone-stimulated chloride transport. Chloride ion secretion, and venous adenosine and inosine concentrations increased in parallel in response to hormone stimulation. From a basal rate of 157 +/- 26 mu eq/h per g, chloride secretion increased to 836 +/- 96 and 2170 +/- 358 with 1 and 10 microM forskolin, venous adenosine increased from 5.0 +/- 1 to 126 +/- 29 and 896 +/- 181 nM, and inosine increased from 30 +/- 9 to 349 +/- 77 and 1719 +/- 454 nM (all P less than 0.01). Nitrobenzylthioinosine (NBTI), a nucleoside transport inhibitor, completely blocked the release of adenosine and inosine. Inhibition of chloride transport with bumetanide, an inhibitor of the Na+/K+/2Cl- cotransporter, or ouabain, an inhibitor of Na+/K+ ATPase activity, reduced venous adenosine and inosine to basal values. When the interaction of endogenous adenosine with extracellular receptors was prevented by adenosine deaminase, NBTI, or 8-phenyltheophylline, the chloride transport response to secretagogues increased by 1.7-2.3-fold. These studies demonstrate that endogenous adenosine is released in response to hormone-stimulated cellular work and acts at A1 adenosine receptors as a feedback inhibitor of chloride transport.

Adenosine↗
PubMed1990

Sodium-adenosine cotransport in brush-border membranes from rabbit ileum.

To determine the mechanism(s) of transcellular adenosine transport in epithelial tissues that possess an adenosine receptor response, we studied [3H]adenosine uptake using vesicles prepared from isolated brush-border and basolateral membranes of the rabbit ileum. In the presence of the adenosine deaminase inhibitor deoxycoformycin uptake of [3H]adenosine into brush-border membrane vesicles is stimulated fivefold by an inwardly directed Na gradient. Na-dependent [3H]adenosine uptake is enhanced and concentrative under conditions that increase inside negativity of vesicles, thus providing evidence for an electrogenic carrier. Na-dependent adenosine uptake is a saturable function of adenosine concentration with a Michaelis-Menten constant of 17.3 +/- 7.1 microM and maximum transport rate of 216.9 +/- 20.2 pmol.min-1.mg protein-1. Both uridine and inosine inhibit [3H]adenosine uptake, suggesting that the Na-dependent transporter has broad substrate specificity for both purine and pyrimidine ribonucleosides. Na-dependent adenosine uptake is inhibited by dipyridamole but is insensitive to 6-(4-nitrobenzyl)thio-9-beta-D-ribofuranosylpurine. We conclude that adenosine is transported across ileal brush-border membranes by a Na-ribonucleoside cotransport system. In contrast, adenosine uptake in basolateral membranes is not stimulated by a Na gradient. These studies show asymmetry in the distribution of transport systems for adenosine in polarized intestinal epithelia.

Adenosine↗
PubMed1990

A1 adenosine receptors inhibit chloride transport in the shark rectal gland. Dissociation of inhibition and cyclic AMP.

In the in vitro perfused rectal gland of the dogfish shark (Squalus acanthias), the adenosine analogue 2-chloroadenosine (2Clado) completely and reversibly inhibited forskolin-stimulated chloride secretion with an IC50 of 5 nM. Other A1 receptor agonists including cyclohexyladenosine (CHA), N-ethylcarboxamideadenosine (NECA) and R-phenylisopropyl-adenosine (R-PIA) also completely inhibited forskolin stimulated chloride secretion. The "S" stereoisomer of PIA (S-PIA) was a less potent inhibitor of forskolin stimulated chloride secretion, consistent with the affinity profile of PIA stereoisomers for an A1 receptor. The adenosine receptor antagonists 8-phenyltheophylline and 8-cyclopentyltheophylline completely blocked the effect of 2Clado to inhibit forskolin-stimulated chloride secretion. When chloride secretion and tissue cyclic (c)AMP content were determined simultaneously in perfused glands, 2Clado completely inhibited secretion but only inhibited forskolin stimulated cAMP accumulation by 34-40%, indicating that the mechanism of inhibition of secretion by 2Clado is at least partially cAMP independent. Consistent with these results, A1 receptor agonists only modestly inhibited (9-15%) forskolin stimulated adenylate cyclase activity and 2Clado markedly inhibited chloride secretion stimulated by a permeant cAMP analogue, 8-chlorophenylthio cAMP (8CPT cAMP). These findings provide the first evidence for a high affinity A1 adenosine receptor that inhibits hormone stimulated ion transport in a model epithelia. A major portion of this inhibition occurs by a mechanism that is independent of the cAMP messenger system.

2-Chloroadenosine↗
PubMed1986

Treatment of severe lithium-induced polyuria with amiloride.

One of the major side effects of lithium is nephrogenic diabetes insipidus. The established treatment for the disorder is thiazide diuretics, which are associated with hypokalemia and reduced lithium excretion, predisposing the patient to lithium toxicity. Amiloride is a new diuretic that reduces lithium-induced polyuria in animals without affecting lithium or potassium levels. The authors found that 10-20 mg/day of amiloride given to eight patients who had become hypokalemic while being treated with hydrochlorothiazide for lithium-induced nephrogenic diabetes insipidus increased renal concentrating ability and reduced polyuria. They conclude that amiloride can be useful in treating lithium-induced nephrogenic diabetes insipidus and merits future randomized clinical trials.

Adult↗
PubMed1986

Aminoglycoside toxicity: pH dependent inhibition of ADH response.

The effect of aminoglycoside antibiotics on the response of the isolated toad urinary bladder to antidiuretic hormone (ADH) was investigated. Gentamicin and neomycin both acidify the serosal bathing solution and cause a dose-dependent inhibition of the hydroosmotic response to ADH, while streptomycin has minimal effect on media pH and causes no inhibition of the response to ADH. Detailed studies employing gentamicin indicate that acidification stimulates production of PGE2, a known inhibitor of the hydroosmotic response of the toad bladder to ADH. When media pH is rigidly controlled or PGE2 production is inhibited by indomethacin, the inhibitory effect of gentamicin on the response to ADH is ameliorated. These studies suggest that the defect in renal concentrating ability seen as part of aminoglycoside nephrotoxicity could be due, in part, to an acidification-induced, prostaglandin-mediated resistance to the action of ADH.

Aminoglycosides↗
PubMed1985

Cell solute composition and potassium effects in slices of the rectal gland of the dogfish shark (Squalus acanthias).

Cell solute composition and apparent intracellular ionic concentrations (subscript i) were determined in rectal gland slices of the dogfish shark (Squalus acanthias). On aerobic incubation in elasmobranch Ringer's at 15 degrees C, the steady state values were: Ki+, 151 mM, close to its apparent electrochemical equilibrium; Cai2+, 1.7 mM; Mgi2+, 7.5 mM; Cl1-, 97 mM (maintained against its electrochemical potential gradient); the sum (Nai+ + Ki+) was about 66 meq/kg cell water lower than in the medium. Of free amino acids, only the effectively impermeable anionic taurine (32 mM) and glutamate (3.7 mM) significantly contribute to the Donnan system. Variations of the Ringer's K+ concentrations, or the presence of 0.5 mM ouabain, demonstrated the permeability of the cell membrane to na+, K+, and Cl-. Cell volume changes: 0.5 mM ouabain produced a gradual swelling and ionic changes consistent with an inhibition of the sodium pump. In high-K+ media the cells swelled massively and the membrane depolarized. A linear relationship between cell volume changes and the Nernst potential for K+ was found. These properties are consistent with a pump-and-leak system for cell volume maintenance. The K+ -induced cell swelling was found to be reversible on transfer of the tissue to regular Ringer's. The extrusion of cell water was associated with an influx of Na+ into, and an efflux of K+ from, the cells (both fluxes down-hill), as well as extrusion of Cl- against its concentration gradient.2+

Amino Acids↗
PubMed1985

Adenosine stimulates sodium transport in kidney A6 epithelia in culture.

The effects of adenosine receptor agonists and antagonists were examined in epithelia formed in culture by A6 cells, a continuous cell line derived from Xenopus laevis kidney. A6 epithelia have a high electrical resistance and a short-circuit current that is equal to net sodium flux from mucosal to serosal surface. Adenosine, 2-chloroadenosine, 5'-(N-ethyl)carboxamidoadenosine, and N6-(L-2-phenylisopropyl) adenosine produced concentration-dependent increases in short-circuit current. Stimulation of short-circuit current by 2-chloroadenosine occurred at concentrations of 0.05 microM and above, with half-maximal stimulation occurring at 0.3 microM. 5'-(N-ethyl)carboxamidoadenosine was more potent than N6-(L-2-phenylisopropyl)adenosine, the usual order of potency for activation of stimulatory adenosine receptors. Theophylline (100 microM), an adenosine receptor antagonist, reduced the short-circuit current response to adenosine and 2-chloroadenosine by 85-90%. Amiloride, an agent that inhibits both basal and adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated short-circuit current in A6 epithelia, completely and reversibly inhibited short-circuit current stimulated by 2-chloroadenosine. Adenosine and 2-chloroadenosine stimulated adenylate cyclase activity in a crude membrane preparation from A6 cells. Stimulation by adenosine was blocked by adenosine deaminase. 2-Chloroadenosine increased cell cAMP accumulation in intact epithelia. The results provide evidence that adenosine and adenosine receptor agonists stimulate adenylate cyclase and active sodium transport in an epithelial cell line of renal origin.

2-Chloroadenosine↗
PubMed1984

Mechanism of NaCl secretion in rectal gland tubules of spiny dogfish (Squalus acanthias). III. Effects of stimulation of secretion by cyclic AMP.

Segments of rectal gland tubules (RGT) the spiny dogfish (Squalus acanthias) were perfused in vitro to study the cellular mechanism by which NaCl secretion is stimulated. Transepithelial PD (PDte), transepithelial resistance (Rte), the PD across the basolateral membrane (PDbl), the fractional resistance of the lumen membrane (FR1), and the cellular activities for Cl-, Na+, and K+ (alpha cell x) were measured. In series 1 the effects of stimulation (S) (dbcAMP 10(-4, adenosine 10(-4), and forskolin 10(-6) mol x 1(-1) on these parameters were recorded and compared to nonstimulated state (NS). PDte increased from -1.9 +/- 0.2 mV to -11.0 +/- 0.9 mV (n = 51). PDbl depolarized from -86 +/- 1 to -74 +/- 1.4 mV (n =52), Rte fell from 29 +/- 2.8 to 21 +/- 2 omega cm2 (n = 23), and FR1 fell from 0.96 +/- 0.005 to 0.79 +/- 0.04 (n = 9). alpha cell K+ was constant (123 +/- 13 versus 128 +/- 17 mmol x 1(-1) (n = 6), but alpha cell cl- fell significantly from 48 +/- 4 to 41 +/- 3 mmol x 1(-1) (n = 7). alpha cell Na+ increased from 11 +/- 2.1 to 29.5 +/- 6.6 mmol x 1(-1) (n = 4). In series 2 the conductivity properties were examined by rapid K+, and Cl- concentration steps on the basolateral and luminal cell side respectively in NS and S states. In NS-segments reduction of bath K+ led to a hyperpolarization of PDbl with a mean slope of 28 +/- 1.3 mV/decade (n = 9) (as compared to 19 mV/decade for S-state).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Pardaxin increases solute permeability of gills and rectal gland in the dogfish shark (Squalus acanthias).

The action of the ichthyotoxic secretion of the Red Sea flatfish Pardachirus marmoratus and its derived toxin, pardaxin, was examined in the dogfish shark (Squalus acanthias). Pardaxin was more toxic when administered to the bathing medium than when injected into a dorsal artery and it transiently diminished the spiracular rate and caused a severe struggling response in the adult shark only when administered to the head region of the shark. Pardaxin caused a transient leakage to urea and sodium between the shark and the seawater. In the isolated perfused rectal gland pardaxin irreversibly reduced the rate of chloride secretion and concentration gradient of urea between perfusate and rectal gland fluid. In addition, ultrastructural studies on the rectal gland showed that ionic lanthanum penetrated the tight junctions and foci of cell necrosis were observed. These studies indicate that in shark the gills are the most probable target of the toxicity of pardaxin.

Animals↗
PubMed1984

Adenosine and adenosine analogues stimulate adenosine cyclic 3', 5'-monophosphate-dependent chloride secretion in the mammalian ileum.

Adenosine receptors that modulate adenylate cyclase activity have been identified recently in a number of tissues. The purpose of these investigations was to determine the effect of adenosine on ion transport in rabbit ileum in vitro. Adenosine and some of its analogues were found to increase the short circuit current (Isc) and the order of potency was N-ethylcarboxamide-adenosine greater than or equal to 2-chloroadenosine greater than phenylisopropyladenosine greater than adenosine. Purine-intact adenosine analogues had no effect on Isc. The effect of adenosine on Isc was enhanced by deoxycoformycin, an adenosine deaminase inhibitor, and by dipyridamole, an adenosine uptake inhibitor. The increase in Isc induced by 2-chloroadenosine was partially reversed in a dose-dependent manner by 8-phenyltheophylline but not by theophylline or isobutylmethylxanthine. 2-Chloroadenosine increased cyclic AMP content, and stimulated net Cl secretion; these effects were partially blocked by 8-phenyltheophylline. These results suggest that there is an adenosine receptor on rabbit ileal mucosal cells that stimulates adenylate cyclase, which results in secondary active Cl secretion.

1-Methyl-3-isobutylxanthine↗
PubMed1983

Perfusion of isolated tubules of the shark rectal gland. Electrical characteristics and response to hormones.

Both the mammalian thick ascending limb of Henle's loop and the shark rectal gland actively transport Cl against an electrochemical gradient by mechanisms involving hormone-sensitive NaCl transport. In contrast to mammalian renal tubules, individual tubules of the shark rectal gland previously have not been perfused in vitro. Using a combination of renal slice and microdissection techniques we were able to isolate and perfuse single rectal gland tubules without the use of enzyme treatment. Single tubules consistently generated lumen-negative transepithelial voltages (Vt) of -1.8 mV when perfused and bathed with identical shark Ringer's solution. The addition of cyclic AMP, vasoactive intestinal peptide (VIP), and adenosine to the bath increased Vt to -7.5, -9.0, and -4.3 mV, respectively (all P less than 0.02 compared with paired controls). Each stimulation could be reversed by addition by furosemide to the bath. The adenosine response was inhibited by theophylline, a specific inhibitor of adenosine receptors. The tubules had a low transepithelial electrical resistance of 12-26 omega X cm2 and exhibited a transepithelial permselectivity for small cations. These results indicate that tubules of the rectal gland can be perfused in vitro and have receptors for VIP and adenosine. Cyclic AMP and secretagogues hyperpolarize the membrane consistent with electrogenic chloride transport, and these effects are reversed by furosemide, an inhibitor of coupled sodium-potassium-chloride co-transport. The response of Vt to cyclic AMP and furosemide, the transepithelial electrical resistance, and the cation selective permeability of tubules are remarkably similar to measurements in perfused mammalian thick ascending limbs.

Adenosine↗
PubMed1982

Structure of tight junctions during Cl secretion in the perfused rectal gland of the dogfish shark.

In epithelia that secrete sodium chloride, high-conductance tight junctions between cells have been proposed as the primary pathway for transepithelial sodium flux. We examined the properties of tight junctions in the perfused rectal gland of the dogfish shark during basal secretion and following adenosine 3',5'-cyclic monophosphate stimulation of sodium chloride secretion. Freeze-fracture electron microscopy revealed extensive interdigitation of adjacent cells with an associated amplification in the length of tight junctions per area of luminal surface, averaging 102 +/- 4.7 m/cm2 in outer regions of 80 +/- 6.7 in inner regions of the gland. Marked heterogeneity of junctional structure was present with junctional elements varying from single strands to three duplex elements and junctional depth varying from 15 to 60 nm. In glands perfused with lanthanum chloride, ionic lanthanum filled the intercellular space up to but not through the tight junctions. Characteristics of tight junctions were not different during basal and maximally stimulated sodium chloride secretion. These studies define tight junctions in the rectal gland as an anatomical barrier capable of restricting the passage of relatively small molecules such as urea while providing a greatly amplified junctional area for the passive diffusion of sodium and water.

Animals↗
PubMed1982

Role of prostaglandin E2 in mediating the effects of pH on the hydroosmotic response to vasopressin in the toad urinary bladder.

Acidosis inhibits the hydroosmotic response to vasopressin. Since prostaglandins are known to modulate vasopressin-stimulated water flow we investigated the role of endogenous prostaglandin E2(PGE2) production in the pH-dependent response of the toad urinary bladder to vasopressin. Graded acidification of the serosal medial resulted in a progressive decline in vasopressin-stimulated water flow from 26.6 +/- 0.5 mg/min at pH 8.4 to 1.7 +/- 0.6 at pH 6.9. In these bladders basal PGE2 synthesis increased from 5.09 +/- 0.51 pmol/min per g hemibladder at pH 8.4 to 18.8 +/- 2.8 at pH 6.9. The addition of that concentration of PGE2 produced by the bladder at pH 7.4 (4 nM) to bladders at pH 8.4 resulted in 62-71% of the inhibition usually seen at pH 7.4; these data suggest that basal PGE2 production per se and not other products of prostaglandin synthesis or other pH-dependent events is responsible for the effect of acidosis. Preincubation with prostaglandin synthesis inhibitors reversed in major part the effect of serosal acidification on the response to submaximal concentrations of vasopressin and completely abolished the effect of pH on near maximal concentrations of the hormone. An increase in PGE2 synthesis after vasopressin was not seen at any pH. These studies establish that increased basal PGE2 synthesis plays a critical role in the pH dependence of the hydroosmotic response to vasopressin and demonstrate that factors that modulate the response to vasopressin may exert this effect by changing the basal rate of prostaglandin synthesis.

Acidosis↗