Inhibition Of Fluid Secretion Research Articles

Luminal capsaicin inhibits fluid secretion induced by enterotoxin E. coli STa, but not by carbachol, in vivo in rat small and large intestine.

The enterotoxin E. coli STa induced fluid secretion in the rat jejunum, ileum and proximal colon in vivo that was greatly inhibited by the co-presence of luminal capsaicin, which is a specific neural toxin of afferent C fibres. The same dose of capsaicin had no effect on the fluid secretion activated by carbachol in the jejunum, ileum and proximal colon. Afferent C fibres appear to be involved in the activation by STa of fluid secretion in the rat intestine in vivo.

An ammonium pulse inhibits fluid secretion

Exposing perfused mandibular salivary glands to a 5-min pulse of NH4Cl almost totally inhibited fluid secretion evoked by acetylcholine. Recovery from exposure to 0.5 mM NH4Cl occurred within 10 min, but after exposure to 20 mM NH4Cl recovery was not complete even after 30 min. The inhibition could not be explained by changes in intracellular pH as there was no acidosis measurable after the 0.5 mM NH4Cl pulse. NH4Cl did not inhibit intracellular calcium release by acetylcholine so that receptor inhibition could not be the explanation for the inhibition of secretion. These observations indicate that data obtained from ammonium pulse experiments should be interpreted with caution.

Anion secretion drives fluid secretion by monolayers of cultured human polycystic cells

We have investigated the hypothesis that active anion transport drives fluid secretion by the cystic epithelium in autosomal dominant polycystic kidney disease (ADPKD). We prepared monolayers of a primary culture derived from cystic tissue removed from ADPKD patients. The monolayers were grown on permeant supports, and fluid secretion was initiated by forskolin. The results were compared with those obtained with monolayers of Madin-Darby canine kidney (MDCK) cells, known to secrete Cl-. In the absence of the agonist, ADPKD monolayers absorbed fluid (0.20 +/- 0.02 microliter.cm surface area-2.h-1). Forskolin reversed this to secretion (0.60 +/- 0.03 microliter.cm-2.h-1). Control MDCK monolayers did not transport fluid in either direction, but forskolin induced secretion (0.48 +/- 0.03 microliter.cm-2.h-1). The electrical properties of the monolayers were monitored in Ussing chambers. Forskolin increased the transepithelial potential difference (Vte) of ADPKD monolayers (-0.9 +/- 0.1 to -1.1 +/- 0.1 mV) and the short-circuit current (Isc) (6.6 +/- 0.7 to 9.2 +/- 0.8 microA/cm2). The transepithelial resistance (Rte) fell (156 +/- 9 to 138 +/- 10 omega.cm2). Similar results were obtained with MDCK monolayers. The polarity of Vte and the direction of the Isc are compatible with the hypothesis that active secretion of anion drives fluid secretion. Basolateral application of the Na-K-2Cl cotransporter, bumetanide, reduced forskolin-stimulated fluid secretion by ADPKD monolayers (0.56 +/- 0.05 to 0.28 +/- 0.03), depolarized Vte, and inhibited Isc without affecting Rte. Apical application of the Cl- channel blocker, diphenylamine-2-carboxylate, also inhibited fluid secretion by ADPKD monolayers (0.65 +/- 0.03 to 0.27 +/- 0.02 microliter.cm-2.h-1).(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro fluid secretion by epithelium from polycystic kidneys.

The size of the kidneys in patients with autosomal dominant polycystic kidney disease (ADPKD) is due in large measure to the accumulation of secreted fluid within thin-walled epithelial sacs. We measured the net transepithelial movement of liquid in response to forskolin in isolated, intact cysts excised from the surface of human ADPKD kidneys and in cultured, polarized monolayers of epithelial cells derived from ADPKD cysts. 10 excised cysts bathed symmetrically in control culture medium secreted fluid at a rate of 0.19 +/- 0.03 microliter/cm2 per hour after stimulation with forskolin (10 microM). Ouabain (100 microM) addition to the cavity fluid did not change the rate of fluid secretion of 10 forskolin-treated cysts, but addition of the glycoside to the external bathing medium fluid of nine cysts decreased secretion to -0.004 +/- 0.05 microliter/cm2 per hour. 24 monolayers absorbed fluid (range -0.029 to -0.412 microliter/cm2 per hour); by contrast, fluid was secreted (range 0.074 to 1.242 microliters/cm2 per hour) after stimulation with forskolin (10 microM). Ouabain (0.1 microM) in the basolateral but not in the apical medium inhibited fluid secretion. Forskolin increased the intracellular cyclic AMP content of ADPKD and MDCK monolayers by 236 and 196%, respectively. Six ADPKD monolayers had stable lumen negative transepithelial electrical potential differences (PDte) of -1.4 +/- 0.3 mV, positive short circuit currents (SCC) of 11.9 +/- 2.1 microAmp/cm2 and a tissue resistance (Rte) of 116 +/- 14 ohm.cm2. Forskolin increased SCC to 15.5 +/- 1.9 microAmp/cm2 (P < 0.005) and decreased Rte to 95 +/- 13 ohm.cm2 (P < 0.05); PDte remained stable at -1.4 +/- 0.3 mV. Ouabain (10 microM) had no effect when added to the apical medium, but in the basolateral medium decreased SCC to 1.7 +/- 0.3 microAmp/cm2 and PDte to -0.2 +/- 0.1 mV. We conclude that ADPKD cells in surface cysts have the potential to absorb or to secrete solutes and fluid. cAMP-mediated fluid secretion from the basolateral medium into the lumen of surface ADPKD cysts may be driven by anion transport.

Mechanisms of K+ Uptake Across the Basal Membrane of Malpighian Tubules of Formica Polyctena: The Effect of Ions and Inhibitors

In the presence of 6 mmol l-1 Ba2+, known to block the K+ channels in the basal membrane, a rise in bath [K+] ([K+]bl) induced an increase in intracellular K+ concentration ([K+]i) similar in amount and in time course to that obtained in the absence of Ba2+. The presence of active and passive (other than through K+ channels) K+ uptake mechanisms across the basal membrane was investigated in different bath K+ concentrations. Dihydro-ouabain (10(-3) mol l-1), a blocker of the Na+/K(+)-ATPase, tested in low bath [K+], and Sch28080 (10(-4) mol l-1), a K+/H(+)-ATPase inhibitor, were without effect on fluid secretion. Dihydro-ouabain was also without effect on electrical potential differences either in the absence or in the presence of Ba2+. Vanadate (10(-3) mol l-1), in contrast, strongly reduced fluid secretion not only in control solution but also in high-K+, Na(+)-free medium and reduced the transepithelial and the apical membrane potential differences but not the basal membrane potential difference of [K+]i. Omitting Na+ from the bathing medium, replacing Cl- by Br- or applying bumetanide (10(-5) mol l-1) inhibited fluid secretion only in a low-K+ (10 mmol l-1) medium. In 51 mmol l-1 [K+]bl, omitting Na+ was without effect and 10(-4) mol l-1 bumetanide was needed to inhibit secretion. Replacing Cl- by Br- stimulated fluid secretion at this K+ concentration. Bumetanide (10(-4) mol l-1) had no effect in 113 mmol l-1 [K+]bl. Bumetanide (10(-4) mol l-1) in 51 mmol l-1 [K+]bl did not affect membrane potentials, did not lower [K+]i and did not affect the rise in [K+]i observed on an increase in [K+]bl. The results were summarized in a model proposing that K+ channels play a dominant role in high-K+ (113 mmol l-1) bathing medium. A K+/Cl- cotransporter may become more important in 51 mmol l-1 [K+]bl and a K+/Na+/2Cl- cotransporter may gain in importance in 10 mmol l-1 [K+]bl. Active mechanisms for K+ uptake across the basal membrane seem to play no detectable role in sustaining fluid secretion. The response to vanadate might be due to an effect on the apical electrogenic H+ pump.

CP-96,345, a substance P antagonist, inhibits rat intestinal responses to Clostridium difficile toxin A but not cholera toxin.

Toxin A from Clostridium difficile mediates acute inflammatory enterocolitis in experimental animals, while cholera toxin causes noninflammatory secretory diarrhea. The purpose of this study was to investigate whether an antagonist to the peptide substance P, a constituent of primary sensory neurons known to participate in inflammatory responses, would inhibit toxin A-mediated enteritis in the rat ileum. Pretreatment of rats with CP-96,345 (2.5 mg per kg of body weight), a substance P antagonist, dramatically inhibited fluid secretion (P < 0.01) and mannitol permeability (P < 0.01) in ileal loops exposed to toxin A. The protective effects, which were dose dependent, caused a significant reduction of inflammation in the lamina propria, reduction of the necrosis of intestinal epithelial cells, and complete inhibition of toxin A-mediated release of rat mast cell protease II, a specific product of rat mucosal mast cells. An inactive enantiomer of the substance P antagonist, CP-96,344, had no effect. In contrast, pretreatment with CP-96,345 had no inhibitory effect on the intestinal effects caused by administration of cholera toxin into the ileal loops. From these data, we conclude that the peptide substance P is involved in the secretory and inflammatory effects of toxin A but not of cholera toxin.

VIP-antiserum inhibits fluid secretion by the inflamed gallbladder mucosa

The inflammatory fluid secretion by the gallbladder mucosa in experimental cholecystitis is induced by an increased prostaglandin formation and is mediated by intramural nerves. In the present study the effect of VIP-antiserum on the inflammatory fluid secretion in the gallbladder was tested in a validated experimental model in cats. The animals were studied in acute experiments 6 weeks after a procedure when the cystic duct was tied and gallstones were implanted in the gallbladder. During basal conditions there was a continuous secretion of fluid into the lumen of the inflamed gallbladder averaging 0.43±0.18 ml/h . Injection of VIP antiserum, obtained from immunized rabbits and diluted with saline 1:10 in a bolus of 4 ml into the coeliac artery reversed this secretion into an absorption of 1.72±0.44 ml h −1 ( P<0.001). VIP-antiserum did not affect the fluid absorption in control animals with an intact gallbladder and injection of control serum from rabbits not immunized to VIP did not affect fluid secretion in the inflamed gallbladders. The results support the idea that the inflammatory fluid secretion in the gallbladder mucosa is mediated by VIP-ergic nerve fibres.

Inhibition of desert locust ( Schistocerca gregaria) Malpighian tubule fluid secretion by destruxins, cyclic peptide toxins from the insect pathogenic fungus Metarhizium anisopliae

Destruxins are cyclic peptide lactone toxins isolated from the insect pathogenic fungus Metarhizium anisopliae. Destruxins A, A 2, B and E all inhibit fluid secretion in vitro by Malpighian tubules of the desert locust Schistocerca gregaria. Inhibition is dose-dependent; the IC 50 for destruxin A is 23 μM. Destruxins A 2, B and E are similar to destruxin A in their effectiveness on fluid secretion at a concentration of 16 μM. Following a brief exposure to destruxin A in vitro, the rate of fluid secretion recovers significantly but incompletely. Fluid secretion was increased to 2.2 times the basal rate when Malpighian tubules were exposed to synthetic Locusta migratoria diuretic peptide. This stimulation of fluid secretion was completely inhibited by destruxin A. Fluid secretion by Malpighian tubules was stimulated by the intracellular second messenger, adenosine 3′,5′-cyclic monophosphate (cAMP), in the presence of the cAMP phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). This stimulation was also abolished by destruxin A. Schistocerca Malpighian tubules continued to secrete fluid in calcium-free conditions (zero calcium saline with added EGTA). Destruxin A inhibited fluid secretion equally well in the absence or presence of external calcium. The calcium channel blocker cadmium chloride did not prevent inhibition of fluid secretion by destruxin A nor did the anion channel blocker, 4-acetamido-4′-isothio-cyanatostilbene-2,2′-disulphonic acid (SITS). It is suggested that the inhibition by destruxin A of desert locust Malpighian tubule fluid secretion involves a cellular mechanism beyond the level of control by calcium or cAMP.

1992 Homer Smith Award. Fluid secretion, cellular proliferation, and the pathogenesis of renal epithelial cysts.

Renal cysts, caused by hereditary or acquired disorders, develop in tubule segments. The central pathogenetic elements of cyst formation include abnormal cellular proliferation, accumulation of intratubular liquid, and remodeling of the extracellular matrix. This review addresses the pathogenetic basis of liquid collection and cellular proliferation. Cavity liquid. At an early stage of growth, most renal cysts become detached from the tubule segment of origin; thus, transepithelial fluid secretion is the source of the liquid in most macroscopic cysts. Evidence from in situ and in vitro studies of intact cysts and epithelium cultured from cyst walls and normal renal tubules indicates that: (1) solutes (NaCl) are secreted into the cysts and water flows secondarily by osmosis; (2) active Na+ transport has a primary or secondary role in the secretion of Na+ and Cl-; and (3) the rate of liquid secretion can be modulated by hormones (arginine vasopressin), autocoids (prostaglandin E1 and E2), growth factors (epidermal growth factor), and unknown factors in cyst fluids. Cellular proliferation. Epithelial cells of renal cysts appear to proliferate more than normal. Each cyst resembles a tumor, except that the mass is composed primarily of liquid rather than cells. The proliferation of cyst epithelial cells is associated with: (1) abnormal expression of proto-oncogenes; (2) abnormal displays of morphologic and biochemical phenotypic markers; and (3) abnormal responsiveness to growth factors. The maturation arrest hypothesis, introduced as a framework to explore the pathogenetic basis of all renal cysts, supposes that the epithelial cells comprising cysts are "locked" in an immature, dedifferentiated state. Therapeutic strategies to control the growth of renal cysts may reasonably target processes that inhibit fluid secretion, maximize fluid absorption, and redifferentiate the immature and abnormally proliferative epithelial cells within cysts.

Unmasking of the Apical Electrogenic H Pump in Isolated Malpighian Tubules &lt;i&gt;(Formica polyctena&lt;/i&gt;&lt;i&gt;)&lt;/i&gt;by the Use of Barium

In the present study evidence is given for the presence of an electrogenic, vacuolar type ATPase (V type ATPase) in the apical membrane of malpighian tubules of Formica. Barium (6 mM), the metabolic inhibitor monoiodo-acetic acid (MIA; 5 10-4M) and two inhibitors of V type proton ATPases, i.e. bafilomycin A1 (Baf-A1; 5·10-6M) and N-ethylmaleimide (NEM; 5 10–4M), all inhibited fluid secretion significantly (p < 0.05). This is in agreement with the hypothesis that K enters passively via K channels in the basolateral membrane and that a V type ATPase is involved in the active transport step at the apical membrane. Also MIA, NEM and Baf-A1 slightly depolarized the apical membrane potential, Vap, by 16, 17 and 30 mV, respectively, whereas they had virtually no effect on the basolateral membrane potential (Vm). The mild effect on Vap, in contrast with the pronounced effect on fluid secretion, can be explained by the high apical over basolateral membrane resistance: the voltage divider ratio, VDR, was 47 ± 9 (n = 6). As a consequence the basolateral membrane will impose its value on the other barriers. VDR was decreased to 1.4 ± 0.2 (n = 19) by Ba. Ba also caused a strong and reversible hyperpolarization of both Vb1 and Vap (from – 16 ± 1 to – 84 ± 4 (n = 8) and from -51 ± 4 to-96 ± 6 mV, respectively). As expected MIA, NEM and Baf-Al now had a much more pronounced depolarizing effect, i.e. they drastically reduced the Ba-induced hyperpolarization of both Vap and Vb1 The reduction in Vap was 67, 67 and 54 mV, respectively. Vb1depolarized from -73 ± 4to-15 ± 4mV (n = 7), from-72 ± 10 to 13 ± 2mV(n≈5)and from – 78 ± 3 to – 30 ± 5 mV (n = 12) in the presence of Ba and MIA, NEM or Baf-A1, respectively. From cable analysis and total transepithelial resistance in the absence and presence of barium it was also possible to make an estimate of the resistances across the different barriers: total basolateral resistance = 10 Ω·cm2, total apical resistance = 475 Ω·cm2, total shunt resistance = 228 Ω·cm2. It was concluded that in malpighian tubules of Formica an H pump of the V type is present in the apical membrane. As suggested in other epithelia this pump can be the prime mover in active K transport: the proton concentration gradient built up across the apical membrane can drive a K/H exchanger.

Characteristics of fluid secretion from isolated rat pancreatic ducts stimulated with secretin and bombesin.

1. Micropuncture techniques were used to study the cellular mechanisms of fluid secretion by interlobular ducts isolated from the pancreas of copper-deficient rats. 2. Perifusing ducts with a calcium-free buffer containing 5 mM-EGTA reduced the volume of fluid secreted in the presence of 10 nM-bombesin by 62%, whereas fluid secretion measured in the presence of 10 nM-secretin was reduced by only 26%. 3. The anion selectivities of the fluid secretions evoked by secretin and bombesin were different. The anion sequence for secretin was: Br- = I- = NO3- = Cl- (1.0) much greater than thiocyanate = gluconate (0.3); whereas the sequence for bombesin was: Br- = Cl- (1.0) greater than I- = NO3- (0.6) greater than thiocyanate = gluconate (approximately 0.3). 4. SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid; mM), reduced fluid secretion measured in the presence of bombesin by 61%, but had no effect on the response to secretin. 5. The K+ channel blockers, barium (3 mM) and tetraethylammonium (TEA; 10 mM), inhibited fluid secretion measured in the presence of both secretin and bombesin by between 52 and 66%. 6. From these results, we conclude that secretin and bombesin may utilize different intracellular signalling pathways and, furthermore, may activate different anion secretory mechanisms within the pancreatic ductal epithelium. However, the effect of the potassium channel blockers is consistent with both peptides activating secretory mechanisms which are electrogenic, and which depend for their operation on potassium efflux across the basolateral membrane of the duct cell.

Effect of vasoactive intestinal peptide, bombesin and substance P on fluid secretion by isolated rat pancreatic ducts.

1. We have used micropuncture techniques to study the regulation of fluid secretion by interlobular ducts isolated from the pancreas of copper-deficient rats. 2. Ducts isolated from different strains of Wistar rats exhibited quantitative differences in basal fluid secretion; however, secretion rates measured in the presence of secretin were similar. 3. Vasoactive intestinal peptide had no effect on fluid transport. 4. Bombesin stimulated fluid secretion, and this effect was abolished by removal of extracellular bicarbonate. 5. Substance P inhibited basal secretion, and that stimulated by bombesin and secretin. These inhibitory effects were partially reversed by spantide. 6. Substance P also inhibited fluid secretion stimulated by dibutyryl cyclic AMP and forskolin. This places the site of inhibition mediated by substance P at a point in the secretory mechanism distal to the generation of cyclic AMP. 7. We conclude that rat pancreatic duct cells possess receptors for bombesin and substance P, in addition to 'secretin-preferring' receptors. Since VIP had no effect on fluid transport, it is unlikely that 'VIP-preferring' receptors are present on rat duct cells.

The role of leucocytes in the induction of fluid secretion by Salmonella typhimurium

Nitrogen mustard (N2M) treatment of rabbits induced neutropenia, and, in ligated ileal loops, it inhibited fluid secretion induced by salmonella or by cholera toxin (CT). Pretreatment of rabbits with indomethacin almost abolished salmonella-induced fluid secretion and significantly reduced that induced by CT. Similar effects of N2M and indomethacin on fluid secretion induced by salmonella, but not by CT, have been reported by other workers and used to implicate prostaglandins, from the salmonella-induced inflammation, as mediators of fluid secretion. In contrast, we show that N2M treatment, in addition to reducing CT-induced secretion, caused severe morphological alterations to ileal mucosa. Irradiation techniques were developed for inducing neutropenia, but they did not totally inhibit salmonella-induced leucocyte influx into ileal mucosa. We propose an alternative mechanism for the inhibitory effect of N2M on salmonella- and CT-induced secretion, based on the known anti-mitotic activity of N2M. Also, the anti-secretory effect of indomethacin cannot be attributed uniquely to its anti-inflammatory activity because it depressed CT-induced secretion as well as salmonella-induced secretion. These results support the concept of pathophysiological secretion in infectious diarrhoea, developed previously for rotavirus and extended to bacterial infections.

Effect of Bacillus thuringiensis kurstaki δ-endotoxin on insect Malpighian tubule structure and function

The effect of Bacillus thuringiensis kurstaki HD-73 δ-endotoxin was determined on the cytoarchitecture and urine transport in isolated Malpighian tubules from larvae of the skipper butterfly Calpodes ethlius. Endotoxins (134 and 64 kDa) were injected into the tubule lumen or applied externally in the bathing medium of Ileac plexus-rectal lead and Yellow region tubules. Mucosal (lumenal) exposure in both tubule regions with the 63 kDa toxin at 16 μg/ml completely inhibited urine secretion and induced massive cytolysis. Serosal (bathing medium) exposure of the 63 kDa toxin in both tubule regions had variable activity and required a 20-fold higher concentration to inhibit fluid secretion and disrupt cell structure. Mucosal exposure to the 134 kDa endotoxin was without effect on cell structure or fluid secretion in both tubule regions confirming its protoxin status. The 63 kDa toxin induced similar cytological alterations in both Ileac plexus-rectal lead and Yellow region Malpighian tubule primary cells. However, mucosal and serosal exposure had different patterns of membrane disorganization, with those membranes closest to the site of application being disrupted first. After 1 hr exposure mucosally, primary cells exhibited variable cytopathological changes with a progression of severity from low to high toxin concentration. Stage 1 (least severe) changes involved enlargement of spaces in the cytoplasm and basal infolds whereas Stage 2 alterations included microvillar disorganization and lysis, mitochondrial alterations, and continued enlargement of cytoplasmic spaces. Stage 3 (most severe) changes included apical and basal membrane lysis, swelling of the rough endoplasmic reticulum and mitochondria, and leakage of cytoplasmic material into the tubule lumen. These results demonstrate that the 63 kDa B. thuringiensis endotoxin affects a fluid-transporting epithelium other than midgut. The similar patterns of activity in two physiologically distinct Malpighian tubule regions by the 63 kDa toxin is most consistent with the idea that the toxin acts by forming pores in cell membranes rather than by poisoning histospecific membrane proteins.

Mechanism of action of calcitonin on secretion in rat submandibular gland

Calcitonin (CT) was found to reduce the initial flow of pilocarpine-stimulated saliva from the submandibular glands in the rat. Although there was a concomitant increase of the concentration of calcium and protein in the saliva, the calcium/protein ratio was not significantly affected. CT also caused a significant increase of the potassium concentration in submandibular saliva. Both in vivo and in vitro, CT inhibited the production of cyclic AMP (cAMP) both in the absence and in the presence of forskolin. This decrease in intracellular cAMP levels could result in an inhibition of mucus secretion, which would explain the previously observed calcitonin-induced intracellular accumulation of mucus in the submandibular gland acinar cells. CT did not affect the cytoplasmic free Ca2+ concentration (as measured with fura 2) in isolated submandibular acini either in the absence or in the presence of cholinergic or adrenergic agonists. These results indicate that the inhibition of fluid secretion in the submandibular gland by calcitonin must be located distal to changes in [Ca2+]i. It can be concluded that CT affects both mucus and fluid secretion in the submandibular gland, but that only the inhibition of mucus secretion can as yet be explained by an effect at the level of the second messenger.

Cytoskeletal involvement in spermiation and sperm transport

The process of spermiation and sperm transport was studied using specific inhibitors of cytoskeletal elements. Within 12–24 hr after the intratesticular injection of taxol, a compound that acts to stabilize microtubules and inhibit microtubule-related processes, an unusually large number of microtubules was seen within the body of the Sertoli cell. At the same time, transport of elements within the seminiferous epithelium was affected. At the end of stage VI of the cycle, step 19 spermatids were maintained in the deep recesses of the Sertoli cell and not transported to the rim of the seminiferous tubule lumen. At stage VIII, residual bodies remained at, or near, the rim of the tubule and were not transported to the base of the tubule. They underwent only partial degradation at this site, indicating that there may have been two phases involved in their dissolution—one autophagic and one phagocytic, but the latter did not occur since the residual bodies were not transported to Sertoli lysosomes at the base of the tubule. The observations suggest that microtubules are involved in transport processes within the seminiferous epithelium. Within 1–12 hr after the intratesticular injection of 500 μM cytochalasin D, a compound which interferes with actin-related processes, normal appearing tubulobulbar complexes were not present. The tubular portion (distal tube) of the complex did not initiate development. It was assumed that filaments (which were identified as such using NBD-phallacidin and the S-1 fragment of myosin) played an important role in the development of this portion of the complex. Cells did not eliminate cytoplasm normally, as evidenced by an enlarged cytoplasmic droplet, further emphasizing the published role for tubulobulbar complexes in cytoplasmic elimination. Although sperm were released normally from stage VIII tubules, many remained within the tubular lumen and did not traverse the duct system. Cytochalasin did not inhibit fluid secretion by the Sertoli cell, as demonstrated by efferent duct ligation, but did alter myoid cell actin cytoskeletal organization, suggesting that myoid cell contractility is primarily responsible for transport of sperm. Overall, the observations suggest that cytoskeletal activity of the Sertoli cell is important for several aspects of the spermiation process as well as sperm transport.

A Novel Target for Antidiuretic Hormone in Insects

Diuresis in insects is controlled by two antagonistic hormone groups: diuretic hormones, which promote water loss, and antidiuretic hormones, which inhibit it. All known antidiuretic factors act solely to promote fluid reabsorption by the hindgut and do not affect secretion by the Malpighian tubules. In the house cricket, Acheta domesticus, an antidiuretic hormone was found that inhibits fluid secretion by the Malpighian tubules but has no effect on the hindgut. Correlations were found between the density of neurosecretory granules and the presence of antidiuretic hormone in the corpora cardiaca, suggesting that the hormone is released from specific axons. Its release is triggered by dehydration; the hormone is detectable in the hemolymph of water-deprived crickets. These results imply that an unusual mechanism regulates water balance in these insects.

Adaptation to ingestion of β-carboline alkaloids by Heliconiini butterflies

Butterflies of the tribe Heliconiini (Nymphalidae) are specialist herbivores in which the larvae utilize the plant family Passifloraceae. Their adaptation to their host plant's toxins was examined by analyzing retention and excretion of three Passiflora alkaloids: norharman, harman and harmine. Three species of adult Heliconiines were analyzed for the presence of these β-carboline alkaloids. The butterflies were found to sequester a substantial proportion of the alkaloids ingested by the larvae. Adults of Spodoptera exiqua (Hubner) (Noctuidae) raised as larvae on an artificial diet containing the β-carboline, harman, were also analyzed and they were found to eliminate harman almost completely. Excretion studies, using isolated Malpighian tubules from larvae of Heliconius ismenius clarescens (Heliconiini), identified concentrations of norharman which reversibly inhibit fluid secretion. Norharman appears to be transported in Malpighian tubules by a saturable system.

Mechanism of fluid secretion in isolated shark renal proximal tubules.

Renal proximal tubules from the glomerular Squalus acanthias were studied for evidence of fluid secretion by closing one end of isolated tubules and leaving the other end open so that secreted fluid could be collected. When tubules were bathed in shark Ringer, fluid secretion rate was 27.6 +/- 3.9 (SE) pl X min-1 X mm-1 (21 tubules). Dibutyryl cAMP stimulated fluid secretion 50% (P less than 0.02, n = 14), furosemide inhibited fluid secretion 50% (P less than 0.01, n = 6), and metabolic inhibitors blocked fluid secretion nearly 100%. Secreted fluid was slightly hyperosmotic to peritubular bath (P less than 0.01, n = 7) but Na, Cl, S, K, and Ca concentrations were not significantly different from bath concentrations (wavelength-dispersive spectroscopy, electron probe analysis). cAMP had no effect on secreted fluid composition, and in some tubules cAMP did not stimulate fluid secretion. In conjunction with previous data we propose that spontaneous fluid secretion is driven by secretion of NaCl. However, finding the mechanism of NaCl and fluid secretion in glomerular renal tubules offers new perspectives of some previously inexplicable phenomena in the renal physiology of fish.

The enteric nervous system participates in the secretory response to the heat stable enterotoxins of Escherichia coli in rats and cats

Intestinal secretion was evoked in periarterially denervated jejunal segments of anesthetized rats and cats by exposing the intestines to the heat stable (ST) toxins from a strain of Escherichia coli producing both STa and STb toxins. The secretion was significantly inhibited and to about the same relative extent by the addition of each one of the three following drugs: hexamethonium (i.V., rats), lidocaine (applied on the serosal surface, rats) and tetrodotoxin (intra-arterial, cats). Atropine inhibited fluid secretion in some experiments. It is proposed that a nervous mechanism is mediating part of the secretory response to Escherichia coli heat stable toxins, since three different drugs, which influence nervous activity in different ways, significantly diminished the secretory response. A model for the secretory nervous reflex(es) within the enteric nervous system is proposed; Escherichia coli heat stable toxins activate a “receptor cell” in the epithelium, which then stimulates surrounding dendritic nerve endings via the release of unknown substance(s). A nicotinic receptor is involved but further characteristics of the nervous reflex(es) remain to be elucidated.