Hormonal control of renal functions in insects

Insect Malpighian tubules secrete an isosmotic, KCl-rich primary urine containing low concentrations of most other blood solutes. Neuropeptide diuretic hormones (DH), possibly related to vasopressin, stimulate tubular fluid secretion by 2- to 200-fold in response to water loading, e.g., feeding. DH acts on tubules through cyclic AMP (cAMP) to stimulate salt transport without measurable change in osmotic permeability. Changes in composition of tubular secretion after stimulation and the possible control of DH release are discussed. Most of the water, ions, and metabolites in tubular secretion are normally reabsorbed by active mechanisms in the rectum, where the urine may finally become either hyposmotic or strongly hyperosmotic to the blood. A newly discovered neuropeptide, chloride transport-stimulating hormone, controls (via cAMP) reabsorption of the principal salt by stimulating K-dependent, electrogenic transport of Cl- across the apical cell border. Passive net absorption of K+ is thereby enhanced. Diuretic and antidiuretic factors may control osmotic permeability of the rectal wall and thereby influence the osmotic concentrations of the rectal absorbate and final urine. The increased recycling of a KCl-rich fluid through the Malpighian tubule-rectal system after feeding probably serves to clear the body of unwanted substances ingested with, and produced by, metabolism of the meal.

Patterns of cloacal water and electrolyte excretion in constantly-loaded intact and colostomized ducks (Anas platyrhynchos)

1. The patterns of cloacal water and electrolyte excretion were qualitatively similar in intact and chronically colostomized ducks given oral loads of either distilled water or electrolyte solution. 2. The rates of Na+ and K+ excretion were each selectively increased in birds loaded with solutions containing a relatively higher concentration of each ion. 3. Compared to the intact birds, the colostomized birds showed lower rates of electrolyte excretion following administration of identical electrolyte loads and thus appeared to be retaining electrolyte. 4. At the same time, a significant antidiuresis of similar magnitude occurred in each group of saline-loaded birds. 5. The possible contributions of renal and post-renal mechanisms to the overall conservation of water in intact and colostomized ducks are discussed.

Salt and water excretion by birds: the lower intestine as an integrator of renal and intestinal excretion

1. In the fowl, the small intestine is important for net absorption of Ca2+ and K+, but not for Na+ nor water (in this and several other species). 2. Net water absorption in birds with large saccate caeca occurs in caeca greater than rectum greater than coprodeum, but net Na+ absorption (an active process motivating other absorptive functions) occurs in rectum less than caeca and coprodeum. 3. Interspecific variability and the scarcity of comparative studies militate against broad, well-founded generalisations in this subject.

Some factors affecting Malpighian tubule fluid secretion and transepithelial potential in Locusta migratoria L

Both fluid secretion and transepithelial potential were stimulated by cAMP. Fluid secretion was unaffected by 5-HT over the concentration range 10(-8)-10(-4) M. The presence of ouabain in the bathing medium effected a decrease in transepithelial potential.

Sodium transport in the hen lower intestine. induction of sodium sites in the brush border by a low sodium diet

1. The fluxes of Na were measured on isolated coprodeal mucosa at 1--220 mM-Na from hens on low (L) and high (H) Na diets with the purpose of finding the location and characteristics of Na sites activated in the cellular pathway by L. 2. The influx across the brush border, JNamc, and the transmural fluxes, JNasm and JNams, were determined. Effects on these fluxes of ouabain, 10(-3) M in the serosal solution, and amiloride, 10(-4) M in the mucosal solution, were studied for both dietary states. 3. JNamc was 5--22 (L) and 0--0.8 (H) muequiv/cm2.hr at 130 mM-Na corrected for the paracellular flux of Na. The JNamc (H) is tenfold smaller than found by Choshniak, Munck & Skadhauge (1977). This discrepancy is at present inexplicable. Amiloride completely inhibited JNamc (L). Preincubation in 0 or 130 mM-Na had no effect on JNamc. Ouabain reduced JNamc (L) by only about 37% after preincubation at 130 mM-Na. The Kt of JNamc was 5.1 (L) and 50.6 (H) mM-Na. 4. JNasm was 50 (H) and 61 (L) n-equiv/cm2.hr at 6.5 mM-Na. Ouabain increased JNasm by 360% in the low Na state. The increased JNasm was inhibited 74--100% by amiloride. This is interpreted as a ouabain induced Na-Na exchange at the basolateral Na-K-ATPase and an almost complete block of JNacm by amiloride. A similar exchange of Na at the basolateral membrane in the high-Na state was revealed by 'opening' the brush border for Na with monensin added to the mucosal solution. Amiloride in itself prevented a 50% recirculation of Na via the paracellular route and back across the cells in the low Na state. 5. JNams was 5.6 (L) muequiv/cm2.hr and 187 (L) microA/cm2 at 6.5 mM-Na. Amiloride reduced these values to 0.4 muequiv/cm2.hr and 5.8 microA/cm2. On addition of amiloride the transmural resistance in (L) coprodea at 130 mM-Na increased from 140 to 190 and it remained unchanged at 260 omega cm2 in (H) coprodea. The resistance of (L) birds, 163, was not affected by ouabain, 166 (L) omega cm2. 6. 20:1 NaCl dilution potentials at the mucosal side of 17--18 mV (L) and nearly zero (H) had half-times around 1 sec. Amiloride eliminated completely these diffusion potentials. The short half-time indicates a location in the brush border of sodium specific sites induced by the low-Na diet. This conclusion is oppsite to that described by Choshniak et al. (1977). 7. Ion selectivity, voltage--current and conductance--concentration relations in the presence of amiloride indicated a weakly cation selective and highly hydrated pathway, which was also thick and with neutral sites. This fits a paracellular route with the limiting barrier for ions at the tight junction.

Estrogen target sites in the cloacal region of female and male chick embryos

After intravenous injection of 3H-estradiol in the 12-day old chick embryo, radioactivity is concentrated in nuclei of certain cells in the cloacal area. The nuclear labeling is observed in mesenchymal cells along the different portions of the cloaca, and in an unidentified tissue located laterally to the cloaca. The labeled mesenchymal cells display a definite pattern of distribution along the epithelial wall of the cloaca, identical both in male and in female embryos. In the adjacent bursa of Fabricius, cells do not concentrate labeled hormone in their nuclei. The presence of estrogen receptors in the cloacal area of embryos of either sex adds evidence, at the cellular level, to support the concept of a "neutral", or undifferentiated, sex with estradiol inhibiting this "neutral" male differentiation.

The influence of the conventional microflora on the body temperature of the chick

1. Deep body temperature was measured daily in germ-free and conventional chicks from 1 to 16 of age. 2. The temperature of germ-free chicks rose rapidly during the first 4 d and thereafter only slowly, whereas that of conventional birds increased gradually throughout. 3. Body temperature attained by the germ-free birds was about 0.3 degrees C higher than that of conventional birds.

Excretion in insects: function of gut and rectum in concentrating and diluting the urine

The diverse excretory systems of insects exhibit several features that appear unusual when comparisons are made with the mammalian kidney. Secretion by the Malpighian tubules of a fluid that is unlike the blood in composition, substitutes for glomerular filtration. Various reabsorptive functions, such as volume reduction, regulation of individual electrolytes, adjustment of osmotic concentration and pH regulation, which are associated with distinct renal segments in the mammalian kidney, all occur simultaneously in the rectum of terrestrial insects. Involvement of an extracellular molecular sieve in selective reabsorption is novel. As far as water transport is concerned, the rectal pads of the cockroach and locust appear to accomplish, across a single layer of cells, the same function as the countercurrent multiplier system of the mammalian kidney with its several epithelial layers. Direct absorption of water vapor in the rectum of some insects from atmospheres of low relative humidity, clearly involves quite different and unknown mechanisms. Finally, saline-water insect larvae produce hyperosmotic excreta by direct secretion of ions into the rectal lumen. They can adjust individual transport processes to form various secretions, which are appropriate to the natural waters of diverse chemical types in which these larvae thrive.

The action of the excretory apparatus of Calliphora vomitoria in handling injected sugar solution

Recent evidence suggests that the isolated Malpighian tubules of Calliphora possess mechanisms which restrict the loss of glucose and trehalose from the insect. This report establishes that the intact, diuresing fly does not excrete glucose or trehalose when solutions of these sugars are injected. When solutions of non-metabolized sugars such as sorbose and xylose are injected into the fly, these sugars are rapidly excreted. High concentrations of sorbose and xylose are found in the urine, suggesting that rapid reabsorption of fluid occurs in the excretory apparatus even during the diuresis which the injections provoke. However, injected sucrose is apparently not excreted in large amounts and it is possible that the Malpighian tubules when functioning in vivo are impermeable to disaccharides.

Structural-functional correlations in the kidneys and observations of colon and cloacal morphology in certain Australian birds

1. Variations in renal microstructure between the zebra finch and Senegal dove were consistent with their relative renal concentrating abilities (urine/plasma ratios of 2-8 and 1-7, respectively). Compared with dove kidneys, those of the finch contained a higher fraction of mammalian-type nephrons (with Henle's loops), and a lower fraction of reptilian-type nephrons (without loops). 2. Singing honeyeaters concentrated their urine almost as well as zebra finches, although honeyeater kidneys were less specialized (fewer mammalian-type nephrons). Such findings emphasize the need to clarify other osmoregulatory parameters. 3. No significant microstructural differences were found in the kidneys of domesticated as compared with those of wild zebra finches. Hence, osmoregulatory differences between tame and wild birds must be related to physiological factors rather than morphological. 4. Thickness of the renal medulla seemed to be directly correlated with urine concentrating ability. However, certain inconsistencies obscure this relationship such that its resolution will require further research. 5. Histological features of the mucosae of the colon and cloaca are described. The galah and kookaburra displayed a mammalian (non-villous) pattern of mucosal organization. Zebra finches, singing honeyeaters, and particularly emus, possessed colonic and cloacal villi and hence an increased surface area per volume in this region of the gut. This raises the possibility that the colon and cloaca are involved in uring concentration and osmoregulatory activities in these species.

Sites of organic acid production and pattern of digesta movement in the gastrointestinal tract of geese

Sixteen geese were used to assess the movement of fluid and particulate digesta through their gastrointestinal tracts and to determine the diurnal variation in organic acid levels for the various segments of the tract. Fluid (polyethylene glycol and chronium-labeled ethylenediaminetetraacetic acid) and particulate markers (2 and 5 mm long) were administered with the meal. Animals were killed at given intervals after the administration of markers. The gastrointestinal tract was divided into nine segments for measurement of markers, pH, volatile fatty acids (VFA), and lactic acid contents. The data indicated a rapid evacuation of fluid marker from the foregut (crop, ventriculus, and proventriculus), while particulate markers were retained for extended periods of time. Retention of fluid marker was observed only within the cecum. Retrograde movement of particulate marker was demonstrated from the duodenum to ventriculus and proventriculus. Retrograde movement of fluid marker was observed from the cloaca to the colon, cecum, and distal third of the small intestine. However, particulate marker showed no retrograde movement in these segments of tract. Highest VFA levels were observed in the cecum. Retention of digesta and production of VFA within the colon were less than those noted for the dog, pig and pony. Lactic acid comprised less than 10% of the organic acids present in the gastrointestinal tract and were at their highest levels in the proximal and mid small intestine.

The control of diuresis in the tsetse fly Glossina austeni: a preliminary investigation of the diuretic hormone

The rate of secretion of the Malpighian tubules of Glossina austeni is controlled by a diuretic hormone. This hormone is present in the nervous tissue of the fly together with a degradative enzyme that can be activated by boiling. It is demonstrated that the Malpighian tubules are able to destroy the diuretic hormone; they may therefore participate in the control of diuresis. The diuretic hormone appears to be a heat-stable, non-dialysable, alcohol-soluble molecule, containing amino acid, glucose and sialic acid residues.

Thermoregulatory behavior and body temperature in chicks of willow grouse (Lagopus lagopus lagopus)

Clocal temperatures (Tcl) of outdoor living captive willow grouse chicks (Lagopus lagopus lagopus) were found to increase from 39.4 +/- 0.5 degrees C. the first day to 40.3 +/- 0.5 degrees C. the twelfth day after hatching. Average Tcl of adults was 40.7 +/- 0.3 degrees C. When left alone for 30 min. in a controlled test environment providing temperatures ranging from 21 degrees C. to 46 degrees C., the one day old chicks preferred significantly higher ambient temperatures than eight day old birds. Tcl was significantly lower in the chicks tested the day after hatching than in the older chicks. It is concluded that the chicks' thermoregulatory behavior is essential for maintaining homeothermia, and that the birds' thermoregulatory set-point is low the day after hatching and climbs to adult level during the first week.

Copulatory plugs in snakes: enforced chastity

The male of some snake species forms a copulatory plug which occludes the oviductal parts of the female's cloaca for a few days. The plug, apparently formed from secretions of the kidney immediately after insemination, probably prevents rival males from copulating with the same female.

Excretory role of the midgut in larvae of the tobacco hornworm, Manduca sexta (L.)

Caterpillars of Manduca sexta use two distinct transport mechanisms for the excretion of dyes. One pump (Type A) has a high affinity for acid (anionic) dyes and occurs in the midgut and medial Malpighian tubules. Acid dyes accumulate rapidly in the lumen of the midgut while the Malpighian tubules appear to play only a minor role in the excretion of these dyes. The other pump (Type B) excretes basic (cationic) dyes and is located primarily in the proximal Malpighian tubules. Evidence is presented that hippuric acid competes with acid dyes for excretion by both midgut and Malpighian tubules. After the final-instar larva purges its gut the ability of the midgut and Malpighian tubules to excrete dyes gradually decreases. Sixty hours after the purge only the Malpighian tubules retain some dye excreting activity.