Expression cloning of human and rat renal cortex Na/Pi cotransport

We have isolated two cDNA clones, NaPi-2 and NaPi-3, by screening rat kidney cortex and human kidney cortex cDNA libraries, respectively, for expression of sodium-dependent phosphate transport in Xenopus laevis oocytes. Substrate specificity and a detailed kinetic analysis (Na, Pi, H+ concentrations) suggested that expressed uptake activities relate to proximal tubular brush border membrane Na/Pi cotransport. NaPi-2 cDNA contains 2464 bp encoding a protein of 637 aa; NaPi-3 cDNA contains 2573 bp encoding a protein of 639 aa. NaPi-2- and NaPi-3-deduced protein sequences show high homology to each other but are different from the protein sequence deduced from the previously cloned NaPi-1 cDNA (from rabbit proximal tubules). Hydropathy profile predictions suggest at least eight membrane-spanning regions in NaPi-2/3-related proteins. In vitro translation results in proteins of the expected size and suggests glycosylation. Northern blot analysis shows corresponding mRNA species (approximately 2.7 kb) in kidney cortex of various species but no hybridization with RNAs isolated from a variety of other tissues (including intestinal segments); a hybridization signal (approximately 4.8 kb) was observed only in the lung (human). We conclude that we have structurally identified two closely related proteins most likely involved in human and rat renal brush border Na/Pi cotransport.

Cellular mechanisms of acute and chronic adaptation of rat renal P(i) transporter to alterations in dietary P(i)

Recently, the cDNA for a Na-P(i) cotransport system of rat kidney cortex (NaPi-2) has been identified by expression cloning. Using polyclonal antibodies raised against this renal Na-P(i) cotransport system, and using the polymerase chain reaction after reverse transcription of mRNA in microdissected nephron segments, we recently demonstrated that NaPi-2-related mRNA and protein is expressed in the brush-border membranes (BBM) of the proximal tubules of rat kidney. The purpose of the present study was to study the cellular mechanisms involved in adaptation of rat renal Na-P(i) cotransporter to acute and chronic alterations in dietary P(i). Compared with rats fed chronically (7 days) a high-P(i) diet (1.2%), in rats fed chronically a low-P(i) (0.1%) diet the 3.4-fold increase in BBM Na-P(i) cotransport rate (chronic upregulation) was associated with a 2.2-fold increase in renal cortical NaPi-2 mRNA and a 4.9-fold increase in BBM NaPi-2 protein abundances. In contrast, compared with rats fed chronically (7 day) a high-P(i) diet, in rats fed acutely (2 h) a low-P(i) diet the 1.5-fold increase in Na-P(i) cotransport rate (acute upregulation) was associated with a 1.8-fold increase in NaPi-2 protein but no change in NaPi-2 mRNA abundance. Similarly, compared with rats fed chronically a low-P(i) diet, in rats fed acutely (2 h) a high-P(i) diet the 1.9-fold decrease in Na-P(i) cotransport rate (acute downregulation) was associated with a 3.8-fold decrease in NaPi-2 protein but no change in NaPi-2 mRNA abundance.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of thyroid hormone in regulation of renal phosphate transport in young and aged rats

In the present study, we have examined the cellular mechanisms mediating the regulation of renal proximal tubular sodium-coupled inorganic phosphate (Na/Pi) transport by thyroid hormone (T3) in young and aged rats. Young hypothyroid rats showed a marked decrease in Na/Pi cotransport activity, which was associated with parallel decreases in type II Na/Pi cotransporter (NaPi-2) protein and messenger RNA (mRNA) abundance. In contrast, administration of long-term physiological and supraphysiological doses of T3 resulted in significant increases in Na/Pi cotransport activity, protein, and mRNA levels. Nuclear run-on experiments indicated that thyroid hormone regulates NaPi-2 mRNA levels by a transcriptional mechanism. In aged rats, although there were no changes in T3 serum levels (when compared with young animals), there were significant decreases in serum Pi concentration, renal Na/Pi cotransport activity, and NaPi-2 protein and mRNA abundance. These effects were mediated, at least in part, by a reduction in the transcriptional rate of the NaPi-2 gene, probably caused by, among other factors, a smaller response to the stimulatory action of T3. Compared with young rats, the old rats exhibited less sensitivity of the Na/Pi cotransporter to thyroid hormone, with-decreased effects in both hypothyroid (inhibitory) and hyperthyroid (stimulatory) animals.

Interactions of the growth-related, type IIc renal sodium/phosphate cotransporter with PDZ proteins

Despite similar molecular structures, the growth-related sodium/phosphate cotransporter NaPiIIc is regulated differently than the main NaPiIIa phosphate transporter. Using two-hybrid systems and immunoprecipitation, we identified several proteins that interact with NaPiIIc that might account for this differential regulation. NaPiIIc interacted with the PDZ domain-containing sodium-hydrogen exchange-regulating factor (NHERF) 1 and NHERF3 through novel binding motifs in its C terminus. NaPiIIc from brush-border membranes coprecipitated with both NHERF1 and NHERF3, with more NHERF3 co-precipitated in rats fed a low-phosphorus diet. NaPiIIc colocalizes with both NHERF1 and NHERF3 in brush-border membranes of rats fed either a low- or high-phosphorus diet. When mouse NaPiIIc was transfected into opossum kidney cells, it was localized mainly in apical microvilli and the trans-Golgi. Both confocal and total internal reflection microscopy show that NaPiIIc colocalizes with NHERF1 and NHERF3 in the apical microvilli, and this was not altered by truncation of the last three amino acids of NaPiIIc. Interactions of NaPiIIc with NHERF1 and NHERF3 were modulated by the membrane-associated 17 kDa protein (MAP17) similarly to NaPiIIa, but only the MAP17-NaPiIIc-NHERF3 complexes were internalized to the trans-Golgi. Our study shows that NaPiIIc interacts with a limited number of PDZ domain proteins, and the mechanisms and consequences of such interactions differ from those of NaPiIIa.

Regulation of opossum kidney (OK) cell Na/Pi cotransport by Pi deprivation involves mRNA stability

Renal proximal tubular Na-dependent phosphate transport (Na/Pi cotransport) has been studied extensively in the opossum kidney (OK) cell line. Recently, we cloned a complementary deoxyribonucleic acid (cDNA) (NaPi-4) from OK cells encoding an apical NaPi cotransport system. OK cells exposed to a low-Pi medium, as compared to high-Pi media, responded with an increase in Na/Pi cotransport, which was followed by an increase in NaPi-4 messenger ribonucleic acid (mRNA) abundance; maximal stimulation of Na/Pi cotransport was reached in 2 h, with no further increase for up to 16 h. NAPi-4 mRNA abundance was unaltered for 2 h, then increased to a maximum after 6-16 h in cells treated with low Pi medium. NaPi-4 mRNA decay rate was lowered by low-Pi media when compared to high-Pi media, with no increase in the NaPi-4 mRNA transcription rate. These data suggest that the upregulation of Na/Pi cotransport in OK cells by low-Pi media involves two regulatory mechanisms: an immediate (early) increase (after 2 h) in the expression of Na/Pi cotransport, independent of mRNA synthesis or stability, and a delayed (late) effect (after 4-6 h), resulting in an increase in NaPi-4 mRNA abundance, due to an increased stability.

Cellular mechanisms of the age-related decrease in renal phosphate reabsorption

The aging process in humans and in the rat is associated with an impairment in renal tubular reabsorption of Pi and renal tubular adaptation to a low Pi diet. The purposes of the present study were to determine whether changes in the abundance of type II Na-Pi contransporter (NaPi-2) protein and/or mRNA play a role in the age-related decrease in Na-Pi cotransport activity, and to further determine the cellular mechanisms of impaired adaptation to a low Pi diet. In studies performed in 3- to 4-month-old young adult rats and 32-to 16-month-old aged rats we found that there was an age-related twofold decrease in proximal tubular apical brush border membrane (BBM) Na-Pi cotransport activity, which was associated with similar decreases in BBM NaPi-2 protein abundance and renal cortical NaPi-2 mRNA level. Immunohisto-chemistry showed lower NaPi-2 protein expression in the BBM of proximal tubules of superficial, midcortical, and juxtamedullary nephrons. We also found that in response to chronic (7 days) and/or acute (4 hr) feeding of a low Pi diet there were similar adaptive increases in BBM Na-Pi cotransport activity and BBM NaPi-2 protein abundance in both young and aged rats. However, BBM Na-Pi cotransport activity and BBM NaPi-2 protein abundance were still significantly lower in aged rats, in spite of a significantly lower serum Pi concentration in aged rats. The results indicate that impaired expression of the type II renal Na-Pi cotransporter protein at the level of the apical BBM plays an important role in the age-related impairment in renal tubular reabsorption of Pi and renal tubular adaptation to a low Pi diet.

The chick embryo appears as a natural model for research in beta-amyloid precursor protein processing

This study reveals that the chick embryo has active the machinery for the production and degradation of the amyloid beta peptide characteristic of Alzheimer's disease. We cloned the principal beta-amyloid precursor protein isoforms in the chick embryo and observed that they are highly homologous to the human sequences and identical at the C-terminal sequence, including the amyloid beta domain. Mammals such as rat or mouse, more commonly used as animal models of human diseases, have a distinct amyloid beta sequence. The distribution of beta-amyloid precursor protein isoforms in the chick embryo revealed that, as in humans, their expression is ubiquitous and the prototype beta-amyloid precursor protein-695 predominated in the nervous system. We also found that the chick embryo expresses the genes for the main proteolytic proteases implicated in the production of amyloid beta, including BACE-1, BACE-2, presenilin-1, presenilin-2 and nicastrin, as well as the amyloid beta-degrading enzyme neprilysin, or ADAM-17, a protease implicated in the non-amyloidogenic processing of beta-amyloid precursor protein. We have also found that between amyloid beta40 and amyloid beta42, this latter seems to be the major amyloid beta peptide produced during chick embryogenesis. The chick embryo appears as a suitable natural model to study cell biology and developmental function of beta-amyloid precursor protein and a potential assay system for drugs that regulate beta-amyloid precursor protein processing.

Thyroid hormone stimulation of Na/Pi-cotransport in opossum kidney cells

Thyroid hormone (T3), a known stimulator of renal proximal tubular brush border membrane Na-dependent phosphate (Pi) uptake (Na/Pi-cotransport), stimulated Na-dependent Pi transport in opossum kidney (OK) cells. Na/Pi-cotransport was stimulated in a time- and dose-dependent manner with maximal effects (57%) at 24 h and 10(-10) M T3. This stimulation was related to an increase in the apparent capacity (Vmax) of Na/Pi-cotransport. Treatment with T3 had no effect on Na-independent transport of Pi or of L-arginine. The stimulation of Na/Pi-cotransport was paralleled by an increase in the messenger ribonucleic acid (mRNA) encoding the OK cell apical Na/Pi-cotransporter (termed NaPi-4); the mRNA levels related to the activity of Na-independent L-arginine transport (rBAT) were unaffected by T3. Actinomycin D (10(-7) M) completely prevented the stimulatory effect of T3 on OK cell Na/Pi-cotransport and on NaPi-4 mRNA content. In conclusion, T3 stimulates apical Na/Pi-cotransport in OK cells most likely by enhancing its transcription.

Cloning of a rabbit renal Na-Pi cotransporter, which is regulated by dietary phosphate

Previously, we isolated a cDNA (NaPi-1) related to a rabbit renal proximal tubular Na-Pi cotransporter (A. Werner, M.L. Moore, N. Mantei, J. Biber, G. Semenza, and H. Murer. Proc. Natl. Acad. Sci. USA 88:9608-9612, 1991.). In this study, we isolated an additional (rabbit renal) cDNA (NaPi-6), which induces Na-dependent Pi uptake in Xenopus laevis oocytes. Substrate specificity and kinetic properties corresponded to those known for rabbit renal brush-border membrane (BBM) Na-Pi cotransport. NaPi-6 was cloned by homology using NaPi-2 cDNA, a rat renal BBM Na-Pi cotransporter (S. Magagnin, A. Werner, D. Markovich, V. Sorribas, G. Stange, J. Biber, and H. Murer. Proc. Natl. Acad. Sci. USA 90: 5979-5983, 1993). NaPi-6 encodes a protein of 642 amino acids, exhibiting at least eight transmembrane domains. NaPi-6 mRNA and protein in kidneys of rabbits fed a low-Pi diet (LPD; 0.11% Pi) for 1 wk were increased by 1.5- and 4-fold, respectively, compared with those of rabbits fed a high-Pi diet (HPD; 1.20% Pi). This effect was correlated with an increase in Na-Pi cotransport of BBM vesicles isolated from animals adapted to LPD (2.5-fold with respect to HPD). In contrast, NaPi-1 mRNA and protein were not altered in response to LPD. Thus rabbit proximal tubular BBMs contain two different Na-Pi cotransport systems: NaPi-1 (type I) and NaPi-6 (type II). Only the type II transport system seems to be under regulatory control in response to low-Pi dietary intake.

Alzheimer beta-amyloid precursor proteins display specific patterns of expression during embryogenesis

The beta-amyloid precursor proteins (betaAPPs) are a family of glycosylated transmembrane proteins that include in their sequences the beta-amyloid peptide, a major component of the characteristic amyloid deposits or senile plaques found in the brains of Alzheimer's disease patients and aged Down's syndrome subjects. Various betaAPP isoforms, mainly betaAPP-695, betaAPP-714, betaAPP-751 and betaAPP-770, the number corresponding to the number of amino acids they encode, resulting from the alternative splicing of a single primary transcript have been described. Using oligonucleotides recognizing each of the four major Alzheimer's betaAPP mRNAs, we have found that each betaAPP mRNA displays a specific temporal and spatial pattern of expression. The prototype isoform betaAPP-695 occurs early in cells actively implicated in morphogenetic events, as those mesodermal cells invaginating at the level of the primitive streak, and it is later restricted to the neurectodermal (neural tube, neural crest and neurogenic placode) derivatives. By contrast, the longest isoform betaAPP-770 appears later and restricted to mesodermal and endodermal derivatives. The isoforms betaAPP-714 and betaAPP-751 are still expressed later than the other two isoforms and distributed ubiquitously, though betaAPP-714 transcripts predominate typically within the neural tube.

Gentamicin causes endocytosis of Na/Pi cotransporter protein (NaPi-2)

BACKGROUND

Renal toxicity is a major side-effect of aminoglycoside antibiotics and is characterized by an early impairment in proximal tubular function. In a previous study, we have shown that gentamicin administration to the rat causes an early impairment in sodium gradient-dependent phosphate (Na/Pi) cotransport activity. The purpose of our current study was to determine the molecular mechanisms of the impairment in Na/Pi cotransport activity, specifically the role of the proximal tubular type II Na/Pi cotransporter.

METHODS

Rats were treated for one, two, and three days with two daily injections of 30 mg/kg body weight gentamicin or the vehicle.

RESULTS

Gentamicin caused a progressive decrease in superficial cortical apical brush-border membrane (SC-BBM) Na/Pi cotransporter activity (856 +/- 93 in control vs. 545 +/- 87 pmol/mg BBM protein in 3-day gentamicin, P < 0.01). Western blot analysis showed a parallel and progressive decrease in SC-BBM Na/Pi cotransporter protein abundance, a 50% decrease after one day of treatment, a 63% decrease after two days of treatment, and an 83% decrease after three days treatment with gentamicin. In contrast, gentamicin treatment had no effect on Na/Pi cotransport activity or Na/Pi cotransporter protein abundance in BBM isolated from the juxtamedullary cortex (JMC-BBM). Immunofluorescence microscopy showed a major decrease in the expression of Na/Pi cotransporter protein in the apical membrane of the proximal convoluted tubule, with progressive intracellular accumulation of Na/Pi protein. Colocalization studies showed that in gentamicin-treated rats, Na/Pi protein was colocalized in the early endosomes and especially in the lysosomes. Northern blot analysis of cortical RNA interestingly showed no reduction in Na/Pi cotransporter mRNA abundance even after three days of gentamicin treatment.

CONCLUSION

We conclude that gentamicin inhibits Na/Pi cotransport activity by causing a decrease in the expression of the type II Na/Pi cotransport protein at the level of the proximal tubular apical BBM and that inhibition of Na/Pi cotransport activity is most likely mediated by post-transcriptional mechanisms.

Expression of Na/Pi cotransport from opossum kidney cells in Xenopus laevis oocytes

Xenopus laevis oocytes have been used for the expression of Na/Pi-cotransport activity by injections of poly(A)+ RNA (mRNA) isolated from an established renal cell line (OK cells). 3-5 days after mRNA injection, Na-dependent phosphate (Pi) uptake by oocytes was increased in a dose-dependent manner; there was no increase in Na-independent Pi uptake. Sucrose density-gradient fractionation indicated that the mRNA species encoding this activity is 2.4-2.8 kb in length. In Northern blots, using a cDNA probe related to human kidney-cortex Na/Pi-cotransport activity (NaPi-3), hybridization with a mRNA-species of 2.4-2.6 kb was obtained. Kinetic characterization ([Pi], [Na]) showed that expressed transport activity has properties similar to apical Na/Pi cotransport in OK cells.

Effect of VIP on sugar transport in rabbit small intestine in vitro

The vasoactive intestinal peptide (VIP) has shown to be widely distributed in the gastrointestinal mucosa, submucosa and nerves, and the existence of VIP receptors on the basolateral membrane of enterocytes has been recently reported for many species. The interaction of VIP with its receptors seemed to increase cyclic AMP level, and this nucleotide has been shown to be responsible for the intestinal secretion produced by VIP. The present study confirms that VIP inhibits the intestinal absorption of D-galactose. This effect seems to be due to the inhibition of the Na(+)-independent basolateral intestinal sugar transport system. RMI 12330A, described as adenylate cyclase inhibitors, blocked the VIP action. These findings suggest that cyclic AMP might be responsible for the inhibition of Na(+)-independent transport of D-galactose across the basolateral membrane. Moreover, results obtained to determine the possible role of calcium in the action of VIP suggest that Ca2+ play a part, directly or indirectly, in the inhibition of the D-galactose transport across the basolateral membrane produced by VIP.

Expression and molecular characterization of rat renal D-mannose transport in Xenopus oocytes

Renal reabsorption appears to play a major role in d-mannose homeostasis. Here we show that in rat kidney, the transport of d-mannose by brush border membrane vesicles from tubular epithelial cells involves an uphill and rheogenic Na-dependent system, which is fully inhibited by d-mannose itself, incompletely inhibited by d-glucose, d-fructose, phloridzin, and phloretin, and noninhibited by l-mannose or disaccharides. In addition, this system exhibits both low capacity (112.9+/-15.6 pmol/mg/second) and high affinity (0.18+/-0.04 mm), with a 2:1 stoichiometry for the Na:d-mannose interaction, and low affinity for sodium (16.6+/-3.67 mm). We also show expression of d-mannose transport by Xenopus laevis oocytes injected with rat renal polyA(+) RNA. Kinetic analysis of the expressed transport was performed after RNA enrichment by fractionation through a sucrose density gradient and was shown to be identical to that measured in membrane vesicles. The RNA species encoding the expressed transport has a small mean size, 1 kb approximately, and shows no homology with the SGLT family of Na-dependent d-glucose transporters, as shown by low stringent RT-PCR and northern analysis. The expressed transport is specific for d-mannose, since in spite of a significant inhibition by d-glucose and d-fructose, neither of these two substrates was transported above the level of the water-injected oocytes.

Study of serotonin interactions with brush border membrane of rabbit jejunum enterocytes

Recent studies have demonstrated that serotonin (5-hydroxytryptamine, 5-HT) may interact with either specific receptors or with a specific transporter that takes up 5-HT in the gastrointestinal tract. The purpose of the present work was to study the 5-HT interactions with brush border membrane from rabbit jejunum enterocytes. The results obtained showed that 5-HT did not seem to be transported by any specific system of transport in brush border membrane vesicles. Nevertheless, [3H]5-HT seemed to bind specifically to this membrane. The kinetic analysis indicated a saturable and dissociable specific binding with a dissociation constant K(D)=14x10(-9) M. The saturation studies with [3H]5-HT indicated the presence of one specific site in the brush border membrane. The results of displacement of [3H]5-HT specific binding from the brush border membrane showed that both unlabelled 5-HT and unlabelled GR113080 ([1-[(2-methyl sulphonyl) amino] ethyl-4-piperidinyl] methyl-1-methyl-1H-indole-3-carboxylate), a specific competitive antagonist of 5-HT(4) receptors, inhibited the specific binding of [3H]5-HT to this membrane.

Effect of erythromycin on D-galactose absorption and sucrase activity in rabbit jejunum

Erythromycin, an antibiotic used in the treatment of infectious diseases, produces gastrointestinal side effects such as diarrhea. The mechanisms by which erythromycin produces these effects are not known. However, erythromycin has been shown to increase gastrointestinal motor activity and to inhibit intestinal neutral amino acid absorption. Both effects could contribute to the gastrointestinal side effects observed. Because the intestinal systems of amino acid and sugar transport present similar characteristics, the aim of the present work was to determine whether erythromycin also alters D-galactose absorption and sucrase activity in rabbit jejunum. The results show that erythromycin diminishes intestinal D-galactose absorption. This effect seems to be due to an action mainly located on the Na(+)-dependent sugar transport of the mucosal border of the intestinal epithelium. Erythromycin also inhibits the Na(+)-K+ ATPase activity of the enterocyte, which might explain the inhibition of the D-galactose Na(+)-dependent transport. However, a direct action of the erythromycin molecule on the Na(+)-dependent carrier cannot be excluded. Erythromycin did not alter sucrase activity.

Effect of erythromycin on L-threonine transport in rabbit jejunum in vitro

Several antibiotics characterized by different molecular structures are known to affect some intestinal activities. Some of them have been described as inhibitors of the intestinal sugar and amino acid transport with different mechanisms. Erythromycin (EM) is a macrolide antibiotic acting as a motilin agonist and thus stimulating the gastrointestinal motor activity. Since several substances which increase the motor activity of the gastrointestinal tract may produce effects on the intestinal absorption of nutrients, the present study has been carried out to determine whether erythromycin affects the L-threonine intestinal absorption. The results obtained indicate that erythromycin diminishes the L-threonine intestinal transport, probably at the mucosal border level. Two groups of experiments carried out, with Na(+)-deprived medium and ouabain-enriched medium, might indicate that erythromycin action could be due to either a direct or an indirect action on the Na(+)-dependent L-threonine transport located in the brush border.

Expression of rat ileal Na(+)-sulphate cotransport in Xenopus laevis oocytes: functional characterization

Small-intestinal sulphate absorption is a Na(+)-dependent process having its highest rate in the ileum; it involves brush-border membrane Na(+)-sulphate cotransport. Injection of rat ileal mRNA into Xenopus laevis oocytes induced Na(+)-dependent sulphate uptake in a dose-dependent manner, with no apparent effect on Na(+)-independent sulphate uptake. For mRNA-induced transport, the apparent Km value for sulphate interaction was 0.6 +/- 0.2 mM and that for sodium interaction was 25 +/- 2 mM (Hill coefficient: 2.3 +/- 0.3). mRNA-induced transport, was inhibited by thiosulphate, but not by phosphate or 4,4,'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). Using a rat renal Na(+)-sulphate cotransporter cDNA as a probe [NaSi-1; Markovich et al. (1993) Proc Natl Acad Sci USA 90:8073-8077], the highest hybridization signals (2.3 kb and 2.9 kb) were obtained in size fractions showing the highest expression of Na(+)-dependent sulphate transport in oocytes. Hybrid depletion experiments using antisense oligonucleotides (from the NaSi-1 cDNA sequence), provided further evidence that rat small-intestinal (ileal) Na(+)-sulphate cotransport is closely related to rat proximal-tubular brush-border membrane Na(+)-sulphate cotransport.

Effect of somatostatin on D-galactose transport across the small intestine of rats

Somatostatin was found to diminish control and theophylline-treated tissue sugar accumulation as well as control and also to diminish theophylline mucosal to serosal D-galactose fluxes. When Na+ was removed from the bath solution, sugar transport was unaltered by the presence of somatostatin. The same results were obtained with phlorizin in the medium. These results seem to suggest that the action of somatostatin is restricted to the Na+-dependent sugar carrier located on the brush border of the intestinal epithelium.

The two mature transcripts of the chick calcitonin gene are expressed within the central nervous system during embryogenesis

Calcitonin mRNA and calcitonin gene-related peptide (CGRP) mRNA both are generated from the calcitonin gene because of tissue-specific alternative splicing of the primary transcript. It is currently established that, of the two mature transcripts, calcitonin mRNA is far the predominant transcript produced in thyroid C-cells whereas only CGRP mRNA is produced in the nervous system. However, here we provide evidence that the two splicing forms of the chick calcitonin primary transcript are found within the developing central nervous system, although displaying specific patterns of expression. While CGRP mRNA is first expressed in motor neurons at rather advanced stages of embryogenesis, calcitonin mRNA is expressed in the floor plate and dorsal rhombencephalon from earliest stages.

Effect of motilin on the L-leucine transport in rabbit jejunum

Motilin is a gastrointestinal peptide that stimulates the gastrointestinal motility in several species. The aim of the present work has been to determine the effect of motilin on the L-leucine absorption in rabbit jejunum. The results show that motilin inhibits the L-leucine Na(+)-dependent system of transport located in the mucosal border mainly by diminishing the apparent Vmax. Motilin did not directly affect the Na(+)-dependent system of transport, but it seems to act across the protein kinase C (PKC). These results suggest that motilin may act as a regulatory hormone of the intestinal absorption of nutrients.

Action of robenidine on the intestinal transport and digestion of nutrients in rabbit

Robenidine is an anticoccidial guanidine used as an additive in rabbit fodder. Because its action is restricted to the small intestine, the present work addresses the question whether robenidine affects the growth of the animals, sugar and amino acid intestinal transport and membrane-bound intestinal digestion. For this purpose we have determined the intestinal transport of the substrates, and the enzymatic activity of neutral aminopeptidase and sucrase. We have found that robenidine diminishes the tissue accumulation of L-leucine and D-galactose at long incubation times, and increases the transepithelial mucosal to serosal flux of both substrates. These results suggest that robenidine may stimulate the enterocyte basolateral membrane flux of sugars and neutral amino acids. These results have been corroborated by means of isolated brush border and basolateral membrane vesicles. Apart from these effects, robenidine has also been shown to increase the enzymatic activity of neutral aminopeptidase and sucrase and thus resulting in a better digestion of nutrients.

Nicarbazin stimulates intestinal transport of L-leucine in rabbit

Nicarbazin is an anticoccidial drug used mainly in birds, but also in rabbits. Besides the anticoccidial activity, nicarbazin has shown several other effects such as inhibition of growth and feed efficiency in poultries, and stimulation of sugar intestinal absorption in rabbits. The present work has been performed in order to check whether nicarbazin also affects L-leucine intestinal absorption. The results obtained show that nicarbazin decreases L-leucine accumulation in the jejunal tissue, and increases mucosal to serosal transepithelial fluxes of this amino acid in a dose-dependent way, without modifying its diffusion across the intestinal epithelium. The drug stimulates the amino acid uptake in brush-border and basolateral membrane vesicles, thus suggesting that nicarbazin increases the absorption of L-leucine mediated by carriers.