Abstract-We present a recursive method for evaluating the impulse response of an indoor free-space optical channel with Lambertian reflectors. The method, which accounts for multiple reflections of any order, enables accurate analysis of the effects of multipath dispersion on high-speed indoor optical communication systems. We present a simple algorithm for computer implementation of the technique. We present computer simulation results for both line-of-sight and diffuse transmitter configurations. In both cases, we flnd that reflections of multiple order are a significant murce of intersymbol interference. We also report experimental measurements of optical multipath, which help verify the accuracy of our simulations.

The intestinal hormone guanylin and bacterial heat-stable enterotoxins (STs) are members of a peptide family that activates intestinal membrane guanylate cyclase. Two different peptides that activate the human intestinal T84 cell guanylate cyclase have been purified from urine and intestinal mucosa of opossums (Didelphis virginiana). The highly acidic peptide, QEDCELCINVACTGC, was named uroguanylin because it was isolated from urine and shares 53% identity with guanylin. A second peptide, SHTCEICAFAA-CAGC, was purified from urine and intestinal mucosa. This alanine-rich peptide was 47% identical to uroguanylin and 73% identical to human guanylin, suggesting that it may be an opossum homologue of guanylin. Synthetic uroguanylin-(2-15) (i.e., EDCELCINVACTGC) was 10-fold more potent than synthetic rat guanylin, but both peptides were less potent than Escherchia coli ST in the T84 cell cGMP bioassay. Uroguanylin-(2-15) and guanylin inhibited 12'I-ST binding to T84 cell receptors in competitive radioligand binding assays. Transepi-

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Mucins of the gastroduodenal junction are secreted by the mucous surface and mucusproducing glandular cells in the stomach, and by goblet cells and Brunner's glands in the duodenum. Developmental studies have demonstrated that Brunner's glands can arise from undifferentiated gastric epithelium and/or intestinal epithelium in the proximal duodenum. The aim of this study was to investigate the carbohydrate composition of mucins from this region and compare it with that of mucins from Brunner's glands to evaluate the probable evolution of mucins from these glands. Toward that end, paraffin sections from 13 mammalian species were stained by classic carbohydrate histochemistry and treated with 13 lectins. In general, the mucous surface cells of the stomach, pyloric glands, duodenal goblet cells, and Brunner's glands secretory epithelium had different lectin-binding patterns. However, the lectin-binding profile of the secretory epithelium of Brunner's glands resembled that of pyloric glands more closely than that of duodenal goblet cells and mucous surface cells of the stomach. Mucins from Brunner's glands and pyloric glands showed a greater terminal carbohydrate residue diversity than those of gastric mucous surface cells or duodenal goblet cells. The lectin-binding profile argues for the evolution of similar mucins from the epithelia of Brunner's glands and pyloric glands. The greater diversity of carbohydrate residues in mucins secreted by Brunner's glands suggests that their mucus is more adaptable. This may explain why Brunner's glands metaplasia rather than goblet cell metaplasia is seen in the mucosa adjacent to chronic intestinal ulcers. Anat Rec Part A 278A:540 -550, 2004. Key words: Brunner's glands; carbohydrate histochemistry; comparative anatomy; gastroduodenal junction; histogenesis; lectin; mucinThe epithelial cells that line the gastrointestinal tract of mammals are protected in part from the somewhat harsh environment of acid, proteolytic enzymes, and abrasives in the lumen by a mucus layer. In the stomach the mucus layer, which is several microns thick, is secreted by both the epithelium lining the gastric mucosa and mucus-producing cells of the gastric glands, while in the duodenum it is secreted by goblet cells and Brunner's glands, which are located in the submucosa. Mucus is a highly hydrated gel that consists of about 95% water, 5% mucins, and minor components, such as electrolytes (Allen, 1981;Neutra and Forstner, 1987). Mucins are high-molecularweight glycoproteins that have gel-forming properties which are highly glycosylated and which in turn are responsible for their protective function.A particular morphological feature of the duodenal submucosa is the presence of Brunner's glands. Brunner's glands are specific to mammals and have been observed in all mammals examined to date (Krause, 1988). They are

Heat-stable enterotoxins activate guanylate cyclase, whereas heat-labile enterotoxins stimulate adenylate cyclase. Both classes of toxins cause secretory diarrhea at least in part by stimulating Cl- secretion in the intestine. The mechanism for regulation of Cl- secretion by guanosine 3',5'-cyclic monophosphate (cGMP) was investigated using cultured T84 intestinal cells as a model for intestinal crypt cells. Escherichia coli heat-stable enterotoxin (ST) markedly stimulated cGMP production in T84 cells. Cl- secretion across T84 cell monolayers cultured on permeable filters was stimulated by E. coli ST, cholera toxin, or 8-BrcAMP, but 8-BrcGMP was ineffective. cGMP analogues that are known to be potent and specific activators of cGMP-dependent protein kinase (cG-kinase) also had little effect on 36Cl- uptake by T84 cells cultured in plastic dishes. E. coli ST, forskolin, cholera toxin, or membrane-permeant cAMP analogues markedly increased 36Cl- uptake into T84 cells. The general protein kinase inhibitor, staurosporine, inhibited the stimulation of Cl- permeability elicited by E. coli ST, vasoactive intestinal peptide (VIP), or 8-BrcAMP. DEAE-Sephacel chromatography revealed a predominant type II isoform of cAMP-dependent protein kinase (cA-kinase) in T84 cells, whereas little or no cytosolic cG-kinase activity was found. Treatment of T84 cells with E. coli ST or VIP resulted in an increase in the cA-kinase activity ratio (-cAMP/+cAMP) if the cytosolic enzyme was assayed at reduced temperature (on ice).(ABSTRACT TRUNCATED AT 250 WORDS)

Pathogenic strains of enteric bacteria secrete small heat-stable toxins (STs) that activate membrane guanylyl cyclase receptors found in the intestine. The intestinal peptide agonists, guanylin and uroguanylin, are structurally related to STs. Receptors for ¹²⁵I-ST were found throughout the entire length of the intestinal tract of all the birds examined. These receptors were restricted to intestinal epithelial cells covering villi and forming intestinal glands and were not observed in other strata of the gut wall. The most intense labeling of receptors by ^!25I-ST occurred in the region of the microvillus border of individual enterocytes. There appeared to be a decrease in receptor density distally along the length of the small intestine, although labeling of receptors by ^l25I-ST was observed throughout the small intestine and colon. Cellular cGMP accumulation responses to Escherichia coli ST and rat guanylin in the domestic turkey and duck were greater in the proximal small intestine compared to the distal small intestine or colon. Brush border membranes (BBM) isolated from the mucosa of proximal small intestine of turkeys exhibited agonist-stimulated guanylyl cyclase activity. The rank order potency for enzyme activation was E. coli ST > uroguanylin > guanylin. Competitive radioligand binding assays using ¹²⁵I-ST and turkey intestine BBM revealed a similar rank order affinity for the receptors that was exemplified by the K_d values of ST 2.5 nM, uroguanylin 80 nM and guanylin 2.6 µM. It may be concluded that functional receptors for the endogenous peptides, guanylin and uroguanylin, occur in the apical membranes of enterocytes throughout the avian intestine. The receptor-guanylyl cyclase(s) of proximal small intestine were preferentially activated by uroguanylin relative to guanylin, but both endogenous peptides were less potent than their molecular mimic, E. coli ST.

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Three clonal subpopulations of opossum kidney (OK) cells were derived from the parental line. The distribution of apical microvilli suggested that the OK cell line was heterogeneous. The clonal OK sublines appeared homogeneous as reflected by microvilli, which were uniformly distributed on the apical surface. Parathyroid hormone (PTH), forskolin (FSK), and prostaglandin E1 (PGE1) increased adenosine 3',5'-cyclic monophosphate (cAMP) formation in OK cells and all of the clones. PTH inhibited sodium-dependent phosphate transport in parental cells and in OK/B and OK/P clones with maximal effects appearing at 4, 2, and 1 h, respectively. PTH had no effect on phosphate transport in OK/H cells. FSK inhibited phosphate transport in parental cells and OK/B and OK/P clones but was relatively ineffective in OK/H cells. PGE1 decreased phosphate transport in OK/B and OK/P cells but was ineffective in the parental line and in OK/H cells. Phorbol 12-myristate 13-acetate, a potent inhibitor of phosphate transport in the parental OK cell line, had little effect in the clonal sublines. These clonal lines have remained phenotypically stable for 10 passages and should prove useful in studying the regulation of phosphate transport by PTH as well as addressing the question of whether PTH receptor subclasses exist which couple to cAMP and/or calcium effector systems in kidney cells.