A novel disialoganglioside (IV3NeuAcIII6NeuAcLc4) of human adenocarcinoma and the monoclonal antibody (FH9) defining this disialosyl structure

This ganglioside is highly immunogenic, and immunization of mice with this disialoganglioside fraction coated on Salmonella minnesota followed by fusion of immunized spleen cells with mouse myeloma and selection of the hybridoma by positive reactivity with the purified disialoganglioside resulted in the establishment of a hybridoma secreting immunoglobulin G2a antibody FH9 that reacts specifically with the ganglioside antigen above but not with monosialosyllactotetraosylceramide I (IV3NeuAcLc4), monosialosyllactotetraosylceramide II (III6NeuAcLc4), or any other gangliosides tested.

Changes of Lex and dimeric Lex haptens and their sialylated antigens during development of human kidney and kidney tumors

The carbohydrate antigen termed Lex (Gal beta 1----4[Fuc alpha 1----3]GlcNAc beta 1----R), its di- or trimeric form, and their sialylated antigens have been characterized as developmentally regulated, tumor-associated antigens in human gastrointestinal epithelia. In this paper, remarkable changes of these antigens, defined by respective monoclonal antibodies FH3, FH4, and FH6, in fetal kidney (mesonephros and metanephros) and other urogenital organs, as well as in various types of kidney tumors, have been investigated. During the development of each organ and tissue, the antigens were found to be maximally expressed at a defined period of organogenesis, and a shifting of expression from one locus to another was observed. Each antigen showed a slightly but clearly different stage of maximum expression. The following changes in metanephros development are of particular interest. Expression of the antigen defined by FH3 followed by the antigen defined by FH4 appeared only after six weeks of gestation in the convoluted tubuli at the central region of metanephros, and propagated rapidly into those at the peripheral cortex region with a simultaneous regression at the central region. The regression of FH4 antigen was more rapid than that of FH3. All three antigens were expressed in the medullar thin tubuli, which develop into the thin-limb of Henle's loop, in which only the antigens defined by FH3 and FH4 persisted and FH6 antigen disappeared. Well-differentiated, but not undifferentiated, renal adenocarcinomas strongly expressed the antigens defined by FH4 and FH6, although the antigen defined by FH6 was expressed in more differentiated tumor cells than the antigen defined by FH4. Well-differentiated cells organized into tubular structures showed a strong expression of these differentiation antigens. However, some tumor cells that were organized into tubular structures, but were characterized by undifferentiated cytomorphology (larger nucleus and smaller volume of cytoplasm), did not express FH4 and FH6 antigens. Thus, cytodifferentiation and histotypic differentiation proceed independently within kidney tumors. The fucosylated type 2 chain structures defined by these three monoclonal antibodies are useful markers that indicate the degree of tumor differentiation.

Lewisx- and sialylated Lewisx-related antigen expression in human malignant and nonmalignant colonic tissues

Biochemical studies have revealed that some normal cells express the LeX trisaccharide Gal beta 1----4(Fuc alpha 1----3)GlcNAc either on short-chain fucolipids or as a single immunodeterminant on glycolipid oligosaccharide side chains. Cancer cells, including those from colonic adenocarcinomas, express this antigen on longer type 2 blood group side chains as difucosylated or trifucosylated fucolipids. Moreover, sialylated forms of difucosylated LeX also accumulate in colon cancer but not in normal colonic mucosa. In the present study, six monoclonal antibodies which selectively recognize the various LeX-related antigens were used for immunohistochemical examination of these antigens in serial sections of human colonic tissue. All of these antigens were oncodevelopmental in human colon. Monoclonal antibodies anti-SSEA-1 and AH8-183, directed against short-chain, monofucosylated LeX, were unable to discriminate well between normal and malignant colonic tissue. However, the other four antibodies were much better at distinguishing cancer from normal tissue. FH6 was the most specific in that no normal tissues bound this antibody. However, FH6 failed to stain poorly differentiated cancers and some colloid-type carcinomas. FH4, which was also highly specific, stained almost all cancers, regardless of the degree of differentiation. FH4 primarily stained cancer cell cytoplasm, whereas the sialylated antigen defined by FH6 predominantly stained cell membranes. Differences were noted between the expression of LeX-related antigens in autopsied normal mucosa compared to mucosa of benign colonic diseases. Monoclonal antibodies recognizing long-chain polyfucosylated and sialylated LeX-related antigens appear to be useful tools for detection of colon cancer.

Quantitative and qualitative characterization of human cancer-associated serum glycoprotein antigens expressing fucosyl or sialyl-fucosyl type 2 chain polylactosamine

The quantity of tumor-associated antigens carrying type 2 chain polylactosamines with four types of fucosyl determinants, LeX (X-hapten), poly-LeX, sialyl LeX, and LeY (Y-hapten), present in sera of patients with various malignant and non-malignant disorders, as well as the qualitative chemical properties of the carrier molecules in sera, have been investigated using four monoclonal antibodies, each of which defines one of these determinants. The following findings are of particular importance: the serum levels of LeX defined by antibody FH2 and poly-LeX defined by ACFH18 in patients with cancer were occasionally high (incidence about 10%); however, the majority of patients did not show elevated levels; the serum level of the antigen, defined by monoclonal antibody FH6 (termed sialyl LeX-i since this determinant is carried by i antigen), was significantly high in patients with cancers originating from organs from which adenocarcinomas often develop. For example, among various types of lung cancer, only adenocarcinoma but not squamous cell carcinoma, small cell carcinoma, or large cell carcinoma showed a high level of sialyl LeX-i antigen in sera. The incidence of high antigen levels in sera of patients with adenocarcinomas of lung was as high as 76% of the observed cases; the serum level of Ley (Y-hapten) was frequently high in patients with hepatoma (incidence, 34%); sialyl LeX-i antigen was separated on gel filtration as a glycoprotein with an average molecular weight greater than 10(6). It was characterized by its susceptibility to basehydrolysis, Pronase digestion, and sialidase and endo-beta-galactosidase treatment and is assumed to be a high molecular weight mucin-type glycoprotein; sialyl LeX-i antigen expressed in sera of patients with cancer was soluble in perchloric acid, while the same antigen in sera of patients with noncancerous diseases and normal subjects was mostly insoluble in perchloric acid. LeX, a poly-LeX, and essentially all LeY antigens in sera of patients with cancer were perchloric acid-insoluble.

Novel fucolipids of human adenocarcinoma: disialosyl Lea antigen (III4FucIII6NeuAcIV3NeuAcLc4) of human colonic adenocarcinoma and the monoclonal antibody (FH7) defining this structure

A new fucoganglioside, disialosyl Lea, has been found in the disialoganglioside fraction of human colonic adenocarcinoma. The ganglioside has been isolated from four other disialogangliosides by high-performance liquid chromatography followed by preparative high-performance thin-layer chromatography. The structure of the antigen was characterized by its conversion to lactofucopentaosyl(II)ceramide (Lea-active ceramide pentasaccharide), methylation analysis, and high-mass range electron-impact as well as field-desorption mass spectrometry of the permethylated derivative, as shown below: (Formula; see text) Specific removal of alpha 2----3-linked sialosyl residue by influenza virus A2 sialidase, or preferential hydrolysis of the same residue by Clostridium perfringens sialidase in the absence of detergent, resulted in the formation of an intermediate product, monosialosyl LeaII (III4FucIII6NeuAcLc4), which reacted with anti-Lea antibody and with monoclonal antibody FH7 and may have a sialic acid linked at the 6 position of GlcNAc. The IgG3 monoclonal antibody FH7 was established, which reacts specifically with disialosyl Lea and monosialosyl LeaII as above, but does not react with disialosyllactotetraosylceramide (IV3NeuAcIII6Neu-AcLc4), monosialosyl LeaI (IV3NeuAcIII4FucLc4), and other mono- and disialogangliosides isolated from the same cancer tissue. The antibody FH7 may be useful in the detection of human cancer.

[Cancer-associated mucin detected by monoclonal anti-carbohydrate antibodies]

SSEA-1 antigen (stage-specific embryonic antigen-1) has been shown to be a series of carbohydrate antigens having type-2 chain and X-hapten structures. These carbohydrate antigens are remarkably accumulated in various human cancer tissues, and activity secreted into the blood stream. Frequently SSEA-1 antigens are further modified with fucoses or sialic acid in human cancer tissues, thus forming various subgroups of antigens such as fucosyl SSEA-1, sialyl SSEA-1 or polyfucosylated antigens. Many monoclonal antibodies are established which can discriminate each subgroup of antigens. Assay systems for these antigens in the sera of cancer patients have been developed using these monoclonal antibodies. Sialyl SSEA-1 is especially elevated in the sera of patients with adenocarcinoma of the lung. The antigens detected with these monoclonal antibodies are mucin-like glycoproteins(cancer-associated mucin, CAM). Various types of cancer-associated mucins can be characterized by respective monoclonal antibodies. It is therefore possible to classify cancer-associated mucins according to the structure of their carbohydrate side chains using these monoclonal antibodies.

Coordinate expression of X and Y haptens during murine embryogenesis

The X hapten (Gal beta 1----4[Fuc alpha 1----3]GlcNAc) may play an important role in the adhesion of blastomeres during compaction. Therefore, we have investigated more thoroughly developmental changes in the fucosylation of lactoseries carbohydrate chains and the enzymatic basis of these fucosylation changes using well-characterized monoclonal antibodies. The Y hapten (Fuc alpha 1----2Gal beta 1----4[Fuc alpha 1----3]GlcNAc) and polymeric X haptens were detected by fluorescence-activated flow cytometry on murine embryonal carcinoma cells. In paraffin sections of postimplantation mouse embryos, the Y hapten was detected in the embryonic ectoderm and visceral endoderm on Days 5.5-7.5; this pattern of antigen expression is identical to that previously reported for the X hapten (SSEA-1). Thus, the Gal:alpha 1----2 (H) and GlcNAc:alpha 1----3 (X) fucosyltransferases appear to be co-regulated during embryogenesis. Reciprocal changes in X and Y hapten expression were observed, however, during preimplantation development. Unlike the X hapten, the Y hapten is expressed maximally on 16-cell morulae and 32- to 64-cell blastocysts. Eight-cell embryos cultured to the blastocyst stage in vitro did not acquire the Y hapten, however, suggesting a role for the uterine environment in carbohydrate antigen expression. Homogenates of F9 embryonal carcinoma cells were found to possess a potent GlcNAc:alpha 1----3 fucosyltransferase activity, as well as a weaker Gal:alpha 1----2 fucosyltransferase activity, using paragloboside as a substrate. The results suggest that embryonic cell surface carbohydrate phenotypes represent a balance in the competition between glycosyltransferases for available substrates. Rapid changes in carbohydrate expression during development may reflect intermediate states of cellular commitment and determination that are critical for lineage formation and morphogenesis.

Attenuation by isosorbide dinitrate of coronary occlusion-induced acidosis in the dog myocardium

In dogs anaesthetized with pentobarbitone, the thorax was opened and myocardial pH measured continuously by the use of a glass pH electrode inserted in the left ventricular wall. The left anterior descending coronary artery (LAD) was partially occluded so that the LAD flow could be reduced to a half or one-third of the original flow (partial occlusion). LAD partial occlusion was continued for 90 min, drug or saline being infused for the last 60 min of this period. LAD occlusion decreased myocardial pH significantly by 0.41 to 0.67 pH units, and increased ST segment of the surface electrocardiogram from 11.7 to 12.1 mV. In dogs with non-ischaemic normal hearts, isosorbide dinitrate (ISDN; 1 mg kg-1) did not change markedly either the LAD flow, myocardial pH or heart rate, whereas it decreased myocardial contractile force (determined by a strain gauge arch) slightly and both the systolic and diastolic blood pressure markedly. In dogs with partial LAD occlusion, ISDN (1 mg kg-1) increased myocardial pH significantly and decreased blood pressure, but did not change ST segment elevation in an epicardial lead. These results indicate that ISDN attenuates ischaemia-induced acidosis without attenuating ischaemia-induced ST elevation in the dog myocardium.

Human hepatocellular carcinoma-associated changes of glycosphingolipids detected by two monoclonal antibodies FH2 and IB9

The changes of minor glycosphingolipids associated with human hepatocellular carcinomas were studied using monoclonal antibodies. Glycosphingolipids of twelve patients were isolated from the tumor tissues and their adjacent cirrhotic liver tissues. The immunoreactivities of glycosphingolipids with two monoclonal antibodies FH2 and IB9, that define x-structure (Gal beta 1-4[Fuc alpha 1-3]GlcNAc) and sialosyl alpha 2-6galactosyl residue, respectively, were studied by a solid phase enzyme-linked immunosorbent assay and a chromatogram immunobinding assay. X-structure was detected in the upper-phase neutral glycosphingolipids from both the cirrhotic liver tissues and the tumor tissues, but was generally expressed more strongly in the latter tissues. The thin-layer chromatography patterns of x-active glycosphingolipids were more complex in the tumor tissues. Sialosyl alpha 2-6galactosyl residue was detected in two ganglioside fractions. One ganglioside with faster migration on a thin-layer chromatogram was found in both the non-tumorous tissues and the tumor tissues. Another ganglioside with slower migration was more often detected in the tumor tissues. It was concluded that some of minor glycosphingolipids detected by monoclonal antibodies FH2 and IB9 could be the 'tumor-associated' antigens.

Inhibitory effect of trimetazidine on utilization of myocardial glycogen during coronary ligation in dogs

The left anterior descending coronary artery (LAD) of the dog was ligated completely for 1.5 min, and immediately after LAD ligation the heart was taken for determination of the glycogen phosphorylase and glycogen. Trimetazidine was injected intravenously 20 min before LAD ligation. LAD ligation increased the activity of glycogen phosphorylase and decreased the level of glycogen in both ischemic (LAD) and nonischemic (circumflex) areas. Trimetazidine at the dose of 0.3 or 1.0 mg/kg, being the dose that did not affect blood pressure and heart rate markedly, inhibited the ischemia-induced changes in glycogen phosphorylase and glycogen level. It is concluded that trimetazidine inhibits the ischemia-induced increase in the utilization of glycogen in the dog myocardium.

Inhibitory effect of methylmercuric chloride on the contraction mediated by muscarinic receptor of intestinal smooth muscle of the guinea-pig

Effects of methylmercuric chloride (MMC) on the contractile responses to nerve and drug stimulation in the guinea-pig taenia coli were examined. MMC (25-50 microM) severely reduced both responses to electrical and pharmacological (nicotine) stimulation of cholinergic nerves without affecting the responses to direct stimulation of muscles. MMC also suppressed markedly the response to acetylcholine (ACh), but did not change the maximum response to ACh. Dose-inhibition curve for MMC on the response to nerve stimulation was quite similar to that on the response to externally applied ACh. MMC only at a high concentration slightly reduced the response to histamine. MMC depolarized the membrane of the smooth muscle by about 8 mV with the sucrose-gap method. However, conditioning depolarization of the muscle strip by a 25 mM-K solution did not suppress the response to ACh. It was suggested that MMC-induced inhibition of the responses of the guinea-pig taenia coli to cholinergic nerve stimulation resulted from the inactivation of the muscarinic receptor of the smooth muscle.

Location and distribution of difucoganglioside (VI3NeuAcV3III3Fuc2nLc6) in normal and tumor tissues defined by its monoclonal antibody FH6

The distribution of a novel difucoganglioside (6B ganglioside, NeuAc alpha 2----3Gal beta 1----4[Fuc alpha 1----3]GlcNAc beta 1----3Gal beta 1----4[Fuc alpha 1----3]GlcNAc beta 1----3Gal beta 1----4Glc beta 1----1Cer) in various normal adult and fetal tissues, as well as in cancer tissues, has been studied by immunoperoxidase staining with a specific monoclonal antibody, FH6, directed to this antigen. A large variety of embryonic and fetal tissues (stomach, colon, small intestine, pancreas, esophagus, lung, and heart) showed a diffuse, weakly positive staining, particularly in the epithelial layer, up to the 70th to 80th day of gestation. However, no staining was observed in various normal adult tissues, including gastrointestinal and glandular epithelial tissues which were stained positively by antibody N-19-9 (directed to sialyl-Lea) or CSLEXI (directed to sialyl-Lex). FH6-positive loci were limited to the proximal convoluted tubuli in kidney and granulocytes. In contrast, 44 of 76 cases of cancer tissue tested, including gastric, colonic, lung, breast, and renal cancers, showed clearly positive staining. The intensity of staining in gastric and colonic cancer tissues by FH6 antibody was weaker and less frequent, although the incidence of positive staining for lung (50%) and breast cancer (86%) was significantly higher than that of the antigen stained by monoclonal antibody FH4 (Y. Fukushi, S. Hakomori, and T. Shepard, J. Exp. Med., 159: 506-520, 1984), which is directed to the asialo core of the FH6 antigen. The antigen levels in the serum of patients with various cancers, inflammatory diseases, and normal subjects were determined by radioimmunoassay. The antigen level was found to be significantly higher in the serum of some patients with cancer, particularly lung, liver, and pancreatic cancers, as compared with the serum levels in other types of cancer, noncancerous diseases, and normal subjects.

Effects of methylmercuric chloride and bis (methylmercuric) selenide on responses of isolated intestinal smooth muscle of the guinea-pig

The neurologically toxic substance, methylmercury, is detoxified by simultaneous application of selenite. In vivo formation of bis (methylmercuric) selenide has been suggested as the mechanism of detoxication by selenite. By using isolated guinea-pig taenia coli preparations, the effects of methylmercuric chloride and bis (methylmercuric) selenide on contractile responses to nerve and direct stimulation were investigated. Application of either drugs scarcely affected contractile responses to direct stimulation. However, contractile responses to nerve stimulation were severely reduced by methylmercuric chloride. The effect of bis (methylmercuric) selenide was considerably weaker than methylmercuric chloride. The site blocked by methylmercuric chloride and bis (methylmercuric) selenide in the taenia preparation was discussed.

Characterization of sialosylated Lewisx as a new tumor-associated antigen

A monoclonal antibody CSLEX1 which reacts with sialosyl Lex but not with sialosyl Lea has been produced. The CSLEX1 antigen has a tissue distribution similar to that of Lex, appearing characteristically in the proximal tubules of the kidney and on granulocytes. It is tumor associated in that 14 of 34 (41%) of tumor lines tested reacted with the CSLEX1 antibody, and 50 of 74 (68%) of tumor tissues tested reacted with the antibody. Loss of immunoperoxidase staining of tissues after neuraminidase treatment showed that the antibody is reacting to sialyl derivatives. The antibody reacted in solid-phase radioimmunoassay to sialosyllactofucopentaosyl(III)ceramide and sialosyldifucosylganglioside (6B). These results indicate that the CSLEX1 epitope has the following structure: (formula: see text) This structure had not previously been known to be tumor associated.

Novel fucolipids accumulating in human adenocarcinoma. III. A hybridoma antibody (FH6) defining a human cancer-associated difucoganglioside (VI3NeuAcV3III3Fuc2nLc6)

A new fucoganglioside, 6B, which accumulates in human colonic adenocarcinoma but is absent in normal colonic mucosa, was isolated from a monosialoganglioside fraction of colonic adenocarcinomas. The structure of this ganglioside was identified as shown below by methylation analysis, direct probe mass spectrometry, and enzymatic degradation followed by examination of the degradation products with specific monoclonal antibodies. (formula; see text) The hybridoma (FH6) secreting a monoclonal IgM antibody directed to this glycolipid was selected by reactivity of the antibody with this ganglioside and lack of reactivity with other glycolipids having a closely related structure, such as sialosyllactoneotetraosylceramide (IV3NeuAcnLc4), sialosyllactofucopentaosy(III)ceramide (IV3NeuAcIII3FucnLc4), sialosyllactofucopentaosy(II)ceramide (sialosyl-Lea glycolipid; IV3NeuAcIII4FucLc4), and 6C fucoganglioside (sialosyl 2 leads to 6 fucoganglioside; VI6NeuAcIII3FucnLc6). The antibody was highly reactive with a large variety of human cancer cells, but was less reactive or did not react with a variety of normal cells.

Localization and alteration of mono-, di-, and trifucosyl alpha 1----3 type 2 chain structures during human embryogenesis and in human cancer

Distribution patterns of specific fucose-containing antigens having X determinant (Gal beta 1----4[Fuc alpha 1----3]GlcNAc) as well as the di- or trimeric X determinants (Gal beta 1----4[Fuc alpha 1----3]GlcNAc beta 1----3Gal beta 1----4[Fuc alpha 1----3]-GlcNAc) in the developing human embryo and fetus and in human cancer have been examined using immunohistological techniques. Tissue sections were stained with monoclonal antibody FH3, which defines X determinant, and with monoclonal antibody FH4, which defines di- or trimeric X determinant. The following general trends in the expression of the antigens defined by FH3 and FH4 have been observed: (a) A well-organized, orderly appearance and disappearance of the antigens was observed during the histogenesis of various epithelia of gastrointestinal and other organs. The developmental stage exhibiting the maximum antigen expression is different for each organ. (b) The X determinant defined by FH3 was expressed approximately 2 wk earlier than the di- or trimeric X determinant defined by FH4, and the antigen defined by FH4 regressed more rapidly and more completely than the X determinant defined by FH3 on further development of epithelial tissue. Thus, expression of the FH4 antigen is highly limited to specific types of cells in newborn and adult epithelial tissues. (c) The antigen defined by FH4 was strongly expressed in the majority of tubular and papillary adenocarcinoma of stomach, adenocarcinoma of colon, and infiltrating ductal carcinoma of breast and its metastatic lesions. No antigen was found in poorly differentiated stomach adenocarcinoma, squamous lung carcinoma, and many other types of tumors from ovary, testis, prostate, skin, and muscle. The presence of the antigen defined by FH4 is therefore limited to carcinoma of the stomach, colon, and breast and can be regarded as a retrograde expression of the antigen to a certain stage of fetal development in which expression of this antigen was maximal.

Novel fucolipids accumulating in human adenocarcinoma. II. Selective isolation of hybridoma antibodies that differentially recognize mono-, di-, and trifucosylated type 2 chain

A series of glycolipids having the X determinant (Gal beta 1----4 [Fuc alpha----3]GlcNAc) at the terminus and a fucosyl alpha 1----3 residue at the internal GlcNAc residue have been isolated and characterized from tumor tissues (Hakomori, S., Nudelman, E., Levery, S.B., and Kannagi, R. (1984) J. Biol. Chem. 259, 4672-4680. A series of monoclonal antibodies that differentially recognize glycolipids with mono-, di-, and trifucosylated type 2 chain have been isolated and characterized. The antibody FH4 shows a remarkable preferential reactivity towards di-/or trifucosylated type 2 chain, i.e. it does not react with monofucosylated structures, including lactofucopentaosyl (III) ceramide (III3FucnLc4), monofucosyl neolactonorhexaosylceramide (y2, V3FucnLc6), and monofucosyl neolactonoroctaosylceramide (Z1, VII3FucnLc8), but reacts well with di- and trifucosylated type 2 chain structures such as difucosyl neolactonorhexaosylceramide (III3V3Fuc2nLc6) and trifucosyl neolactonoroctaosylceramide (III3V3VII3Fuc3nLc8). Two other monoclonal antibodies, FH5 and ACFH18, preferentially react with trifucosylated type 2 chain structure (III3V3VII3Fuc3nLc8), although cross-reactivity with difucosylated type 2 chain (III3V3Fuc2nLc6) was observed. They showed a minimal cross-reaction with monofucosylated type 2 chain. In contrast, the antibody FH1 does not react with III3FucnLc4 but reacts with V3FucnLc6, III3V3Fuc2nLc6, and III3V3VII3Fuc3nLc8. Two monoclonal antibodies, FH2 and FH3, do not discriminate among various glycolipids having fucosylated type 2 chain, and their reactivities are essentially similar to previously established antibodies directed to the X determinant, such as anti-SSEA-1, WGHS 29, VEP8 and 9, My-1, etc. This series of antibodies will be useful to detect the specific type of glycolipid with fucosylated type 2 chain accumulating in human cancer and in undifferentiated cells.

Electrical and mechanical events through activation of muscarinic receptor in the smooth muscle of guinea-pig vas deferens

Electrical and mechanical responses of the guinea-pig vas deferens to exogenous acetylcholine (ACh) were observed by means of a single sucrose-gap method. ACh at various concentrations evoked a contraction consisting of two components. The initial component seemed to be independent of a depolarization, while the second one was dependent on it. All ACh responses were abolished by Ca removal from the external solution. The results suggest the existence of a pathway by which ACh even at low concentrations causes a Ca influx without potential changes.

Effects of homopantothenic acid on intestinal smooth muscles

The effects of homopantothenic acid (HOPA) on intestinal smooth muscles were investigated and compared with those of gamma-aminobutyric acid (GABA). HOPA contracted the rat ileum and produced a biphasic response (relaxation followed by contraction) in the guinea-pig ileum and taenia coli. Half-maximal effects were obtained with 0.3-0.9 nM HOPA and maximal responses with 2-4 nM in the rat and the guinea-pig ileum. In the guinea-pig taenia, half-maximal effects were obtained with 0.2-0.25 microM HOPA and maximal responses with 2-4 microM HOPA. Tachyphylaxis occurred with higher concentrations. All these responses were not significantly affected by tetrodotoxin (TTX), atropine, diphenhydramine or metiamide. The relaxation elicited by HOPA was not reduced by simultaneous application of phentolamine and propranolol. Unlike HOPA, GABA produced a biphasic response (a relaxation followed by a contraction) in the guinea-pig ileum, which was almost abolished by TTX. These results suggest that HOPA mainly acts on intestinal smooth muscle directly, whereas GABA does indirectly through nerves.