Substructure and higher structure of chicken smooth muscle alpha-actinin molecule

Substructure of chicken gizzard smooth muscle alpha-actinin molecule was deduced by domainal mapping of the proteolytic fragments with alpha-chymotrypsin. There were three chymotryptic cleavage sites (Sites I, II, and III, from the amino terminus). Cleavage at Site I generated two fragments, i.e. an NH2-terminal 36-kDa fragment and a COOH-terminal 70-kDa fragment. The 70-kDa fragment generated either a 55-kDa fragment by cleavage at Site II or a 65-kDa fragment by cleavage at Site III. Purified NH2-terminal 36-kDa fragment bound to F-actin, whereas the 55-kDa fragment formed a dimeric molecule. Circular dichroism and electron microscopic experiments demonstrated that the alpha-helical content of the 55-kDa fragment was 14% higher than that of native gizzard alpha-actinin, coinciding with the apparently rod-shaped configuration of this fragment. A 110-kDa product was generated from two 55-kDa fragments in a cross-linking study with the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. Two cross-linkable sites in the 55-kDa, A- and B-site, were shown to be involved in this reaction. Further, it was demonstrated by using N-(7-dimethylamino-4-methyl-3-coumarinyl)maleimide labeling and immunoblotting analyses that the A-site on one 55-kDa fragment was cross-linked to the B-site on the other. These results suggest that smooth muscle alpha-actinin formed an antiparallel dimeric molecule in which the 55-kDa fragments connected the two actin-binding domains composed of the 36-kDa fragments.

Characterization of receptors for calcitonin gene-related peptide on gastric smooth muscle cells

Rat calcitonin gene-related peptide (rat CGRP) and related peptides did not cause contraction of gastric smooth muscle cells; however, preincubation with rat CGRP or human CGRP inhibited smooth muscle contraction caused by carbachol. Rat CGRP and human CGRP were equipotent in opposing contraction with a half-maximal effect produced by 0.1 nM, but rat calcitonin-adjacent peptide (rat CAP) and human calcitonin had no effect. Rat CGRP caused an increase in cellular adenosine 3',5'-cyclic monophosphate (cAMP) of 60%, which was augmented to 155% with 1 mM isobutyl methylxanthine (IBMX). IBMX did not influence the sensitivity of the muscle cells to rat CGRP to increase cAMP, the half maximal effect being produced by 0.4 nM in the presence or absence of IBMX. Rat CGRP and human CGRP were equipotent for stimulating cAMP, but rat CAP and human calcitonin had no effect. Binding of 125I-CGRP was temperature dependent, saturable, reversible, and specific. Rat CGRP and human CGRP were equipotent at inhibiting binding of 125I-CGRP and rat CAP, and human calcitonin did not inhibit binding. For rat CGRP and human CGRP, the dose-response curves for binding of 125I-CGRP were broad, and both peptides accelerated the rate of dissociation of bound 125I-CGRP. Computer analysis of dose-response curves demonstrated two classes of binding sites for CGRP, one with a high affinity (Kd 3 nM) and another with low affinity (Kd 3,700 nM). Our results indicate that CGRP interacts with previously undescribed receptors on gastric smooth muscle cells to increase cellular cAMP and inhibit contraction.

Characterization of muscarinic receptors in guinea pig ileum longitudinal smooth muscle

Heterogeneity in the muscarinic receptor population of guinea pig ileum longitudinal smooth muscle was found in competition binding experiments against N-methyl[3H]scopolamine using either a cardioselective (AF-DX 116) or a smooth muscle-selective (hexahydrosiladifenidol) antimuscarinic compound. AF-DX 116 recognized 65% of the total receptors with high affinity and 35% with low affinity. Hexahydrosiladifenidol distinguished 24% of the total receptors with high affinity and 76% with low affinity. The two affinity binding constants displayed in smooth muscle by the compounds were similar to those of heart and glands, suggesting that the muscarinic receptor population in the smooth muscle is formed of about 30% glandular type and 70% cardiac type of the M2 receptors. In dissociation experiments, the rate of breakdown of the N-methyl[3H]scopolamine receptor complex in the smooth muscle was rapid and similar to the dissociation of N-methyl[3H]scopolamine from muscarinic receptors in cardiac membranes, supporting the evidence for the presence of a large fraction of the cardiac receptor type in smooth muscle. To further characterize the population of the smooth muscle receptors recognized as glandular type, we performed protection experiments with hexahydrosiladifenidol, which binds to glandular M3 receptors with high affinity. Smooth muscle membranes were initially incubated with this compound and then phenoxybenzamine was added to irreversibly alkylate the remaining unprotected receptors. Data from competition and dissociation binding experiments showed that, under these conditions, this protected fraction of the total receptor population in ileum smooth muscle had all the characteristics of the glandular type, i.e., slow N-methyl[3H]scopolamine dissociation and affinity constants for a series of selective and nonselective muscarinic antagonists in the same order of magnitude as those found in the glandular tissue. These findings, together with the known observation that hexahydrosiladifenidol is more potent in inhibiting the functional activation of muscarinic receptors in smooth muscle relative to heart, lead to the hypothesis that smooth muscle contractility is mediated by a muscarinic receptor subtype similar to that found in glandular tissue.

Exaggerated prostaglandin production by colonic smooth muscle in rabbit colitis

Enhanced production of arachidonic acid metabolites by colonic mucosa has been reported in ulcerative colitis as well as in experimental models of colitis. However, production of these compounds by colonic smooth muscle from colitis subjects has not been described. To evaluate arachidonic acid metabolism in colonic tissue, we studied the production of prostaglandin E2 (PGE2) by mucosa and muscularis propria in two experimental models of acute colitis in which inflammation was virtually confined to the mucosa. Colitis was induced in New Zealand white rabbits by either of two methods, dinitrochlorobenzene (DNCB) sensitization or formalin followed by intravenous soluble immune complexes (F-IC). Arachidonic acid metabolites were identified from in vitro incubations of tissue with [14C] arachidonic acid by thin layer chromatography followed by autoradiography. The major eicosanoid metabolites of colitis mucosa and muscularis were 14C-labeled prostaglandin E2, prostaglandin F2a and 6-keto prostaglandin F1 alpha. PGE2 was quantitated from incubations without labeled arachidonic acid by radio-immunoassay. PGE2, expressed as picograms per milligram protein per 20 min (mean +/- SEM), was increased in F-IC mucosa (1093 +/- 141 vs 645 +/- 189, P less than 0.05) and DNCB mucosa (1354 +/- 487 vs 527 +/- 222, P less than 0.05) compared to normals. PGE2 production by uninflamed colitis muscularis propria was also increased five- to eightfold compared to normals for F-IC muscularis (1594 +/- 329 vs 189 +/- 35, P less than 0.005) and DNCB muscularis (1287 +/- 171 vs 225 +/- 72, P less than 0.005). Thus, the adjacent inflammation in colonic mucosa may induce increased eicosanoid production by the uninflamed smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Desmin distinguishes cultured fat-storing cells from myofibroblasts, smooth muscle cells and fibroblasts in the rat

To differentiate cultured rat liver myofibroblasts, fat-storing cells, aortic smooth muscle cells and skin fibroblasts from each other, desmin and vimentin stainings were undertaken by indirect immunofluorescence using monoclonal antibodies. In myofibroblasts, the reaction with antibodies to vimentin was positive but that with antibodies to desmin was virtually negative. In primary cultures as well as subsequent passage of fat-storing cells, reactions with antibodies to both desmin and vimentin were positive. In primary culture of smooth muscle cells, both reactions were positive, but in the first passage, smooth muscle cells lost the reactivity with antibodies to desmin. Fibroblasts showed a positive reaction with antibodies to vimentin and a negative one with antibodies to desmin. Thus, immunohistochemistry of intermediate filaments allows for the differentiation between fat-storing cells, which are desmin- and vimentin-positive, and myofibroblasts or fibroblasts, which are desmin-negative but vimentin-positive. Smooth muscle cells are also vimentin-positive and become desmin-negative after the first passage.

Identification of bombesin receptors on isolated muscle cells from human intestine

Smooth muscle cells were isolated separately from the longitudinal and circular muscle layers of human jejunum obtained at surgery and used to determine whether amphibian bombesin-14 and 3 mammalian homologues, GRP-(1-27), GRP-(18-27) and neuromedin B, can cause contraction by acting directly on muscle cells. Circular and longitudinal muscle cells contracted identically in response to bombesin-14 (C50 2 x 10(-12) M). The contractile response was not affected by selective muscarinic, opioid, CCK or serotonin antagonists but was inhibited by the substance P (SP) derivative, [D-Arg1, D-Pro2, D-Trp7,9, Leu11]SP. All 3 mammalian bombesins were less potent than bombesin-14. GRP-(1-27) and GRP-(18-27) were equipotent (C50 4 x 10(-11) M) but 20 times less potent than bombesin-14. Neuromedin B (C50 6 x 10(-12) M) was 3 times less potent than bombesin-14. All bombesins, however, were more potent than other enteric neuropeptides (e.g., tachykinins, opioid peptides). The study demonstrates conclusively the ability of bombesins to cause direct contraction of intestinal smooth muscle cells.

Two smooth muscle myosin heavy chains differ in their light meromyosin fragment

Smooth muscle myosin heavy chains [SM1, approximately 205 kilodaltons (kDa), and SM2, approximately 200 kDa] were separated on sodium dodecyl sulfate (SDS)-polyacrylamide gels. Peptide maps of the two heavy chains showed unique patterns. Limited proteolytic cleavage of purified swine stomach myosin was performed by using a variety of proteases to produce the major myosin fragments which were resolved on SDS gels. A single band was obtained for heavy meromyosin in the soluble fraction following chymotrypsin digestion. However, a variable number of bands were observed for light meromyosin fragments in the insoluble fraction after chymotrypsin digestion. Peptide mapping indicated that the two bands observed after short digestion times with chymotrypsin had relative mobility and solubility properties consistent with approximately 100- and 95-kDa light meromyosin (LMM) fragments. These results indicate that the region of difference between SM1 and SM2 lies in the LMM fragment.

Smooth-muscle differentiation in stromal cells of malignant and non-malignant breast tissues

A mouse monoclonal antibody (MAb) recognizing alpha-smooth-muscle actin has been used to study smooth-muscle differentiation features in the stromal cells of desmoplastic reactions accompanying mammary tumors. We have studied, by the same immunohistochemical technique, a series of malignant and non-malignant human breast tissues. Cells composing the desmoplastic reaction were found to express alpha-smooth-muscle actin in all the 11 breast carcinomas examined, whereas no immunostain was demonstrated in the stromal cells of 7 breast tissue samples histologically defined as normal. Three of 9 cases of fibrocystic disease showed a minority of positively stained stromal cells, generally in association with epithelial hyperplasia. All the 7 cases of sclerosing adenosis, 3 of 4 cases of diffuse papillomatosis and all 3 intraductal papillomas exhibited a majority of immunoreactive stromal cells. Numerous stromal cells in 3 of 11 circumscribed fibroadenomas analyzed expressed low amounts of alpha-smooth-muscle actin. The factor(s) responsible for smooth-muscle differentiation in stromal cells are presently unknown, but the detection of this previously unsuspected stromal cell phenotype in nonmalignant mammary tissues might help in characterizing the variant morphological aspects designated under the label "fibrocystic disease" and in understanding the biology of premalignant or early malignant lesions of the breast.

An 82-kilodalton membrane protein that inhibits the activity of neurite outgrowth factor

Neurite outgrowth factor (NOF), an extracellular matrix glycoprotein of 700 kilodaltons (kDa), promoted neurite outgrowth from cultured ciliary ganglion (CG) neurons of chicken embryo. A fraction solubilized with Nonidet P-40 of chicken gizzard muscle membranes inhibited the neurite-promoting activity of NOF in a dose-dependent manner, but not that of laminin. Binding of CG neurons to the substratum and their survival were not affected by the extract. The inhibitory activity of the extract was abolished by treatment with trypsin or heat. The molecular size was determined to be about 82 kDa by ligand blotting. The active component was partially purified by column chromatography. It is suggested that this molecule interacts with the domain of NOF responsible for its neurite-promoting activity and may modulate NOF activity during development in vivo.

Free amino acid pools in the spontaneously hypertensive rat: a longitudinal study

To investigate abnormalities of metabolism in spontaneously hypertensive rats (SHR) that might be related to the pathogenesis of hypertension, we measured concentrations of free amino acids in plasma and in homogenates of skeletal muscle from SHR and age-matched normotensive Wistar-Kyoto (WKY) rats. These pools were evaluated in rats aged 3.5, 6, 8 and 28 wk, corresponding to time points before, during and after onset of hypertension. Amino acid content of aortic tissue also was examined at 3.5 and 6 wk. In plasma, amino acid concentrations were relatively unchanged throughout the study. Free amino acid content of muscle, on the other hand, decreased markedly with age in both strains. The most consistent and quantitatively important difference between strains was the much smaller muscle pool of lysine in SHR at 3.5, 6 and 8 wk of age compared with WKY controls. The arginine pool was also smaller in SHR but only at 3.5 and 6 wk. Other urea cycle amino acids were also lower in muscle of SHR at 3.5 wk. These alterations in the muscle amino acid pool were mirrored in plasma and were also found in aortic tissue. Glutamine was higher in muscle and plasma of SHR at 6 wk and thereafter. At 28 wk, however, many amino acids, including the branched-chain amino acids and tyrosine and glutamine, were present at higher concentrations in muscle and plasma of SHR than in those of WKY rats. These differences, because they occur most strikingly in SHR during the prehypertensive state, may be related to the development of hypertension.

Gastrointestinal stromal tumors. An immunohistochemical study of cellular differentiation

Forty-five benign and 11 malignant gastrointestinal stromal tumors (GIST) were immunohistochemically studied for the presence of desmin, muscle actins (MA) and S-100 protein. To facilitate the analysis, the tumors were divided into four groups by light microscopy: (1) typical leiomyomas comparable to peripheral leiomyomas (n = 9); (2) cellular spindle cell tumors (n = 29); (3) round cell tumors ("leiomyoblastomas" n = 7); and (4) sarcomas (n = 11). The typical leiomyomas were desmin- and MA-positive throughout, and showed well-differentiated smooth muscle cells by electron microscopy, similar to the normal gastric smooth muscle cells. All esophageal leiomyomas belonged to this group. Nineteen of 29 of the Group 2 tumors showed desmin positivity and 20 of 29 showed MA positivity, but usually only in less than 10% of the tumor cells, and in many instances it was very difficult to determine whether the positive cells were real tumor cells or entrapped muscle cells. Only 5 of 29 of Group 2 tumors showed widespread desmin positivity and 11 of 29 showed similar MA positivity. Of round cell tumors, only 1 of 7 showed desmin-positive cells and 3 of 7 MA-positive cells. None of the sarcomas showed desmin, while MA positivity was found in 6 of 11 cases, often in a large number of tumor cells. Seven tumors showed a significant number of S-100 positive tumor cells, but four of these also showed a high number of desmin- and MA-positive cells, suggesting that these tumors represented complex proliferations of muscle and Schwann cell elements. Two purely S-100 positive benign probably Schwann cell-like tumors were found, both in the small bowel. Small number of S-100 positive cells were commonly found in GIST, and these probably represented entrapped Schwann cells, because many tumors showed simultaneous proliferation of non-neoplastic nerves.

Method for simultaneous determination of creatine phosphate and adenine nucleotides in the intestinal smooth muscle of guinea-pig taenia caeci using high-performance liquid chromatography

A method for simultaneous determination of adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP) and creatine phosphate (PCr) by high-performance liquid chromatography is described. This method was applied to the isolated intestinal smooth muscle tissue of guinea pig taenia caeci weighing approximately 30 mg. It was found that one g of the muscle tissue contained 3.55 mumol PCr, 2.40 mumol ATP and 0.477 mumol ADP.

Indications for P2-purinoceptor subtypes in guinea pig smooth muscle

The effects of ADP, ATP, 2-methylthio ATP, alpha,beta-methylene ADP (alpha,beta meADP) and alpha,beta meATP on smooth muscle tone and the responses to transmural nerve stimulation were studied in isolated longitudinal ileum muscle, in vas deferens and in taenia coli of the guinea pig. The nucleotides evoked contractile responses in the ileum and vas deferens preparations which were blocked by p-chloromercuribenzene sulfonic acid (PCMBS) and evoked relaxation in the taenia which was not affected by PCMBS. The potency order of the agonists in the ileum (2-methylthio ATP was more potent than ATP which was equipotent with ADP) is consistent with the suggestion that there are P2Y receptors in the taenia. The antagonist pattern, including the inefficacy of reactive blue 2, instead suggests a similarity between the ileum receptors and the P2X receptors in vas deferens. However, alpha,beta meATP did not antagonize the contractile effect of ATP in the ileum, in contrast to its effect in the vas deferens where alpha,beta meATP did abolish the contractile effect of ATP. The possibilities for classification of a new receptor/site of purine nucleotide action, P2S, are considered.

Autoradiographic localization of leukotriene C4 and D4 binding sites in guinea-pig lung

Binding of [3H]leukotriene C4 and D4 to guinea-pig lung sections was characterised and binding sites were localized by autoradiography. Both leukotrienes bound to guinea-pig lung sections and membranes with high affinity and with similar characteristics to binding in a membrane preparation. Autoradiography revealed that the distribution of LTC4 and D4 binding sites was markedly different. Smooth muscle and epithelium of central and peripheral airways were densely labelled with [3H]LTC4; vascular smooth muscle and alveolar walls were also labelled. With [3H]LTD4, however, there was no detectable labelling of airways or vessels but substantial labelling of alveolar walls. This lends further support that LTC4 and LTD4 binding sites differ and may not be identical with functional receptors.

Beta-endorphin-like immunoreactivity in normal mucosa, muscle layer, adenocarcinoma, and polyp of the colon

beta-Endorphin-like immunoreactivity was detected in the mucosa and muscle layer of normal colon, adenocarcinomas derived from the colon mucosa, and colon polyps which were histologically confirmed to be adenoma without a focus of carcinoma or with in situ carcinoma. The contents of beta-endorphin-like immunoreactivity in adenocarcinomatous tissue (11.94 +/- 1.77 pmol/g wet wt) and colon polyps without focus of carcinoma (10.71 +/- 1.50 pmol/g wet wt) were found to be significantly higher than those in the mucosal layer (6.86 +/- 0.64 pmol/g wet wt) and muscle layer (8.30 +/- 0.68 pmol/g wet wt) of normal colon. These data suggest that the production of beta-endorphin-like immunoreactivity is specifically increased in some adenocarcinomas and adenomatous polyps and may be related to the alteration of bowel habits. Gel exclusion chromatography of beta-endorphin-like immunoreactivity revealed three peaks corresponding to beta-endorphin, beta-lipotropin, and an immunoreactive form between the two. In the mucosal layer and muscle layer of the colon, a broad major peak was eluted at the position of beta-endorphin, and minor peaks were eluted at the position of beta-lipotropin and between beta-endorphin and beta-lipotropin. In adenocarcinoma and polyp, the peak size corresponding to authentic beta-lipotropin was greater than that of beta-endorphin. This study demonstrated that beta-endorphin-like immunoreactivity existed at a high concentration in some colon adenocarcinomas and polyps whose elution patterns were different from those of normal colon tissue.

Regional distribution of immunoreactive dynorphin A in the human gastrointestinal tract

Immunoreactive dynorphin A (ir-Dyn A) was detected throughout the human gastrointestinal tract by a validated radioimmunoassay. Moreover, the stability of 125I-Dyn A during extraction procedures was confirmed by high performance liquid chromatography. Levels of ir-Dyn A were higher in the stomach and in the small bowel. In tissue samples separated into the main layers composing the gut wall (muscularis externa, submucosa and mucosa) ir-Dyn A was uniformly distributed. An exception was the colon, where concentrations were higher in the muscular portion. Gel permeation chromatography on samples of mucosa and muscularis externa extracts of ileum and gastric fundus, showed immunoreactive material eluting in several forms of apparently higher molecular weight than Dyn A, while only a minor peak was found to coelute with authentic Dyn A.

Binding of caldesmon to smooth muscle myosin

Caldesmon, a major calmodulin binding protein, was found to bind smooth muscle myosin. Addition of caldesmon to smooth muscle myosin induced the formation of small aggregates of myosin in the absence of Ca2+-calmodulin, but not in the presence of Ca2+-calmodulin. The binding site of myosin was studied by using caldesmon-Sepharose 4B affinity chromatography. Subfragment 1 was not retained by the column, while heavy meromyosin and subfragment 2 were bound to the caldesmon affinity column in the absence of Ca2+-calmodulin but not in its presence. It was therefore concluded that the binding site of caldesmon on myosin molecule was the subfragment 2 region and that binding of caldesmon to myosin was abolished in the presence of Ca2+ and calmodulin. Cross-linking of actin and myosin mediated by caldesmon was studied. While actomyosin was completely dissociated in the presence of Mg2+-ATP, the addition of caldesmon caused aggregation of the actomyosin. By low speed centrifugation at which actomyosin alone was not precipitated in the presence of Mg2+-ATP, the aggregate induced by caldesmon was precipitated and the composition of the precipitate was found to be actin, caldesmon, and myosin. In the presence of Mg2+-ATP, pure actin did not bind to a myosin-Sepharose 4B affinity column, while all of the actin was retained when the actin/caldesmon mixture was applied to the column. These results indicate that caldesmon can cross-link actin and myosin.

Amino acid sequence of the regulatory light chain of clam foot muscle myosin

The amino acid sequence of the regulatory light chain of foot muscle myosin from surf-clam (Spisula sachalinensis) was determined by conventional methods. It was: xS-D-D-K-K-A-K-A-A-T-S-S-V-L-T-K-F-T-Q-N-Q-I-Q-E-M-K-E-A-F-T-M-I-D-Q-N-R -D-G-L- I-D-V-S-D-L-K-E-M-Y-S-N-L-G-T-A-P-Q-D-S-V-L-Q-A-M-V-K-E-A-P-Q-M-N-F-T-G- F-L-S-L- F-S-E-K-M-S-G-T-D-P-E-E-T-L-R-N-A-F-Q-M-F-D-S-D-N-T-G-Y-I-P-E-E-Y-M-K-D- L- L-E-N-M-G-D-N-F-S-K-D-E-V-R-Q-T-W-K-E-A-P-I-A-G-G-K-V-D-Y-N-A-F-V-S-K-I- K- G-K-E-Q-D-D-A. The alpha-amino group of the light chain was blocked, and a typical calcium binding structure was recognized at the 33rd to 44th residues, as in other myosin regulatory light chains. The sequences of regulatory light chains from various muscle myosins were arranged according to the well known four-domain structure, and structural homologies were obtained for each of the domains. Based on the homologies, the relationships between the structure, function, and molecular evolution were discussed.

Evidence for multiple prejunctional receptor sites in rat isolated anococcygeus muscle

In rat isolated anococcygeus muscle, dopamine (2.6 x 10(-11) M to 8.3 x 10(-10) M) produced a concentration-dependent inhibitory effect on field stimulated contractions. The effect of dopamine was blocked by pimozide (2.2 x 10(-8) M) but not by yohimbine (2.6 x 10(-10) M) or propranolol (1.0 x 10(-7) M), suggesting a specific prejunctional inhibitory effect. Higher concentrations of dopamine (1.4 x 10(-7) M to 1.4 x 10(-4) M) elicited concentration-dependent contractions which were blocked competitively with higher concentrations of pimozide (2.2 x 10(-7) M to 1.4 x 10(-4) M) and 2.2 x 10(-6) M), suggesting a postjunctional activity. ACh in very low concentrations (2.8 x 10(-11) M to 4.4 x 10(-10) M) blocked the field stimulated contractions. Atropine (2.6 x 10(-9) M) per se augmented them and also antagonized the inhibitory effects of ACh, suggesting a prejunctional activity of ACh. Higher concentrations of ACh (5.5 x 10(-7) M to 7.0 x 10(-5) M) produced contractions which were not altered by atropine in a concentration (2.6 x 10(-9) M) which antagonized the prejunctional activity of ACh. Histamine, in a wide range (3.1 x 10(-12) M to 2.6 x 10(-8) M), did not modify field stimulated contractions. Very low concentrations of 5-HT (1.2 x 10(-11) M to 3.8 x 10(-10) M) had an inhibitory effect on field stimulated contractions. Methysergide (2.8 x 10(-9) M) enhanced the responses to electrical stimulation and antagonized the 5-HT-induced inhibitory effect. Still higher concentrations of 5-HT (1.9 x 10(-6) M to 1.0 x 10(-3) M) produced concentration-dependent contractions. Methysergide (8.5 x 10(-7) M) failed to antagonize, whereas phentolamine (1.0 x 10(-6) M) antagonized 5-HT competitively. Dopamine (8.3 x 10(-10) M), ACh (4.4 x 10(-10) M) or 5-HT (3.9 x 10(-10) M), in concentrations which produced a maximal prejunctional inhibitory effect, did not alter the EC50 value of NA, ruling out a post-junctional effect. Moreover, the concentration ratios of these agents for EC50 pre to EC50 post were less than 1, suggesting their preferential prejunctional site of action. It is concluded that multiple prejunctional site of action. It is concluded that multiple prejunctional receptor activities for DA, ACh (muscarinic) and 5-HT, which modify the release of neurotransmitter, may be operative in this preparation.

Evidence for a Na+/H+ antiport in stomach smooth muscle cells

Single smooth muscle cells were isolated from circular muscle of the canine gastric corpus by collagenase incubation. Cytoplasmic pH (pHi) of these cells was measured fluorometrically using the trapped dye 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein. Cells were examined for their Na+/H+ exchange activity after intracellular acidification. Cells acid-loaded by propionate exposure, the NH4+ prepulse technique or suspension in a Na+-depleted medium regained almost normal pHi upon exposure to a Na+ medium. The Na+-dependent alkalinization was amiloride sensitive. As well, addition of amiloride to cells suspended in a Na+ medium caused a concurrent decrease in pHi. The study indicates that a Na+/H+ antiport is present in these smooth muscle cells.

Changes in myosin heavy chain stoichiometry in pig tracheal smooth muscle during development

The stoichiometry of the myosin heavy chains (MHCs) has been measured in the tracheal smooth muscle of the pig after electrophoresis on SDS 4% polyacrylamide gel. The ratio of slower migrating MHC to the faster migrating MHC was 2.1 neonates, 1.5 in young and 0.95 in old pigs (P less than 0.01) showing that MHC composition changes with development. The unequal proportion of MHCs was not compatible with a heterodimeric arrangement of the MHCs in the native molecule as proposed earlier by Rovner et al. [(1986) Am. J. Physiol. 250, C861-870] and it is suggested that native molecules may be composed of homodimer heavy chains.

Analysis of the birefringence of the smooth muscle anococcygeus of the rat, at rest and in contraction. I

The birefringence of the rat anococcygeus muscle was measured at rest and in contraction. A large increase (+30%) of the optical retardation was observed in muscles fully stimulated by Noradrenaline, in isometric conditions. This was associated with a reduction of the muscle thickness (-12%), so that the birefringence increased by 48%. These changes were reversed upon relaxation. The relationship between the birefringence increase and the mechanical response was investigated as a function of time and of Noradrenaline concentration. Possible origins for the birefringence increase are considered: an increased density of birefringent material, mainly filaments, seems the most likely explanation of the results.