Identification and in situ localization of the insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor in the rat gastrointestinal tract: comparison with the IGF-I receptor.

In the present study we investigated pharmacological, biochemical, and immunological characteristics as well as the tissue distribution of the insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor in the rat gastrointestinal tract, and compared the data with those from corresponding experiments for the IGF-I receptor. Competitive binding and affinity cross-linking studies with [125I]IGF-II, and [125I]IGF-I respectively, in rat jejunum yielded results analogous to those previously obtained for IGF-II/M6P and IGF-I receptors in intestinal epithelial membranes and other tissues. Furthermore, the IGF-II/M6P receptor antibody no. 3637 completely inhibited the association of [125I]IGF-II with receptor protein but nonimmune antibody did not, providing additional evidence for the presence of the IGF-II/M6P receptor in the rat gut. Also, analysis of the IGF-II/M6P receptor by immunoblotting using antiserum no. 3637 identified a specific band of mol wt 220.000 throughout the gastrointestinal tract with the highest content of immunoreactivity being present in colon and ileum. Autoradiographic mapping of the distribution of IGF-receptors in the rat gut showed that the expression of IGF-II/M6P receptors was in general 2-3 times greater than that of IGF-I receptors. IGF-II/M6P receptors were found 1) in greatest densities in colon and ileum, 2) more abundantly in the mucosa than in the muscularis propria, and 3) predominantly in the luminal part of the mucosal epithelial cells. Radioimmunocytochemistry employing anti-IGF-II/M6P receptor antibody no. 3637 and [125I]protein A demonstrated an IGF-II/M6P receptor distribution analogous to that shown by autoradiography with [125I]IGF-II). IGF-I receptors were present 1) in greatest densities in ileum and colon, 2) more abundantly in the muscularis propria than in the mucosa, and 3) within the mucosa in greater densities in the lamina propria than in the surface epithelium. For both receptor types densities were greater in crypt than in villous epithelial cells. We conclude: 1) the presence of IGF-II/M6P receptors throughout the rat gastrointestinal tract points to an important role for IGF-II in this organ, 2) the finding of different patterns of distribution for IGF-II/M6P and IGF-I receptors supports the concept of their different principal functions, 3) a high degree of expression of both receptor types in crypt epithelium suggests an essential role for both IGF receptors in the regulation of cell mitogenesis and growth.     

Substance P receptor binding sites are expressed by glia in vivo after neuronal injury.

In vitro studies have demonstrated that glia can express functional receptors for a variety of neurotransmitters. To determine whether similar neurotransmitter receptors are also expressed by glia in vivo, we examined the glial scar in the transected optic nerve of the albino rabbit by quantitative receptor autoradiography. Receptor binding sites for radiolabeled calcitonin gene-related peptide, cholecystokinin, galanin, glutamate, somatostatin, substance P, and vasoactive intestinal peptide were examined. Specific receptor binding sites for each of these neurotransmitters were identified in the rabbit forebrain but were not detected in the normal optic nerve or tract. In the transected optic nerve and tract, only receptor binding sites for substance P were expressed at detectable levels. The density of substance P receptor binding sites observed in this glial scar is among the highest observed in the rabbit forebrain. Ligand displacement and saturation experiments indicate that the substance P receptor binding site expressed by the glial scar has pharmacological characteristics similar to those of substance P receptors in the rabbit striatum, rat brain, and rat and canine gut. The present study demonstrates that glial cells in vivo express high concentrations of substance P receptor binding sites after transection of retinal ganglion cell axons. Because substance P has been shown to regulate inflammatory and immune responses in peripheral tissues, substance P may also, by analogy, be involved in regulating the glial response to injury in the central nervous system.     

Modulation of gut substance P after whole-body irradiation

Exposure to ionizing radiation induces gastrointestinal dysfunction and inflammatory reactions. The present study carried out in the rat, focuses on substance P, an inflammatory mediator implicated in the control of intestinal motility. We have investigated the effects of gamma irradiation on plasma and tissue substance P levels, ileal smooth muscle activity, and properties of specific receptors. Plasma and ileal (mucosa and muscle) substance P concentrations were measured by radioimmunoassay. At doses ranging from 1 to 8 Gy, plasma substance P levels increased in a dose-dependent manner up to four days after irradiation. Ileal mucosal concentration decreased rapidly 1 hr after a 6-Gy irradiation as compared to controls. A second class of binding sites appeared three days after 6 Gy irradiation. In addition, substance P contractile effects measured on isolated ileum showed a fourfold decrease of EC₅₀, three days after 6 Gy irradiation. These results indicated that gamma irradiation induced an increase of plasma levels concomitant with a modification of gastrointestinal substance P specific binding sites and contractile activity.     

Glucocorticoid and mineralocorticoid receptors in gut mucosa.

Both glucocorticoid and mineralocorticoid hormones are known to be involved in the physiology and pathophysiology of the gastrointestinal tract. Studies on the mechanism of action of steroid hormones indicate an initial obligatory step of binding to stereospecific receptor proteins in the cytoplasm of target tissue cells. Mucosal cells from the gastrointestinal tract of adrenalectomized, gonadectomized rats contain cytoplasmic glucocorticoid receptors which bind tritiated dexamethasone with an affinity (Kdiss4C) of approximately 10(-8)M. The concentration of glucocorticoid receptors per unit cytoplasmic protein is in order duodenum greater than jejunum greater than ileum=stomach greater than colon, and their affinity for steroid hormones is in order dexamethasone greater than corticosterone greater than deoxycorticosterone=aldosterone. No glucocorticoid receptors could be demonstrated in esophageal mucosal cells. Binding sites for tritiated aldosterone, with affinity characteristics appropriate for physiological mineralocorticoid receptors, were demonstrated in duodenum, jejunum, ileum and colon. No similar sites could be shown in the mucosa of the gastric antrum.     

Induction of colitis causes inflammatory responses in fat depots: evidence for substance P pathways in human mesenteric preadipocytes

Intracolonic administration of trinitrobenzene sulfonic acid in mice causes inflammation in the colon that is accompanied by increased expression of proinflammatory cytokines and of the substance P (SP), neurokinin 1 receptor (NK-1R) in the proximal mesenteric fat depot. We also investigated whether human mesenteric preadipocytes contain NK-1R and examined the functional consequences of exposure of these cells to SP as it relates to proinflammatory signaling. We found that human mesenteric preadipocytes express NK-1R both at the mRNA and protein levels. Exposure of human mesenteric preadipocytes to SP increased NK-1R mRNA and protein expression by 3-fold, and stimulated IL-8 mRNA expression and protein secretion. This effect was abolished when these cells were pretreated with the specific NK-1R antagonist CJ 012,255. Moreover, human mesenteric preadipocytes transfected with a luciferase promoter/reporter system containing the IL-8 promoter with a mutated NF-kappaB site lost their ability to respond to SP, indicating that SP-induced IL-8 expression is NF-kappaB-dependent. This report indicates that human mesenteric preadipocytes contain functional SP receptors that are linked to proinflammatory pathways, and that SP can directly increase NK-1R expression. We speculate that mesenteric fat depots may participate in intestinal inflammatory responses via SP-NK-1R-related pathways, as well as other systemic responses to the presence of an ongoing inflammation of the colon.     

Tachykinin antagonists inhibit nerve-mediated contractions in the circular muscle of the human ileum. Involvement of neurokinin-2 receptors.

The effects of some newly developed tachykinin antagonists that are selective for the neurokinin (NK)-1 (L 668,169) or the NK-2 (MEN 10,207, L 659,877 and R 396) tachykinin receptor on the cholinergic and noncholinergic contraction and on the nonadrenergic noncholinergic relaxation produced by electrical field stimulation (50 Hz) were investigated in mucosa-free circular strips of the human ileum. The strips were contracted by substance P and neurokinin A as well as by selective NK-2-receptor ligands, [beta Ala8]neurokinin A(4-10), and MDL 28,564, the latter peptide being capable of discriminating between NK-2-receptor subtypes. The selectivity of the antagonists for NK-1 or NK-2 receptors was confirmed in pharmacological experiments using substance P, neurokinin A, and [beta Ala8]neurokinin A(4-10) as stimulants. Among the NK-2-selective antagonists, MEN 10,207 displayed the highest affinity, followed by L 659,877 and R 396. The antagonists MEN 10,207 and L 659,877 inhibited the noncholinergic contraction to electrical stimulation in a concentration-dependent manner; L 668,169 and R 396 were poorly effective. Thus the potency of antagonists toward the noncholinergic response closely paralleled their rank order of potency at NK-2 receptors. The cholinergic contraction and nonadrenergic noncholinergic relaxation were not inhibited by the antagonists. Both substance P- and neurokinin A-like immunoreactivities were detected in extracts of the human ileum, and the identity of the corresponding peptides was confirmed by reverse-phase high-performance liquid chromatography. It was concluded that in addition to NK-1 receptors, the circular muscle of the human ileum also contains NK-2 receptors. Activation of the latter is chiefly responsible for the noncholinergic contraction to nerve stimulation.     

Endothelin in the gastrointestinal tract. Presence of endothelinlike immunoreactivity, endothelin-1 messenger RNA, endothelin receptors, and pharmacological effect

The possible production and role of endothelin in the gastrointestinal tract was investigated in rats by radioimmunoassay, Northern-blot hybridization, receptor assay using membrane preparations, and pharmacological study using gut strips. Endothelinlike immunoreactivity was detected in all regions (from stomach to colon) of the rat gastrointestinal tract (13-48 fmol/g wet tissue) including the mucosal layer of the ileum and colon (8.4 +/- 2.0 fmol/g wet tissue and 18.4 +/- 2.1 fmol/g wet tissue, respectively, mean +/- SEM; n = 5). Fast protein liquid chromatographic analysis of the endothelinlike immunoreactivity in jejunum, ileum, colon, and colon mucosa extracts showed peaks in the positions of endothelin-1 and endothelin-3. The presence of endothelin-1 messenger RNA was demonstrated by Northern-blot hybridization in the whole colon and pooled ileal and colonic mucosa, but not in the whole jejunum. Specific binding in the rat gastrointestinal tract was particularly high in the fundus of stomach, jejunum, ileum, and colon. In the ileum, many binding sites were found in the circular and longitudinal muscle layers, but few in the mucosal layer. Endothelin-1 and endothelin-3 caused contraction of rat stomach strips, rat colon, and guinea pig ileum. These findings indicate that endothelin is present in the rat gastrointestinal tract, perhaps produced by both vascular endothelial cells and mucosal epithelial cells, and can cause contraction of gastrointestinal smooth muscle. Thus, endothelin may have a physiological role in the control of gastrointestinal function.     

Effect of feeding on myoelectric activity of the sphincter of oddi and the gastrointestinal tract in the opossum

The effect of different foods on the myoelectric activity of the sphincter of Oddi and gastrointestinal tract was evaluated in the opossum. Gallbladder pressure was also recorded. Feeding fat and mixed food resulted in the greatest incidence of spike activity in the duodenum and jejunum, followed by protein. The lowest incidence of slow waves with spikes in the duodenum and jejunum followed feeding of carbohydrates (P less than 0.01). Likewise, the lowest spike activity in the sphincter of Oddi was observed after carbohydrate feeding (P less than 0.05). There was no significant difference in the incidence of spike potentials in the sphincter of Oddi when fat, protein, or mixed food was fed. No significant change in gallbladder pressure during fasting and following feeding of different aliments was observed. We concluded that there is a correlation between the frequency of spike potentials in the sphincter of Oddi and in the small bowel following feeding. The duration of the fed pattern for each type of food correlated with the number of spikes in the sphincter of Oddi and gastrointestinal tract.     

Tachykinins in the porcine pancreas: potent exocrine and endocrine effects via NK-1 receptors

The localization, release, and effects of substance P and neurokinin A were studied in the porcine pancreas and the localization of substance P immunoreactive nerve fibers was examined by immunohistochemistry. The effects of electrical vagus stimulation and capsaicin infusion on tachykinin release and the effects of substance P and neurokinin A infusion on insulin, glucagon, somatostatin, and exocrine secretion were studied using the isolated perfused porcine pancreas with intact vagal innervation. NK-1 and NK-2 receptor antagonists were used to investigate receptor involvement. Substance P immunoreactive nerve fibers were localized to islets of Langerhans, acini, ducts, and blood vessels. Vagus stimulation had no effect on substance P and neurokinin A release, whereas capsaicin infusion stimulated release of both. Substance P and neurokinin A infusion increased release of insulin, glucagon, and exocrine secretion, whereas somatostatin secretion was unaffected. The effect of substance P on insulin, glucagon, and exocrine secretion was blocked by the NK-1 receptor antagonist. The effect of electrical stimulation of vagus nerves on insulin and exocrine secretion was not influenced by tachykinin receptor antagonists. We conclude that tachykinins stimulate both endocrine and exocrine pancreatic functions through NK-1 receptors. Tachykinins are not involved in vagal regulation of pancreatic secretion in pigs but could constitute part of an alternative stimulatory system.     

Distribution and coexistence of peptides in nerve fibers of the external muscle of the human gastrointestinal tract

The nerve fibers that supply the external muscle of the human gastrointestinal tract were examined for their immunoreactivity to the neuropeptides enkephalin, neuropeptide Y, somatostatin, substance P, and vasoactive intestinal peptide, for tyrosine hydroxylase (a catecholamine-synthesizing enzyme), and for coexistence between immunoreactivities in nerve fibers. Studies on coexistence revealed that the majority of reactive nerve fibers could be placed in one of two classes: (a) those fibers with reactivity to enkephalin or substance P, or both, and (b) fibers containing one or both of the peptides neuropeptide Y and vasoactive intestinal peptide. Many fibers immunoreactive for vasoactive intestinal peptide or neuropeptide Y, or both, were found throughout the external smooth muscle of the gastrointestinal tract, but neuropeptide Y-reactive fibers were less common in the small and large intestines than in the stomach and esophagus. Fibers immunoreactive for enkephalin or substance P, or both, were sparse in the esophagus, increased in numbers to reach maximal frequency in the pylorus, and maintained a similar frequency in the small and large intestines. Fibers with somatostatin or tyrosine hydroxylase immunoreactivity were rare. In general, sphincter regions were similar to nonsphincter regions in peptide-immunoreactive fiber numbers and types, except that the internal anal sphincter had no enkephalin-immunoreactive fibers and very few substance P-reactive fibers. Moderate numbers of fibers reactive for neuropeptide Y and vasoactive intestinal peptide were found in the internal anal sphincter. It is suggested that enkephalin and substance P are in excitatory fibers and that vasoactive intestinal peptide and neuropeptide Y are in fibers inhibitory to the external muscle.     

Tachykinin receptors and noncholinergic bronchoconstriction in the guinea-pig isolated bronchi.

The aim of the study was to assess which type(s) of tachykinin receptor mediate the noncholinergic bronchoconstriction produced by activation (electrical field stimulation) of capsaicin-sensitive primary afferents in epithellum-denuded guinea-pig isolated bronchi. Experiments with natural and synthetic tachykinin agonists indicated the presence of both NK-1 and NK-2 receptors at this level. Experiments with the putative NK-1 (L668, 169) or NK-2 (MEN 10,207, MEN 10,376, L659,877, and R396) selective antagonists against NK-1 and NK-2 selective agonists further supported this conclusion. All the tachykinin antagonists tested reduced the noncholinergic bronchoconstriction to field stimulation with the order of potency MEN 10,207 = MEN 10,376 greater than L659,877 greater than L668,169 congruent to R396. In the presence of peptidase inhibitors, the activity of MEN 10,376 toward the noncholinergic bronchoconstriction was slightly reduced, whereas that of L668,169 was increased. These findings demonstrate that both NK-1 and NK-2 receptors mediate the noncholinergic constriction produced by endogenous tachykinins in guinea-pig bronchi and that the relative contribution of NK-2 receptors is greater than that of NK-1. These findings implicate a major role for neurokinin A rather than for substance P as an endogenous bronchoconstrictor in the guinea-pig isolated bronchi. In the presence of peptidase inhibitors, the relative contribution of NK-1 receptors is increased.     

Neurokinin 1 and 2 receptors mediate cholera toxin secretion in rat jejunum

BACKGROUND & AIMS: Substance P, a member of the tachykinin family, is a prosecretory neuropeptide distributed widely throughout the enteric nervous system. Implicated in inflammatory states, its role in enterotoxigenic water and electrolyte secretion is unclear. We assessed the effect of substance P antagonists and neurokinin receptor antagonists on cholera toxin-, Escherichia coli heat-labile enterotoxin (LT)-, and heat-stable enterotoxin (STa)-induced water secretion in an in vivo rat jejunal perfusion model. METHODS: Anesthetized adult male Wistar rats were pretreated with substance P antagonists (D-Pro(2), D-Trp(79), substance P, 0.1-3.0 mg/kg; or CP 96,345/4, 0.3-3 mg/kg) or neurokinin (NK)-1 (sendide, 1.0 mg/kg), NK-2 (GR83074, 1.0 mg/kg), or NK-3 ([Trp(7),betaAla(8)]NKA(4-10), 1.0 mg/kg) receptor antagonists. In a subgroup, extrinsic sensory afferents were ablated by pretreatment with capsaicin. Jejunal perfusion, with a plasma electrolyte solution containing a nonabsorbable marker, was undertaken after exposure to cholera toxin (25 microg), LT (25 microg), STa (200 microg/L), or saline. Results: Cholera toxin-induced water and electrolyte secretion was inhibited by the substance P antagonists and the NK-1 and NK-2 receptor antagonists, but not by the NK-3 receptor antagonist or by pretreatment with capsaicin. Neither LT- nor STa-induced secretions were affected by the pretreatments. CONCLUSIONS: Prosecretory pathways involving NK-1 and NK-2 receptors specifically mediate the actions of cholera toxin in the small intestine.     

Neurokinins modulate hyperventilation-induced bronchoconstriction in canine peripheral airways

This study was designed to test the hypotheses that (1) neurokinin (NK) receptor activity modulates hyperventilation-induced bronchoconstriction (HIB) in canine peripheral airways and (2) NK receptor activity is stimulated via hyperventilation-induced eicosanoid production and release. A bronchoscope was used in anesthetized dogs to record peripheral airway resistance (Rp); to test airway reactivity to NK A (NKA), substance P, and hypertonic saline; and to examine HIB before and after combined treatment with NK-1 (CP 99,994) and NK-2 (SR 48,968) receptor antagonists. Bronchoalveolar lavage fluid cells, prostaglandin D2, and cysteinyl leukotrienes from hyperventilated airways pretreated with either vehicle or NK antagonists were also measured. Pretreatment with NK-1 and NK-2 antagonists significantly attenuated HIB and the response to substance P, virtually abolished the response to NKA, and had little effect on the response to HS. Blockade of NK-1 and NK-2 receptors did not affect either the cell profiles or the mediator concentrations recovered in bronchoalveolar lavage fluid after hyperventilation. We conclude that NKs modulate the development of HIB and appear to do so via hyperventilation-induced eicosanoid production and release.     

Neuromuscular function of the human lower oesophageal sphincter in reflux disease and Barrett's oesophagus

BACKGROUND: Columnar lined (Barrett's) oesophagus is often considered a sequel to chronic severe reflux disease. Aberrant lower oesophageal sphincter (LOS) motility associated with Barrett's oesophagus includes reduced basal LOS pressures. The aim of this study was to characterise neuromuscular function of the LOS in normal (squamous cell carcinoma (SCC) with uninvolved LOS) and reflux affected (Barrett's) oesophagus in vitro. METHODS: Strips of LOS muscle were prepared at biopsy following oesophagectomy from 16 patients with SCC and seven patients with oesophageal adenocarcinoma and Barrett's oesophagus associated with a history of reflux disease. LOS smooth muscle responses were recorded in response to electrical field stimulation (EFS), potassium chloride (KCl), DMPP, isoprenaline, capsaicin, bethanechol, and tachykinins. RESULTS: Basal LOS tone and LOS relaxations in response to isoprenaline, EFS, and DMPP were not significantly altered in the Barrett's group. After tetrodotoxin pretreatment, responses to KCl and DMPP were significantly reduced in the SCC but not in Barrett's LOS. Maximal contraction in response to bethanechol was significantly decreased in Barrett's LOS while substance P and NK-2 receptor mediated contraction was unaltered. Capsaicin, NK-1, and NK-3 receptor agonists exerted negligible effects on LOS tone. CONCLUSIONS: LOS muscle strips from patients with reflux associated Barrett's oesophagus exhibit a reduction in cholinergic muscle contraction while retaining similar features of basal tone, responses to tachykinins, and inhibitory muscle and neural function. Enteric inhibitory neurones in LOS muscle strips from patients with reflux associated Barrett's oesophagus display resistance to axonal sodium channel blockade. No evidence for functional NK-1 or NK-3 receptors or capsaicin sensitive axon collateral reflexes was observed in the human LOS.     

Neuropeptide K potently stimulates salivary gland secretion and potentiates substance P-induced salivation.

Neuropeptide K (NPK) is an N-terminally extended derivative of neurokinin A (NKA) that can be a final product in the posttranslational processing of beta-preprotachykinin. A rat salivation bioassay was used to demonstrate potent effects of NPK at low doses, while effects due to NKA were much weaker at higher doses. The rank order of potency of beta-preprotachykinin-derived peptides on salivation responses was NPK greater than substance P greater than NKA much greater than beta-preprotachykinin-(72-96)-peptide. The time course of the NPK response was longer than that observed with substance P. The responses elicited by NPK were blocked by the tachykinin antagonist [D-Pro2,D-Trp7,9]substance P but not by atropine. In peptide coinfusion studies, NPK strikingly potentiated the salivation responses elicited by substance P. NPK in vitro displayed a 100 times lower potency than substance P in displacing 3H-labeled substance P binding in submandibular gland membranes, a tissue rich in SP-P type (NK-1) receptors. The possible cellular mechanisms by which NPK stimulates salivary gland secretion are discussed. We conclude that NPK and substance P may be cotransmitters derived by posttranslational processing of beta-preprotachykinin.     

Distribution of vasoactive intestinal polypeptide and substance P receptors in human colon and small intestine

Vasoactive intestinal polypeptide (VIP) and substance P are found in neurons in the lamina propria and submucosa and muscularis propria of human small intestine and colon. VIP receptors coupled to adenylate cyclase are present on epithelial, smooth muscle, and mononuclear cells. This study analyzes the distribution of[¹²⁵I]VIP binding and [¹²⁵]substance P in human colon and small intestine using autoradiographic techniques. [¹²⁵I]VIP binding was present in high density in the mucosal layer of colon and small intestine. [¹²⁵I]VIP binding was not significantly greater than nonspecific binding in smooth muscle layers or the lymphoid follicles. In contrast, [¹²⁵I]substance P binding was present in high density over the colonic muscle but was not present over the mucosal layer. In human colon cancer, [¹²⁵I]VIP grain density over the malignant tissue was only slightly higher than background. These autoradiographic studies of [¹²⁵I]VIP binding indicate that the highest density of VIP receptors was found in the small intestine and superficial colonic mucosa, whereas the density of substance P receptors was highest over the smooth muscle layers. These findings suggest a mismatch between immunochemical content of the peptide and autoradiographic density of the receptor.This work was supported by funds from the Research Service of the Veterans Administration.     

Regional Variations in Neurokinin Receptor Subtype Contributions to Muscularis Mucosae and Epithelial Function in Rat Colon

It is known that the muscularis mucosae and mucosa are not pharmacologically homogeneous throughout the rat colon. The aim of this study was to simultaneously characterize all three neurokinin (NK) receptors in the muscularis mucosae and mucosa along the length of the rat colon. Strips of proximal, mid, and distal colonic muscularis mucosae were prepared for isometric recording or sheets of muscle-free mucosa were mounted in Ussing chambers for measurement of short-circuit current. In both muscularis mucosae and mucosa the greatest responses to substance P were found in the proximal region. Use of selective agonists revealed the presence of all three NK receptors in both structures, however, selective antagonism suggests that only NK2 receptors in the muscularis mucosae and NK1 receptors in the mucosa are physiologically relevant. In conclusion, substance P–induced responses in the rat colon are region-specific and not mediated by a single NK receptor subtype common to both structures.     

Insulinlike growth factor I receptors in rabbit gastrointestinal tract. Characterization and autoradiographic localization

Insulinlike growth factor I is a potent mitogen with insulinlike metabolic effects. Insulinlike growth factor I is synthesized in the liver, intestine, and other organs. Insulinlike growth factor I receptors are widely distributed and structurally similar to insulin receptors. Frozen sections of rabbit gastrointestinal tract were incubated in buffer containing 40 pmol/L [125I]insulinlike growth factor I. Binding was saturable, temperature- and time-dependent, and reversible. Saturation binding experiments showed a single class of high-affinity receptors (Kd = 0.9 nmol/L, Bmax = 0.36 pmol/mg protein). The IC50s for insulinlike growth factor I and insulinlike growth factor II were 3 nmol/L and 90 nmol/L, respectively; whereas insulin at 1-3 mumol/L displaced 50% of specific binding. Autoradiography of insulinlike growth factor I binding demonstrated significant differences in receptor density in gastrointestinal smooth muscle, epithelium of the esophagus, stomach, small intestine, and colon. These results indicate that a single class of specific, high-affinity insulinlike growth factor I receptors were distributed in muscular and mucosal layers of the entire rabbit gastrointestinal tract. Insulinlike growth factor I is likely to be an important local mediator of intestinal growth and metabolism.     

Substance P receptors in rat spleen: characterization and autoradiographic distribution

The interaction of substance P with intact lymphatic tissue was quantified and autoradiographically visualized, using slide-mounted tissue sections of rat spleen. Radiolabeled substance P binds rapidly to an apparently single class of noninteracting high affinity sites (Kd = 2.4 nmol/L; Bmax = 9.4 fmol/mg protein). The ligand selectivity pattern suggests that substance P binding sites are similar to substance P receptors found in other tissues, including the brain, T lymphocytes, and macrophages. Substance P receptors are highly concentrated in the antigen-trapping spleen marginal zone, with low densities being found in the red pulp. No specific binding of radiolabel to T cell-dependent immunologic domains of the spleen is seen. The distribution of substance P receptors suggests that substance P is probably involved in the control of sensory functions of the immune system.