Molecular cloning and functional expression of the pituitary adenylate cyclase-activating polypeptide type I receptor

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide belonging to the vasoactive intestinal peptide (VIP)/secretion/glucagon family of peptides, interacts with a distinct high-affinity receptor (type I receptor) on a number of tissues. These PACAP type I receptors have a high affinity for PACAP and a low affinity for VIP and are present in the hypothalamus and anterior pituitary, where they regulate the release of adrenocorticotropin, luteinizing hormone, growth hormone, and prolactin, and in the adrenal medulla, where they regulate the release of epinephrine. Type I PACAP receptors are also present in high concentrations in testicular germ cells, where they may regulate spermatogenesis, and some transformed cell lines, such as the rat pancreatic acinar carcinoma cell AR4-2J. Here we report the molecular cloning and functional expression of the PACAP type I receptor isolated from an AR4-2J cell cDNA library by cross-hybridization screening with a rat VIP receptor cDNA. The cDNA sequence encodes a unique 495-amino acid protein with seven transmembrane domains characteristic of guanine nucleotide-binding regulatory protein-coupled receptors. A high degree of sequence homology with the VIP, secretin, glucagon-like peptide 1, parathyroid, and calcitonin receptors suggests its membership in this subfamily of Gs-coupled receptors. Results of binding studies and stimulation of cellular cAMP accumulation in COS-7 cells transfected with this cDNA are characteristic of a PACAP type I receptor. Cloning of the PACAP type I receptor will enhance our understanding of its distribution, structure, and functional properties and ultimately increase our understanding of its physiological role.

Cibacron blue-induced enhancement of agonist binding to cholecystokinin (CCK) receptors in solubilized pancreatic membranes

The pancreatic receptor for cholecystokinin (CCK) typifies many G protein-coupled receptors in that its ability to bind agonist can be reduced by GTP or the solubilization of membranes. We found, however, that a dye, cibacron blue, caused up to a 6-fold increase in binding of the CCK receptor agonist, 125I-CCK-8, to rat pancreatic membranes solubilized with digitonin. Binding optimally enhanced in this manner was comparable to binding of 125I-CCK-8 to native membranes with respect to time-course, maximal amount bound, reversibility, and sensitivity to inhibition by various CCK receptor ligands. Increases in affinity of the CCK receptor for CCK-8 accounted fully for the enhancement of binding of 125I-CCK-8. Cibacron blue did not enhance binding of 125I-CCK-8 to native membranes, and also failed to enhance binding of the CCK receptor antagonist, [3H]L-364,718, to solubilized or native membranes. The ability of cibacron blue to enhance binding of agonist but not that of antagonist suggests that this dye may mimic or perhaps stimulate the effects of G protein on CCK receptors. Such a phenomenon may provide new insights into the mechanisms by which receptors distinguish agonists from antagonists.

Molecular cloning of the human brain and gastric cholecystokinin receptor: structure, functional expression and chromosomal localization

The receptors for the brain and gastrointestinal peptide, cholecystokinin, can be classified into CCKA and CCKB subtypes. Having recently cloned the rat CCKB receptor, we used it's cDNA to isolate the human CCKB receptor homologue from brain and stomach which encodes a 447 amino acid protein with 90% identity to both rat CCKB and canine gastrin receptors. Northern hybridization identifies transcripts from stomach, pancreas, brain and gallbladder. The CCKB receptor gene maps to chromosome 11. Expression of the receptor cDNA in COS-7 cells was characteristic of a CCKB receptor subtype pharmacology. These data confirm that we have cloned a novel gene for the human brain and stomach CCKB receptor.

Brain and gastrointestinal cholecystokinin receptor family: structure and functional expression

Cholecystokinin was one of the first gastrointestinal peptides discovered in the mammalian brain. In the central nervous system there is evidence for CCKA and CCKB receptor subtypes. The CCKA receptors occur in a few localized areas of the central and peripheral nervous systems where they modulate feeding and dopamine-induced behavior. CCKB receptors occur throughout the central nervous system where they modulate anxiety, analgesia, arousal, and neuroleptic activity. We have recently purified and cloned a CCKA receptor cDNA from rat pancreas that allowed isolation of an identical cDNA from rat brain by using the polymerase chain reaction. Using low-stringency hybridization screening of cDNA libraries from rat brain and AR42-J cells, which possess large numbers of CCKB receptors, we identified previously unreported cDNAs, the sequence of which were identical in both tissues. The cDNA sequence encodes a 452-amino acid protein that is 48% identical to the CCKA receptor and contains seven transmembrane domains characteristics of guanine nucleotide-binding regulatory protein-coupled receptors. COS-7 cells transfected with this cDNA expressed binding sites for agonists and antagonists characteristic of a CCKB receptor subtype. We conclude that this cDNA isolated from rat brain and AR42-J cells is a receptor of the CCKB subtype and that the respective cDNAs for both CCKA and CCKB are identical in the brain and gastrointestinal system.

Purification, molecular cloning, and functional expression of the cholecystokinin receptor from rat pancreas

The cholecystokinin (CCK) family of peptides and their receptors are widely distributed throughout the gastrointestinal and central nervous systems where they regulate secretion, motility, growth, anxiety, and satiety. The CCK receptors can be subdivided into at least two subtypes, CCKA and CCKB on the basis of pharmacological studies. We report here the purification of the CCKA receptor to homogeneity from rat pancreas by using ion-exchange and multiple affinity chromatographic separations. This allowed partial peptide sequencing after chemical/enzymatic cleavage and synthesis of degenerate oligonucleotide primers. These primers were used for initial cloning of the cDNA from rat pancreas by PCR. The predicted protein sequence of the cDNA clone contained the five partial peptide sequences obtained from the purified protein. Seven putative transmembrane domains suggest its membership in the guanine nucleotide-binding regulatory protein-coupled receptor superfamily. In vitro transcripts of the cDNA clone were functionally expressed in Xenopus oocytes and displayed the expected agonist and antagonist specificity.

Characterization of the bombesin receptor on mouse pancreatic acini by chemical cross-linking

Bombesin (BN), gastrin-releasing peptide (GRP) and GRP(18-27) (neuromedin C) were equipotent and 30-fold more potent than neuromedin B (NMB) in inhibiting binding of 125I-GRP to and in stimulating amylase release from mouse pancreatic acini. In the present study we used 125I-GRP and chemical cross-linking techniques to characterize the mouse pancreatic BN receptor. After binding of 125I-GRP to membranes, and incubation with various chemical cross-linking agents, cross-linked radioactivity was analyzed by SDS-PAG electrophoresis and autoradiography. With each of 4 different chemical cross-linking agents, there was a single broad polypeptide band of Mr 80,000. Cross-linking did not occur in the absence of the cross-linking agent. Cross-linking was inhibited only by peptides that interact with the BN receptor such as GRP, NMB, GRP(18-27) or BN. Dose-inhibition curves for the ability of BN or NMB to inhibit binding of 125I-GRP to membranes or cross-linking to the 80,000 polypeptide demonstrated for both that BN was 15-fold more potent than NMB. The apparent molecular weight of the cross-linked polypeptide was unchanged by adding dithiothreitol. N-Glycanase treatment reduced the molecular weight of the cross-linked peptide to 40,000. The present results indicate that the BN receptor on mouse pancreatic acinar cell membranes resembles that recently described on various tumor cells in being a single glycoprotein with a molecular weight of 76,000. Because dithiothreitol had no effect, this glycoprotein is not a subunit of a larger disulfide-linked structure.

Pancreatic receptors for cholecystokinin: evidence for three receptor classes

For inhibition of binding of 125I-Bolton-Hunter-labeled cholecystokinin octapeptide (125I-BH-CCK-8) to guinea pig pancreatic acini, the potencies for agonists were CCK-8 greater than desulfated [des(SO3)] CCK-8 greater than gastrin-17-I greater than pentagastrin greater than CCK-4 and for the antagonists L 364718 greater than proglumide analogue 10 greater than CBZ-CCK-(27-32)-NH2. For all non-sulfated agonists, the curves were biphasic with 20% of the tracer bound to sites with high affinity for these agonists with the following relative potencies: gastrin-17-I greater than pentagastrin greater than des(SO3)CCK-8 much greater than CCK-4; whereas 80% was bound to low-affinity sites with the following potencies: des(SO3)CCK-8 greater than gastrin-17-I = pentagastrin much greater than CCK-4. For L 364718 and proglumide analogue 10, 80% of 125I-BH-CCK-8 was bound to sites with high affinity for these antagonists and 20% to sites with low affinity. Analysis of the dose-inhibition curve for CCK-8 demonstrated two binding sites; however, comparison with the analysis in the presence of 0.1 microM gastrin-17-I suggested three binding sites. The gastrin-17-I dose-inhibition curve was significantly better fit by a three-site model than by a two-site model. The affinities of the various agonists and antagonists for the three sites were compared with their abilities to inhibit binding of 125I-gastrin-I and either stimulate or inhibit CCK-8-stimulated amylase release. These results demonstrate that 125I-BH-CCK-8 binds to three classes of receptors, not two as reported previously. Two classes are CCK-preferring, bind 83% of 125I-BH-CCK-8 at tracer concentrations, and comprise high- and low-affinity CCK-preferring sites that can be distinguished by all agonists but have equally high affinity for L 364718 and proglumide 10. A third class binds 17% of the tracer, cannot be differentiated from high-affinity CCK-preferring receptors by CCK-8, and has low affinities for L 364718 and proglumide 10. Future studies relating binding of 125I-BH-CCK-8 to biological activity or characterization of the CCK receptor by using radiolabeled agonists should consider CCK interaction with three receptors, not two as was done in the past.

Secretin and calcium provocative tests in the Zollinger-Ellison syndrome. A prospective study

STUDY OBJECTIVE

To evaluate criteria of positivity for and usefulness of both the secretin and calcium gastrin-provocative tests in patients with the Zollinger-Ellison syndrome.

DESIGN

Prospective trial in consecutive patients.

SETTING

Referrals to a clinical research center.

PATIENTS

Consecutive sample of 80 patients with the Zollinger-Ellison syndrome.

INTERVENTION

Kabi-secretin (2 U/kg body weight) given by intravenous bolus and calcium gluconate (10%) (54 mg/kg.h [5 mg/kg.h of calcium]) given by continuous intravenous infusion for 3 hours. Serum gastrin measured at -15, and -1 minutes before, and 2, 5, 10, 15, 20, and 30 minutes after secretin, or every 30 minutes for 3 hours during the calcium infusion. Serum calcium and serum gastrin were measured simultaneously during the calcium infusion.

MEASUREMENTS AND MAIN RESULTS

There was no significant difference in the responses of patients with different extents or locations of the tumor, presence or absence of multiple endocrine neoplasia, type-I, or with fasting gastrin less than or greater than 1000 pg/mL. In patients with fasting gastrin of less than 1000 pg/mL, the sensitivity of the secretin test using the criterion of an increase in gastrin of at least 110 pg/mL was 93% (CI, 76% to 99%) and for an increase of 200 pg/mL it was 85% (CI, 66% to 96%), (P greater than 0.05). With the calcium infusion test, the sensitivity using the criterion of an increase of 395 pg/mL was 43%, (CI, 23% to 66%) and for an increase of 50% was 74% (CI, 52% to 90%), (P less than 0.01). The calcium infusion test was positive in 33% of patients with a negative secretin test. With the secretin test, 75% of patients had a positive response by 5 minutes, 95% by 10 minutes, 100% by 15 minutes, and 6% only at 2 minutes. With calcium infusion, patients had positive responses at 120 to 180 minutes.

CONCLUSIONS

The secretin test is preferred over the calcium test because of its greater sensitivity and simplicity. The recommended criteria are a 200 pg/mL increase for the secretin test and a 395 pg/mL increase for the calcium test. The calcium test should be reserved for patients having a negative secretin test, gastric acid hypersecretion, and a strong clinical suspicion of the Zollinger-Ellison syndrome.

Interaction of CCK with pancreatic acinar cells

Recent studies demonstrate that cholecystokinin-like peptides are widely distributed in the CNS as well as in the peripheral nervous system and gastrointestinal tract. Studies with agonists have demonstrated multiple classes of receptors and recently potent receptor antagonists have been described which will distinguish these classes and should allow a better understanding of the role of CCK in various physiological processes. One of the known peripheral physiological functions of CCK is the stimulation of digestive enzymes from pancreatic acinar cells. In recent years the interaction of CCK with pancreatic acinar cells has been extensively studied and significant advances have been made in understanding its cellular basis of action. Robert Jensen and colleagues report on each of these areas.

Importance of sulfation of gastrin or cholecystokinin (CCK) on affinity for gastrin and CCK receptors

We investigated the importance of sulfation of gastrin or cholecystokinin (CCK) on influencing their affinity for gastrin or CCK receptors by comparing the abilities of sulfated gastrin-17 (gastrin-17-II), desulfated gastrin-17 (gastrin-17-I), CCK-8 and desulfated CCK-8 [des(SO3)CCK-8] to interact with CCK or gastrin receptors on guinea pig pancreatic acini. For inhibiting binding of 125I-gastrin to gastrin receptors, gastrin-17-II (Kd 0.08 nM) greater than CCK-8 (Kd 0.4 nM) greater than gastrin-17-I (Kd 1.5 nM) greater than des(SO3)CCK-8 (Kd 28 nM). For inhibiting binding of 125I-Bolton Hunter-labeled CCK-8 to CCK receptors the relative potencies were: CCK-8 much greater than des(SO3)CCK-8 = gastrin-17-II greater than gastrin-17-I. Each peptide interacted with both high and low affinity CCK binding sites. The relative abilities of each peptide to interact with high affinity CCK receptors showed a close correlation with their abilities to cause half-maximal stimulation of enzyme secretion. These results demonstrate that, in contrast to older studies, sulfation of both CCK and gastrin increase their affinities for both gastrin and CCK receptors. Moreover, the gastrin receptor is relatively insensitive to the position of the sulfate moiety, whereas the CCK receptor is extremely sensitive to both the presence and exact position of the sulfate moiety.

Benzodiazepine analogues L365,260 and L364,718 as gastrin and pancreatic CCK receptor antagonists

We examined the ability of the recently described 3-(benzoylamino)benzodiazepine analogue L365,260 and the 3-(acylamino)benzodiazepine analogue L364,718 to distinguish gastrin from pancreatic cholecystokinin (CCK) receptors. Neither L365,260 nor L364,718 when present alone (1 microM) caused stimulation of amylase release from guinea pig pancreatic acini or caused contraction of smooth muscle cells from guinea pig stomach. Each analogue inhibited CCK-stimulated amylase release, gastrin-17-I-stimulated smooth muscle contraction, binding of 125I-Bolton-Hunter-cholecystokinin octapeptide (125I-BH-CCK-8) to pancreatic CCK receptors, and binding of 125I-gastrin-17-I to gastrin receptors on pancreatic acini. L365,260, (Ki, 7.3 +/- 0.8 nM) was 50-70 times more potent than L364,718 at inhibiting binding of 125I-gastrin to pancreatic acini or gastrin-stimulated smooth muscle contraction. In contrast, L364,718 (Ki, 4 +/- 1 nM) was 145-200 times more potent than L365,260 at inhibiting binding of 125I-BH-CCK-8 to pancreatic acini or CCK-stimulated amylase release. Neither L364,718 nor L365,260 distinguished between high- and low-affinity CCK binding sites. L365,260 and L364,718 did not inhibit binding of radiolabeled vasoactive intestinal peptide, secretin, bombesin, substance P, or N-methylscopolamine to pancreatic acini. These results demonstrate that, in contrast to other gastrin-CCK receptor antagonists described, L365,260 is a selective gastrin receptor antagonist having an 80-fold higher affinity for gastrin than pancreatic CCK receptor, whereas L364,718 has a 125-fold higher affinity for pancreatic CCK receptors. Because of the selectivity of these two antagonists the involvement of CCK and gastrin in various physiological processes can be differentiated even when both receptors occur on the same cell.

Regulation of bombesin receptors on pancreatic acini by cholecystokinin

When guinea pig pancreatic acini are first incubated with the COOH-terminal octapeptide of cholecystokinin (CCK-8), washed, and then reincubated with 125I-[Tyr4]bombesin (125I-[Tyr4]BN) there is a significant decrease in binding of 125I-[Tyr4]BN compared with that observed with pancreatic acini that have been first incubated with no additions. The CCK-8-induced decrease in binding is maximal after 90 min of first incubation is abolished by reducing the temperature of the first incubation from 37 to 4 degrees C or by adding L364,718 to the first incubation and cannot be reproduced by first incubating acini with A23187, 8-bromoadenosine 3',5'-cyclic monophosphate (8Br-cAMP), 8-bromoadenosine 3',5'-cyclic monophosphate (8Br-cGMP), or 12-O-tetradecanoylphorbol 13-acetate. 125I-[Tyr4]BN interacts with a single class of receptors on pancreatic acini, and first incubating acini with CCK-8 decreases the affinity of BN receptors for BN with no change in the maximal binding capacity. CCK-8 does not alter the rate at which bound 125I-[Tyr4]BN dissociates from pancreatic acini; therefore, CCK-8 must alter the rate at which the radiolabeled BN analogue associates with its receptor. Pancreatic acini possess two classes of CCK receptors: one has a high affinity for CCK-8; the other has a low affinity for CCK-8. The dose-response curve for CCK-8-induced inhibition of binding of 125I-[Tyr4]BN appears to to reflect occupation of low-affinity CCK receptors by CCK-8.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of binding of cholecystokinin to pancreatic acini

In the present study we examined the kinetics of binding of iodinated COOH-terminal octapeptide of cholecystokinin (125I-CCK-8) to its receptors on dispersed acini prepared from guinea pig pancreas. At 37 degrees C, binding of 125I-CCK-8 reached a steady state after 60 min of incubation. Dissociation of bound 125I-CCK-8 was biphasic, indicating that the labeled peptide binds in two distinct states: a rapidly dissociating state and a slowly dissociating state. Binding of 125I-CCK-8 in the rapidly dissociating state was maximal within 3 min of incubation, did not depend on incubation temperature or cellular energy metabolism, could be stripped by 0.5 M potassium thiocyanate, and showed accelerated dissociation with CCK-8 or dibutyrylguanosine 3',5'-cyclic monophosphate (Bt2cGMP). Binding of 125I-CCK-8 in the slowly dissociating state was maximal after 60 min of incubation, was decreased by reducing the incubation temperature or inhibiting cellular energy metabolism, was not stripped by 0.5 M potassium thiocyanate, and did not show accelerated dissociation with CCK-8 or Bt2cGMP. Increasing the concentration of 125I-CCK-8 increased the fraction of radioactivity bound in the rapidly dissociating state. When binding of 125I-CCK-8 reached a steady state, nearly all of the bound radioactivity was in the slowly dissociating state. Computer analysis of the inhibition of 125I-CCK-8 by CCK-8 under experimental conditions where the rapidly dissociating state predominates demonstrated a complete loss of high-affinity binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Medical management of patients with Zollinger-Ellison syndrome who have had previous gastric surgery: a prospective study

We examined prospectively the criteria for medical management in 16 patients with Zollinger-Ellison syndrome who had had previous gastric surgery. Each patient received sufficient antisecretory medication to lower gastric acid output to less than 10 mEq/h during the last hour before the next dose of drug. The 7 patients with a vagotomy but no gastric resection were symptom-free and had no mucosal disease. Of 9 patients with a partial gastrectomy, 7 had mucosal disease, with or without symptoms, and 6 of the 7 patients had acid outputs of 5-10 mEq/h. In these patients, antisecretory medication was increased to reduce output to less than 5 mEq/h and symptoms and mucosal abnormalities resolved in each patient. Patients with Zollinger-Ellison syndrome and a vagotomy can be treated safely by reducing acid secretion to less than 10 mEq/h, but in patients with a partial gastrectomy, acid secretion must be reduced to less than 5 mEq/h, and adequacy of therapy must be checked further by endoscopy.

Famotidine: effective treatment of Zollinger-Ellison syndrome

The ability of famotidine to control gastric acid hypersecretion in 32 patients with Zollinger-Ellison syndrome was compared to that of cimetidine or ranitidine. Equipotent doses of each drug were determined in nine patients and famotidine was nine times more potent than ranitidine and 32 times as potent as cimetidine. Famotidine had a 30% longer duration of action than either ranitidine or cimetidine. The 32 patients were treated with famotidine for a mean follow-up of 10.5 months. No hematologic, biochemical, or clinical toxicity occurred. Famotidine appears to be the histamine H2-receptor antagonist of choice in the treatment of Zollinger-Ellison syndrome.

Role of selective angiography in the management of patients with Zollinger-Ellison syndrome

To determine the ability of selective abdominal angiography to localize gastrinoma in patients with Zollinger-Ellison syndrome, selective angiography was performed in 70 consecutive patients and the results were assessed prospectively by either surgery, autopsy, or percutaneous biopsy. In addition, to define the role of angiography in the management of patients with gastrinoma, we compared the results of angiography with those of computed tomography (CT) scanning in 58 patients who underwent both tests. For gastrinoma in the liver, angiography had a specificity of 100% and a sensitivity of 86% with a positive predictive value of 100% and a negative predictive value of 94%. For extrahepatic gastrinoma, angiography had a specificity of 94% and a sensitivity of 68%, a positive predictive value of 97% and a negative predictive value of 53%. Comparison of CT scanning and angiography demonstrated that for hepatic tumor CT demonstrated 72% and angiography 89% of tumors, and the combination detected all tumors with no false-positive results. Outside the liver, CT scanning detected 57%, angiography 70%, and the combination 73% of tumors with a false-positive rate of 7%. These results indicate that if a CT scan is performed first, then the addition of selective angiography will detect a further 28% of hepatic tumors and a further 16% of extrahepatic tumors, but that 24% of extrahepatic tumors will still be missed. Angiography is a useful adjunct to CT particularly in patients in whom surgery is contemplated.

Prospective study of the ability of computed axial tomography to localize gastrinomas in patients with Zollinger-Ellison syndrome

The ability of routine computed tomography (CT) performed with oral and intravenous contrast to localize gastrinomas in 61 consecutive patients with Zollinger-Ellison syndrome was evaluated prospectively. The results of CT scanning were subsequently evaluated in all patients by either surgery, autopsy, or percutaneous biopsy. Thirteen of 14 patients with CT scans positive for hepatic metastases and 5 of 13 patients with CT scans negative for hepatic metastases were found to have gastrinoma in the liver. For gastrinoma metastatic to the liver, CT scanning had a specificity of 98%, a sensitivity of 72%, a positive predictive value of 93%, and a negative predictive value of 90%. Twenty-two of 23 patients with positive extrahepatic CT scans and 15 of 33 patients with negative extrahepatic CT scans were found to have extrahepatic gastrinomas. For extrahepatic gastrinoma, CT scanning had a specificity of 95%, a sensitivity of 59%, a positive predictive value of 96%, and a negative predictive value of 54%. The ability of CT scan to detect gastrinomas both in the liver and extrahepatically was directly related to tumor size, detecting 0% of tumors less than 1 cm and 83%-95% of tumors greater than 3 cm. The location of the extrahepatic gastrinoma was also an important determinant in that approximately 80% of pancreatic gastrinomas but only 35% of extrapancreatic gastrinomas were detected. The present results indicate that because of its convenience and accuracy, CT scanning with oral and intravenous contrast material should be the initial procedure to evaluate the extent of gastrinoma. A positive CT scan is almost always correct; therefore, a CT scan detecting metastatic gastrinoma to the liver would avoid unnecessary surgery and, if positive for extrahepatic gastrinoma, would assist the surgeon in finding the gastrinoma. A negative CT is less reliable; therefore, patients should undergo other localizing studies before exploratory laparotomy.

Famotidine in the therapy of gastric hypersecretory states

The histamine (H2)-receptor antagonist famotidine was compared with ranitidine and cimetidine in its ability to control gastric acid hypersecretion in 33 patients with gastric hypersecretory states (32 patients with Zollinger-Ellison syndrome and one patient with idiopathic hypersecretion). Equipotent doses of each drug were determined in nine patients and used to determine relative onset of action, duration of action, and potency. Each drug had a similar time course of onset with a maximal effect at three to four hours after oral ingestion. The duration of action of famotidine was 30 percent longer than that of either cimetidine or ranitidine. In terms of relative potency, famotidine was nine times more potent than ranitidine and 32 times more potent than cimetidine. Thirty-two patients underwent long-term famotidine treatment for up to 34 months (mean, 10 months) with a duration in 21 patients of at least six months, in nine patients of at least 12 months, and in six patients of at least 24 months. The mean daily maintenance dose with famotidine was 0.33 g per day (range, 0.05 to 0.8 g). Prior to famotidine therapy, 27 patients were taking ranitidine and the mean daily dose required was 2.3 g per day (range, 0.6 to 5.4 g), whereas six patients were taking cimetidine and the mean daily dose was 4.6 g per day (range, 1.2 to 9.0 g). Fourteen of the 32 patients required an anticholinergic agent in addition to ranitidine or cimetidine to maintain control, whereas only five patients required an anticholinergic agent with famotidine. Gastric acid hypersecretion was controlled in seven patients with less frequent dosing with famotidine than with cimetidine or ranitidine. Long-term treatment with famotidine was not associated with any hematologic or biochemical toxicity or clinical side effects. These results demonstrate that famotidine has a similar onset of action to other H2-receptor antagonists but has a 30 percent longer duration of action and is nine times more potent than ranitidine and 32 times more potent than cimetidine. Famotidine is safe and highly effective in the long-term treatment of gastric hypersecretory states.

Phase separation of the receptor for immunoglobulin E and its subunits in Triton X-114

Above its critical micelle concentration, Triton X-114 in solution forms two phases at room temperature: a lower phase containing supramicellar aggregates and an upper phase largely depleted of detergent. This property of the detergent is potentially useful for separating under mild conditions proteins that bind detergent from those that do not (Bordier, C. (1981) J. Biol. Chem. 256, 1604-1607). We studied the distribution of the receptor for immunoglobulin E (IgE) and its subunits in the two phases. IgE and IgE complexed either with intact receptors or with the alpha chains of the receptor alone are principally partitioned into the upper phase, whereas the unliganded receptor as well as the isolated alpha, and especially the beta and gamma chains of the receptor, preferentially partition into the lower detergent phase. Chromatography of IgE and of the subunits of the receptor on a hydrophobic support showed that the beta and gamma chains have a considerably greater hydrophobic surface than the alpha chains or IgE. These results indicate that the distribution of a protein in the two phases of phase-separated Triton X-114 is not an all-or-none effect based upon whether it binds detergent or not. Rather, it reflects the overall balance between the hydrophobic and hydrophilic properties of the protein's surface.

Analysis of the rate-limiting step in a ligand-cell receptor interaction: the immunoglobulin E system

Theory predicts that the kinetics of simple interactions between a ligand and a receptor bound on the surface of a cell will be affected by the occupancy of receptors on the same cell. In a diffusion-limited reaction the effect will be on the rate of dissociation but not on the rate of association until the cell is virtually saturated with ligand. If the rate of reaction is not diffusion limited, then the opposite holds; i.e., the forward velocities will be proportional to the concentration of vacant receptors, but the reverse reactions will not be. We examined the kinetics of reaction between immunoglobulin E (IgE) and its receptor and clearly demonstrated that the reaction is not diffusion controlled. The substantial (congruent to 30-fold) increase in the forward rate constant observed for the reaction of IgE with solubilized receptors as opposed to cell-bound receptors is therefore not an artifact of calculation. Since the reverse rate constants show little difference, we postulate that the presence of other surface components (rather than conformational differences in the receptor) affects the reaction with the cells. As an aid to the analysis, the theory has been extended so that not only the rate constants but also the entire course of the reaction of ligand with cell receptors can be predicted for diffusion-limited vs. non-diffusion-limited interactions.

Phospholipids stabilize the interaction between the alpha and beta subunits of the solubilized receptor for immunoglobulin E

The cell-surface component (alpha) which binds monomeric immunoglobulin E with high affinity is associated with a second polypeptide (beta) in the plasma membrane. The latter component tends to dissociate during purification of the alpha chain from detergent extracts of cells, even at neutral pHs and physiological ionic strengths. We now report that the interaction of alpha and beta can be stabilized by maintaining an appropriate phospholipid to detergent ratio. Under such conditions, other discrete components reproducibly copurify with the alpha and beta chains. These results suggest that the subunits of this membrane protein--or the interaction of it with other constituents in the cell--may be stabilized in ways not observed with ordinary soluble proteins.