ABCA1 overexpression leads to hyperalphalipoproteinemia and increased biliary cholesterol excretion in transgenic mice.

The discovery of the ABCA1 lipid transporter has generated interest in modulating human plasma HDL levels and atherogenic risk by enhancing ABCA1 gene expression. To determine if increased ABCA1 expression modulates HDL metabolism in vivo, we generated transgenic mice that overexpress human ABCA1 (hABCA1-Tg). Hepatic and macrophage expression of hABCA1 enhanced macrophage cholesterol efflux to apoA-I; increased plasma cholesterol, cholesteryl esters (CEs), free cholesterol, phospholipids, HDL cholesterol, and apoA-I and apoB levels; and led to the accumulation of apoE-rich HDL1. ABCA1 transgene expression delayed 125I-apoA-I catabolism in both liver and kidney, leading to increased plasma apoA-I levels, but had no effect on apoB secretion after infusion of Triton WR1339. Although the plasma clearance of HDL-CE was not significantly altered in hABCA1-Tg mice, the net hepatic delivery of exogenous 3H-CEt-HDL, which is dependent on the HDL pool size, was increased 1.5-fold. In addition, the cholesterol and phospholipid concentrations in hABCA1-Tg bile were increased 1.8-fold. These studies show that steady-state overexpression of ABCA1 in vivo (a) raises plasma apoB levels without altering apoB secretion and (b) raises plasma HDL-C and apoA-I levels, facilitating hepatic reverse cholesterol transport and biliary cholesterol excretion. Similar metabolic changes may modify atherogenic risk in humans.

MR virtual angioscopy of thoracic aortic atherosclerosis in homozygous familial hypercholesterolemia.

PURPOSE: The thoracic aorta is an important site of atherosclerotic disease in patients with homozygous familial hypercholesterolemia (HFH). Thoracic aortic atherosclerosis in patients with HFH was assessed with contrast-enhanced MR angiograms using exoscopic and endoscopic virtual angioscopy reconstructions and maximum intensity projections (MIPs). METHOD: Contrast-enhanced MR angiograms of the thoracic aorta of 15 patients with HFH and 8 normal volunteers were obtained. Perspective surface reconstructions of the MR angiograms including virtual angioscopy views were evaluated by three radiologists blinded to the diagnosis. RESULTS: Thoracic wall irregularity was depicted on 8 of 15 (53%) patient scans and only 1 of 8 (13%) normal subject scans using surface reconstructions. Wall irregularity scores of patients with HFH were significantly increased compared with controls (2.0 +/- 0.9 vs. 1.0 +/- 0.6; p = 0.008). There was excellent interobserver agreement (weighted kappa = 0.82 +/- 0.12). Virtual endoscopy views added diagnostic confidence compared with exoscopic surface renderings alone. MIP reconstructions were unable to depict wall irregularity. CONCLUSION: MR angiography with virtual angioscopy of the thoracic aorta depicts nonstenotic wall irregularity of thoracic aortic atherosclerosis in patients with HFH. This may be important for assessing disease progression and response to treatment and may be generalizable to routine (non-HFH) atherosclerosis.

Erdheim-Chester disease: a rare multisystem histiocytic disorder associated with interstitial lung disease.

Erdheim-Chester disease (ECD) is a rare multisystem histiocytosis syndrome of unknown cause that usually affects adults. Histiocytic infiltration of multiple end organs produces bone pain, xanthelasma and xanthoma, exophthalmos, diabetes insipidus, and interstitial lung disease. Differential diagnosis includes Langerhans cell histiocytosis, metabolic disorders, malignancy, and sarcoidosis. ECD can be diagnosed using a combination of clinical and histopathologic findings. Sites of involvement include lung, bone, skin, retroorbital tissue, central nervous system, pituitary gland, retroperitoneum, and pericardium. Symmetrical long bone pain with associated osteosclerotic lesions, xanthomas around the eyelids, exophthalmos, and/or diabetes insipidus suggest ECD. Approximately 35% of patients have associated lung involvement, characterized by interstitial accumulations of histiocytic cells and fibrosis in a predominantly perilymphangitic and subpleural pattern. This pattern distinguishes ECD from other histiocytic disorders involving the lung. The diagnosis is confirmed by tissue biopsies that contain histiocytes with non-Langerhans cell features. In general, the clinical course of patients with this disease varies, and the prognosis can be poor despite treatment. Clinical trials for treatment of ECD have not been conducted and treatment is based on anecdotal experience.

Hepatic lipase deficiency decreases the selective uptake of HDL-cholesteryl esters in vivo.

Recent in vitro studies have provided evidence that hepatic lipase (HL) facilitates the selective uptake of HDL cholesteryl esters (CE), but the in vivo physiological relevance of this process has not been demonstrated. To evaluate the role that HL plays in facilitating the selective uptake of HDL-CE in vivo, we studied the metabolism of [(3)H]CEt, (125)I-labeled apolipoprotein (apo) A-I, and (131)I-labeled apoA-II-labeled HDL in HL-deficient mice. Kinetic analysis revealed similar catabolism of (125)I-labeled apoA-I (as well as (131)I-labeled apoA-II) in C57BL controls and HL deficient mice, with fractional catabolic rates (FCR) of 2.17 +/- 0.15 and 2.16 +/- 0.11 d(-)(1) (2.59 +/- 0.14 and 2.67 +/- 0.13 d(-)(1), respectively). In contrast, despite similar hepatic scavenger receptor BI expression, HL-deficient mice had delayed clearance of [(3)H]CEt compared to controls (FCR = 3.66 +/- 0.29 and 4.41 +/- 0.18 d(-)(1), P < 0.05). The hepatic accumulation of [(3)H]CEt in HL-deficient mice (62.3 +/- 2.1% of total) was significantly less than in controls (72.7 +/- 3.0%), while the [(3)H]CEt remaining in the plasma compartment increased (20.7 +/- 1.8% and 12.6 +/- 0.5%) (P < 0.05, all). In summary, HL deficiency does not alter the catabolism of apoA-I and apoA-II but decreases the hepatic uptake and the plasma clearance of HDL-CE. These data establish for the first time an important role for HL in facilitating the selective uptake of HDL-CE in vivo.

In vivo evidence for both lipolytic and nonlipolytic function of hepatic lipase in the metabolism of HDL.

To investigate the in vivo role that hepatic lipase (HL) plays in HDL metabolism independently of its lipolytic function, recombinant adenovirus (rAdV) expressing native HL, catalytically inactive HL (HL-145G), and luciferase control was injected in HL-deficient mice. At day 4 after infusion of 2 x 10(8) plaque-forming units of rHL-AdV and rHL-145G-AdV, similar plasma concentrations were detected in postheparin plasma (HL=8.4+/-0.8 microg/mL and HL-145G=8.3+/-0.8 microg/mL). Mice expressing HL had significant reductions of cholesterol (-76%), phospholipids (PL; -68%), HDL cholesterol (-79%), apolipoprotein (apo) A-I (-45%), and apoA-II (-59%; P<0.05 for all), whereas mice expressing HL-145G decreased their cholesterol (-49%), PL (-40%), HDL cholesterol (-42%), and apoA-II (-89%; P<0.005 for all) but had no changes in apoA-I. The plasma kinetics of (125)I-labeled apoA-I HDL, (131)I-labeled apoA-II HDL, and [(3)H]cholesteryl ester (CE) HDL revealed that compared with mice expressing luciferase control (fractional catabolic rate [FCR] in d(-1): apoA-I HDL=1.3+/-0.1; apoA-II HDL=2.1+/-0; CE HDL=4.1+/-0.7), both HL and HL-145G enhanced the plasma clearance of CEs and apoA-II present in HDL (apoA-II HDL=5.6+/-0.5 and 4.4+/-0.2; CE HDL=9.3+/-0. 0 and 8.3+/-1.1, respectively), whereas the clearance of apoA-I HDL was enhanced in mice expressing HL (FCR=4.6+/-0.3) but not HL-145G (FCR=1.4+/-0.4). These combined findings demonstrate that both lipolytic and nonlipolytic functions of HL are important for HDL metabolism in vivo. Our study provides, for the first time, in vivo evidence for a role of HL in HDL metabolism independent of lipolysis and provides new insights into the role of HL in facilitating distinct metabolic pathways involved in the catabolism of apoA-I- versus apoA-II-containing HDL.

Cholesteryl ester transfer protein corrects dysfunctional high density lipoproteins and reduces aortic atherosclerosis in lecithin cholesterol acyltransferase transgenic mice.

Expression of human lecithin cholesterol acyltransferase (LCAT) in mice (LCAT-Tg) leads to increased high density lipoprotein (HDL) cholesterol levels but paradoxically, enhanced atherosclerosis. We have hypothesized that the absence of cholesteryl ester transfer protein (CETP) in LCAT-Tg mice facilitates the accumulation of dysfunctional HDL leading to impaired reverse cholesterol transport and the development of a pro-atherogenic state. To test this hypothesis we cross-bred LCAT-Tg with CETP-Tg mice. On both regular chow and high fat, high cholesterol diets, expression of CETP in LCAT-Tg mice reduced total cholesterol (-39% and -13%, respectively; p < 0.05), reflecting a decrease in HDL cholesterol levels. CETP normalized both the plasma clearance of [(3)H]cholesteryl esters ([(3)H]CE) from HDL (fractional catabolic rate in days(-1): LCAT-Tg = 3.7 +/- 0.34, LCATxCETP-Tg = 6.1 +/- 0.16, and controls = 6.4 +/- 0.16) as well as the liver uptake of [(3)H]CE from HDL (LCAT-Tg = 36%, LCATxCETP-Tg = 65%, and controls = 63%) in LCAT-Tg mice. On the pro-atherogenic diet the mean aortic lesion area was reduced by 41% in LCATxCETP-Tg (21.2 +/- 2.0 micrometer(2) x 10(3)) compared with LCAT-Tg mice (35.7 +/- 2.0 micrometer(2) x 10(3); p < 0.001). Adenovirus-mediated expression of scavenger receptor class B (SR-BI) failed to normalize the plasma clearance and liver uptake of [(3)H]CE from LCAT-Tg HDL. Thus, the ability of SR-BI to facilitate the selective uptake of CE from LCAT-Tg HDL is impaired, indicating a potential mechanism leading to impaired reverse cholesterol transport and atherosclerosis in these animals. We conclude that CETP expression reduces atherosclerosis in LCAT-Tg mice by restoring the functional properties of LCAT-Tg mouse HDL and promoting the hepatic uptake of HDL-CE. These findings provide definitive in vivo evidence supporting the proposed anti-atherogenic role of CETP in facilitating HDL-mediated reverse cholesterol transport and demonstrate that CETP expression is beneficial in pro-atherogenic states that result from impaired reverse cholesterol transport.

Hepatic lipase facilitates the selective uptake of cholesteryl esters from remnant lipoproteins in apoE-deficient mice.

We have investigated the role of hepatic lipase (HL) in remnant lipoprotein metabolism independent of lipolysis by using recombinant adenovirus to express native and catalytically inactive HL (HL-145G) in apolipoprotein (apo)E-deficient mice characterized by increased plasma concentrations of apoB-48-containing remnants. In the absence of apoE, the mechanisms by which apoB-48-containing remnants are taken up by either low density lipoprotein (LDL)-receptor or LDL-receptor-related protein (LRP) remain unclear. Overexpression of either native or catalytically inactive HL in apoE-deficient mice led to similar reductions (P > 0.5) in the plasma concentrations of cholesterol (41% and 53%) and non high density lipoprotein (HDL)-cholesterol (41% and 56%) indicating that even in the absence of lipolysis, HL can partially compensate for the absence of apoE in this animal model. Although the clearance of [3H]cholesteryl ether from VLDL was significantly increased (approximately 2-fold; P < 0. 02) in mice expressing native or inactive HL compared to luciferase controls, the fractional catabolic rates (FCR) of [125I-labeled] apoB- very low density lipoprotein (VLDL) in all three groups of mice were similar (P > 0.4, all) indicating selective cholesterol uptake. Hepatic uptake of [3H]cholesteryl ether from VLDL was greater in mice expressing either native HL (87%) or inactive HL-145G (72%) compared to luciferase controls (56%). Our combined findings are consistent with a role for HL in mediating the selective uptake of cholesterol from remnant lipoproteins in apoE-deficient mice, independent of lipolysis. These studies support the concept that hepatic lipase (HL) may serve as a ligand that mediates the interaction between remnant lipoproteins and cell surface receptors and/or proteoglycans. We hypothesize that one of these pathways may involve the interaction of HL with cell surface receptors, such as scavenger receptor (SR)-BI, that mediate the selective uptake of cholesteryl esters.

Plasma phospholipid transfer protein. Adenovirus-mediated overexpression in mice leads to decreased plasma high density lipoprotein (HDL) and enhanced hepatic uptake of phospholipids and cholesteryl esters from HDL.

In vitro studies have shown that plasma phospholipid transfer protein (PLTP) converts isolated human high density lipoprotein-3 (HDL3) into larger HDL particles and generates lipid-poor apoA-I containing nascent HDL. To evaluate the role of PLTP in vivo we generated recombinant adenovirus vectors containing either human PLTP cDNA (rPLTP.AdV) or the reporter luciferase cDNA as a control. After intravenous infusion of 4 x 10(7) plaque-forming units (low dose) and 4 x 10(8) plaque-forming units (high dose) of rPLTP.AdV into mice, PLTP activity in plasma increased from base-line levels of 8.4 +/- 0.2 to 108 +/- 17 and from 8.9 +/- 0.6 to 352 +/- 31 micromol/ml/h, respectively, on day 4 (both p < 0.001). Thus, both low and high doses of rPLTP.AdV led to pronounced overexpression of human PLTP in mice. On day 4 after treatment, mice treated with low and high doses of rPLTP.AdV showed decreased HDL cholesterol (-54% and -91%) and apoA-I (-64% and -98%) (all p < 0.05). Kinetic studies revealed that the fractional catabolic rates of HDL labeled with [3H]phosphatidylcholine, [14C]phosphatidylcholine ether, [3H]cholesteryl ether, and 125I-labeled mouse apoA-I were increased by 8.5-, 8.7-, 3.8-, and 2.8-fold, respectively, in mice treated with low dose rPLTP.AdV (all p < 0.001). After injection of labeled HDL, mice treated with rPLTP.AdV showed an increased accumulation of labeled PC ether (+304%) and cholesteryl ether (+92%) in the liver (both p < 0.05). Two-dimensional gel electrophoresis of plasma 5 min after injection of HDL labeled with 125I-apoA-I demonstrated increased levels of newly generated pre-beta-HDL in mice overexpressing PLTP. In conclusion, HDL remodeling mediated by PLTP generates nascent, lipid-poor apoA-I in vivo and accelerates the hepatic uptake of HDL surface and core lipids in mice treated with rPLTP.AdV. Accelerated catabolism of HDL in mice overexpressing PLTP leads to low HDL levels. Our data indicate an important role for PLTP in modulating reverse cholesterol transport in vivo.

Intracellular trafficking of the free cholesterol derived from LDL cholesteryl ester is defective in vivo in Niemann-Pick C disease: insights on normal metabolism of HDL and LDL gained from the NP-C mutation.

Niemann-Pick C disease (NP-C) is a rare inborn error of metabolism with hepatic involvement and neurological sequelae that usually manifest in childhood. Although in vitro studies have shown that the lysosomal distribution of LDL-derived cholesterol is defective in cultured cells of NP-C subjects, no unusual characteristics mark the plasma lipoprotein profiles. We set out to determine whether anomalies exist in vivo in the cellular distribution of newly synthesized, HDL-derived or LDL-derived cholesterol under physiologic conditions in NP-C subjects. Three affected and three normal male subjects were administered [14C]mevalonate as a tracer of newly synthesized cholesterol and [3H]cholesteryl linoleate in either HDL or LDL to trace the distribution of lipoprotein-derived free cholesterol. The rate of appearance of free [14C]- and free [3H]cholesterol in the plasma membrane was detected indirectly by monitoring their appearance in plasma and bile. The plasma disappearance of [3H]cholesteryl linoleate was slightly faster in NP-C subjects regardless of its lipoprotein origin. Appearance of free [14C] cholesterol ill the plasma (and in bile) was essentially identical in normal and affected individuals as was the initial appearance of free [3H]cholesterol derived from HDL, observed before extensive exchange occurred of the [3H]cholesteryl linoleate among lipoproteins. In contrast, the rate of appearance of LDL-derived free [3H]cholesterol in the plasma membrane of NP-C subjects, as detected in plasma and bile, was retarded to a similar extent that LDL cholesterol metabolism was defective in cultured fibroblasts of these affected subjects. These findings show that intracellular distribution of both newly synthesized and HDL-derived cholesterol are essentially unperturbed by the NP-C mutation, and therefore occur by lysosomal-independent paths. In contrast, in NP-C there is defective trafficking of LDL-derived cholesterol to the plasma membrane in vivo as well as in vitro. The in vivo assay of intracellular cholesterol distribution developed herein should prove useful to quickly evaluate therapeutic interventions for NP-C.

Adenoviral delivery of low-density lipoprotein receptors to hyperlipidemic rabbits: receptor expression modulates high-density lipoproteins.

Plasma concentrations of low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs) are inversely related in several dyslipoproteinemias. To elucidate the interactions between these lipoproteins, we used a recombinant adenovirus (hLDLR-rAdV) to express human LDL receptors (hLDLRs) in LDL receptor-deficient rabbits. hLDLR-rAdV administration resulted in hepatocyte expression and a reduction of total, intermediate-density lipoprotein (IDL), and LDL cholesterol. In addition, we found that hLDLR-rAdV treatment induced (1) increased very-low-density lipoprotein (VLDL) cholesterol, (2) increased VLDL, IDL and LDL triglycerides, (3) decreased alpha- and pre-beta-migrating apolipoprotein E (apo E) and decreased pre-beta-migrating apo A-I at 2 to 4 days posttreatment, and (4) increased total plasma apo A-I and pre-beta-migrating apo A-I beginning 8 to 10 days posttreatment. Virtually all plasma apo A-I was present on alpha- and pre-beta-HDL. Pre-beta-HDL particles with size and electrophoretic properties consistent with nascent HDL demonstrated the greatest relative apo A-I enrichment following hLDLR-rAdV treatment. In summary, enhanced expression of hepatocyte LDLRs by hLDLR-rAdV treatment markedly altered apo A-I-containing lipoproteins and IDL and LDL. The use of recombinant viruses to express physiologically relevant genes in intact animals, analogous to transfection of cells in culture, provides a new strategy for the evaluation of effects of specific gene products on metabolic systems in vivo.

Disappearance of two major phosphatidylcholines from plasma is predominantly via LCAT and hepatic lipase.

Metabolism of 1-stearoyl-2-arachidonyl-phosphatidyl-choline (SAPC), a major phosphatidylcholine (PC) species in rat plasma, was compared with 1-palmitoyl-2-linoleoyl-PC (PLPC) metabolism. High-density lipoproteins containing SAPC and PLPC tracers labeled in the sn-2 fatty acid with 3H and 14C isotopes, respectively, were administered. The rats were depleted of endogenous bile acids and infused via the ileum with individual bile acids that ranged widely in hydrophobicity. The half-lives for SAPC and PLPC in plasma were 48 and 57 min, respectively. Most of the 3H activity that disappeared from plasma at 1 h was found in the liver in 1-palmitoyl-2-arachidonyl-PC, SAPC, and 1-oleoyl-2-arachidonyl-PC, indicating phospholipase A1 hydrolysis of plasma SAPC forming 2-arachidonyl-lysophosphatidylcholine, which was reacylated in the liver. Plasma PLPC also underwent phospholipase A1 hydrolysis, as reported previously. The fraction of 3H dose that accumulated in plasma cholesteryl arachidonate was two- to threefold higher than the fraction of 14C dose in cholesteryl linoleate. Multicompartmental models for SAPC and PLPC were developed that included lysophosphatidylcholines and cholesteryl esters. Bile acids did not influence plasma PC metabolism. Lecithin-cholesterol acyltransferase and phospholipase A1 (hepatic lipase) hydrolysis accounted for > or = 90% of the SAPC and PLPC that disappeared from plasma; SAPC and PLPC are comparable as substrates for hepatic lipase, but SAPC is preferred by lecithin-cholesterol acyltransferase.

Selective composition of biliary phosphatidylcholines is affected by secretion rate but not by bile acid hydrophobicity.

Little is known about the mechanisms of: 1) biliary phosphatidylcholine (PC) secretion by the hepatocyte, 2) selectivity for biliary 1-palmitoyl-2-linoleoyl-PC (PLPC) secretion, and 3) exclusion of 1-stearoyl-2-arachidonyl-PC (SAPC) from bile. The experiments were designed to determine, in rats, whether selectivity (for PLPC and against SAPC) is influenced by bile acid hydrophobicity or secretion rate. We examined the effects of bile acid depletion and of ileal infusion of taurocholic acid, tauroursodeoxycholic acid, and taurochenodeoxycholic acid. Compared to bile acid depletion, infusion of each bile acid caused PLPC to decrease from 59% of bile PC to 48%, and SAPC to increase from 2.6% to 5%. Bile acid hydrophobicity had no effect on PC selectivity, but selectivity decreased to a moderate degree as total PC secretion increased. To determine whether selectivity is for preformed molecular species, we used a new method to isotopically label four species of hepatic PC. This was done by intravenous injection of PLPC and SAPC labeled in the linoleate (14C) and arachidonate (3H) moieties. Assuming rapid mixing of each PC species in the hepatocyte as supported by the specific activity data, bile SAPC and SLPC were derived entirely from hepatic preformed SAPC and SLPC; bile PLPC was from both preformed PLPC (55%) and an unlabeled input (45%, probably direct secretion of newly synthesized PLPC). In conclusion, the selective composition of bile PC is not related to bile acid hydrophobicity, but is partially lost as secretion increases within the physiologic range.

Gallbladder mucin, arachidonic acid, and bile lipids in patients who develop gallstones during weight reduction.

BACKGROUND: Arachidonic acid (AA) and hydrophobic bile salts (BS) stimulate gallbladder mucin (GBM) secretion, which is thought to be an essential step in gallstone pathogenesis. The present study was performed to evaluate the relationship between AA, BS, and GBM in patients who develop gallstones following weight reduction. METHODS: Eleven patients who underwent gastric bypass, developed symptomatic gallstones, and then underwent cholecystectomy were evaluated. Gallbladder bile was obtained for analysis during each procedure. Matched patients who did not develop gallstones following gastric bypass served as controls. RESULTS: GBM increased in every patient who developed stones (mean increase: 5000%). The largest increase was observed soon after gastric bypass, and this declined curvilinearly with time. Gallbladder bile cholesterol was initially elevated but then rapidly declined before increasing back to pregastric bypass levels after weight loss was complete. No significant changes in phosphatidylcholine molecular species (including AA) or BS composition were observed following weight reduction. Concentrations of cholesterol, phospholipids, and changes in [AA] over time were each a linear function of [BS]. No relationship between GBM and any of these bile constituents was apparent. CONCLUSIONS: These observations strongly suggest that increases in GBM, which occur with gallstone formation in humans, are not the result of alterations in biliary AA or BS composition.

Pharmacology of gastric acid inhibition.

Gastric acid secretion is precisely regulated by neural (acetylcholine), hormonal (gastrin), and paracrine (histamine; somatostatin) mechanisms. The stimulatory effect of acetylcholine and gastrin is mediated via increase in cytosolic calcium, whereas that of histamine is mediated via activation of adenylate cyclase and generation of cAMP. Potentiation between histamine and either gastrin or acetylcholine may reflect postreceptor interaction between the distinct pathways and/or the ability of gastrin and acetylcholine to release histamine from mucosal ECL cells. The prime inhibitor of acid secretion is somatostatin. Its inhibitory paracrine effect is mediated predominantly by receptors coupled via guanine nucleotide binding proteins to inhibition of adenylate cyclase activity. All the pathways converge on and modulate the activity of the luminal enzyme, H+,K(+)-ATPase, the proton pump of the parietal cell. Precise information on the mechanisms involved in gastric acid secretion and the identification of specific receptor subtypes has led to the development of potent drugs capable of inhibiting acid secretion. These include competitive antagonists that interact with stimulatory receptors (e.g. muscarinic M1-receptor antagonists and histamine H2-receptor antagonists) as well as non-competitive inhibitors of H+,K(+)-ATPase (e.g. omeprazole). The histamine H2-receptor antagonists (cimetidine, ranitidine, famotidine, nizatidine and roxatidine acetate) continue as first-line therapy for peptic ulcer disease and are effective in preventing relapse. Although they are generally well tolerated, histamine H2-receptor antagonists may cause untoward CNS, cardiac and endocrine effects, as well as interfering with the absorption, metabolism and elimination of various drugs. The dominance of the histamine H2-receptor antagonists is now being challenged by omeprazole. Omeprazole reaches the parietal cell via the bloodstream, diffuses through the cytoplasm and becomes activated and trapped as a sulfenamide in the acidic canaliculus of the parietal cell. Here, it covalently binds to H+,K(+)-ATPase, the hydrogen pump of the parietal cell, thereby irreversibly blocking acid secretion in response to all modes of stimulation. The main potential drawback to its use is its extreme potency which sometimes leads to virtual anacidity, gastrin cell hyperplasia, hypergastrinaemia and, in rats, to the development of carcinoid tumours. The cholinergic receptor on the parietal cell has recently been identified as an M3 subtype and that on postganglionic intramural neurones of the submucosal plexus as an M1 subtype.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of gastric acid secretion. Histamine H2-receptor antagonists and H+K(+)-ATPase inhibitors.

Gastric acid secretion is regulated by an intricate interplay of neural (acetylcholine), hormonal (gastrin), and paracrine (histamine, somatostatin) mechanisms. Receptors for each of these agents and the signal transduction pathways to which these receptors are coupled have been identified on the parietal cell. The stimulatory effect of acetylcholine and gastrin is mediated by an increase in cytosolic calcium, whereas that of histamine is mediated by activation of adenylate cyclase and generation of cAMP. Strong potentiation between histamine and either gastrin or acetylcholine reflects postreceptor interaction between the distinct pathways as well as the ability of acetylcholine and gastrin to release histamine from mucosal ECL cells. The inhibitory effects of somatostatin on acid secretion are mediated by receptors coupled by guanine nucleotide-binding proteins to inhibition of adenylate cyclase activity. All the pathways converge on and modulate the activity of the luminal enzyme, H+K(+)-ATPase, the proton pump of the parietal cell. Precise information on the mechanisms involved in gastric acid secretion has led to the development of potent drugs capable of inhibiting acid secretion. These include competitive antagonists that interact with stimulatory receptors (e.g., histamine H2-receptor antagonists) as well as noncompetitive inhibitors of H+K(+)-ATPase (e.g., omeprazole). The histamine H2-receptor antagonists (cimetidine, ranitidine, famotidine, and nizatidine) continue as first-line therapy for peptic ulcer disease and are effective in preventing relapse. Although they are generally well tolerated, histamine H2-receptor antagonists may cause untoward CNS, cardiac, and endocrine effects as well as interference with the absorption, metabolism, and elimination of various drugs. Omeprazole is a weak base that reaches the parietal cell through the bloodstream, diffuses through the cytoplasm, and becomes activated and trapped as a sulfenamide in the acidic canaliculus of the parietal cell. It covalently binds to H+K(+)-ATPase, thereby irreversibly blocking acid secretion in response to all modes of stimulation. The main drawback to its use is its extreme potency, which leads to virtual anacidity, gastrin and ECL cell hyperplasia, hypergastrinemia, and, in rats, to the development of carcinoid tumors.

Control of acid secretion.

Receptors for the main neural (acetylcholine), hormonal (gastrin) and paracrine (histamine) secretory stimulants and the signal transduction pathways to which these receptors are coupled have been identified on the parietal cell. The stimulatory effect of histamine is mediated via an increase in adenylate cyclase activity, whereas the effect of acetylcholine and gastrin are mediated via an increase in cytosolic levels of calcium. Strong synergism between histamine and either gastrin or acetylcholine may reflect postreceptor interaction between the distinct pathways. Acetylcholine and gastrin are also capable of releasing histamine from the gastric mucosa, probably from ECL cells. The inhibitory effects of somatostatin and prostaglandin E on acid secretion are mediated by receptors coupled via guanine nucleotide binding proteins to inhibition of adenylate cyclase activity. All the pathways converge on and modulate the activity of the luminal enzyme, H+K(+)-ATPase, ultimately responsible for acid secretion. The intramural neural and paracrine pathways involved in the regulation of gastrin secretion in the antrum and acid secretion in the fundus have also been identified. Of prime importance is the somatostatin cell, which exerts a paracrine restraint on gastrin secretion and acid secretion. Elimination of this restraint or disinhibition is one of the mechanisms by which the stimulatory influence of cholinergic neurons is exerted on gastrin and parietal cells. Gastrin secretion is regulated by a cholinergic neuron that causes inhibition of somatostatin secretion and thus stimulation of gastrin secretion (disinhibition) and a noncholinergic neuron that causes direct stimulation of gastrin secretion by releasing the neurotransmitter, bombesin (or gastrin-releasing peptide). Acid secretion is regulated by a cholinergic neuron that causes direct stimulation of the parietal cell and indirect stimulation by decreasing somatostatin secretion, thus eliminating its inhibitory effect on the parietal cell (disinhibition). In addition, a regulatory feedback mechanism exists whereby intraluminal acidification stimulates somatostatin secretion, which in turn attenuates acid secretion. Gastric acid secretion may also be regulated by one or more intestinal inhibitory hormones, the most likely candidates being secretin, intestinal somatostatin, and neurotensin. Enterogastrone activity probably reflects the combined effect of all these hormones. Precise information on receptors and signal transduction mechanisms as well as on intramural neural and paracrine regulatory pathways has led to the development of new drugs capable of inhibiting acid secretion. These include antagonists that interact with stimulatory receptors (histamine H2-receptor antagonists, muscarinic receptor antagonists, and gastrin receptor antagonists), agonists that interact with inhibitory receptors (somatostatin and prostaglandin E analogues), and irreversible inhibitors of the luminal enzyme, H+K(+)-ATPase.