Biomedical subjects

M J Smit

Publications and source records attributed to M J Smit.

At least 55 records · Page 3Linked to original sources

PubMed1995

Differential effects of eicosapentaenoic acid on glycerolipid and apolipoprotein B metabolism in primary human hepatocytes compared to HepG2 cells and primary rat hepatocytes.

We compared the effects of eicosapentaenoic acid (EPA) and oleic acid (OA) on glycerolipid and apolipoprotein B (apoB) metabolism in primary human hepatocytes, HepG2 cells and primary rat hepatocytes. Cells were incubated for 1 to 5 h with 0.25 mM bovine serum albumin in the absence (control) or presence of 1 mM of EPA or OA. Synthesis and secretion of [3H]glycerolipid were determined after 1 h incubation with [3H]glycerol. Cellular and medium apoB abundance was semi-quantitatively estimated in human cells by Western blotting. The following observations were made. (1) Compared to control, OA induced a 7-fold increase in [3H]triacylglycerol (TG) synthesis in human hepatocytes and a 4-fold increase in rat hepatocytes and HepG2 cells. EPA enhanced [3H]TG synthesis about 2-fold in all three cell types although it stimulated [3H]diacylglycerol (DG) synthesis to an extent (i.e., 2.5- to 5-fold) similar to OA. (2) In contrast to OA, which stimulated VLDL-associated [3H]TG secretion 2.5- to 3-fold in the three cell types relative to control, EPA did not alter [3H]TG secretion in HepG2 and rat hepatocytes and suppressed [3H]TG secretion by 75% in primary human hepatocytes. (3) In primary human hepatocytes, both OA and EPA did not alter cellular apoB abundance but EPA decreased apoB secretion by 44% as compared to control. In contrast, both EPA and OA increased cellular and medium apoB abundance 2- to 2.5-fold in HepG2 cells, although medium apoB tended to be lower in EPA-treated cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1995

Modelling and mutation studies on the histamine H1-receptor agonist binding site reveal different binding modes for H1-agonists: Asp116 (TM3) has a constitutive role in receptor stimulation.

A modelling study has been carried out, investigating the binding of histamine (Hist), 2-methylhistamine (2-MeHist) and 2-phenylhistamine (2-PhHist) at two postulated agonistic binding sites on transmembrane domain 5 (TM5) of the histamine H1-receptor. For this purpose a conformational analysis study was performed on three particular residues of TM5, i.e., Lys200, Thr203 and Asn207, for which a functional role in binding has been proposed. The most favourable results were obtained for the interaction between Hist and the Lys200/Asn207 pair. Therefore, Lys200 was subsequently mutated and converted to an alanine, resulting in a 50-fold decrease of H1-receptor stimulation by histamine. Altogether, the data suggest that the Lys200/Asn207 pair is important for activation of the H1-receptor by histamine. In contrast, analogues of 2-PhHist seem to belong to a distinct subclass of histamine agonists and an alternative mode of binding is proposed in which the 2-phenyl ring binds to the same receptor location as one of the aromatic rings of classical histamine H1-antagonists. Subsequently, the binding modes of the agonists Hist, 2-MeHist and 2-PhHist and the H1-antagonist cyproheptadine were evaluated in three different seven-alpha-helical models of the H1-receptor built in homology with bacteriorhodopsin, but using three different alignments. Our findings suggest that the position of the carboxylate group of Asp116 (TM3) within the receptor pocket depends on whether an agonist or an antagonist binds to the protein; a conformational change of this aspartate residue upon agonist binding is expected to play an essential role in receptor stimulation.

Amino Acid Sequence↗

PubMed1995

Molecular pharmacological aspects of histamine receptors.

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Amino Acid Sequence↗

PubMed1995

Differences in propionate-induced inhibition of cholesterol and triacylglycerol synthesis between human and rat hepatocytes in primary culture.

Propionate is a short-chain fatty acid formed in the colon and supposedly involved in the cholesterol-lowering effect of soluble fibre. To explore the underlying mechanism(s) of this fibre action, we have used human hepatocytes in primary culture to study the effects of propionate on hepatic lipid synthesis. Initial experiments with mevalonate and mevinolin, a competitive inhibitor of hydroxymethylglutaryl (HMG)-CoA reductase (EC 1.1.1.88) were performed to evaluate basic regulatory mechanisms in these cells; results were compared with those obtained with rat hepatocytes. Incubation for 24 h with mevalonate caused a similar, concentration-dependent inhibition of [14C]acetate incorporation into cholesterol in human and rat hepatocytes. Likewise, mevinolin (100 mumol/l) inhibited the formation of cholesterol from radiolabelled acetate by about 80% in cells from both species. Propionate inhibited cholesterol as well as triacylglycerol synthesis from [14C]acetate with a similar concentration-dependency in rat hepatocytes. Fifty percent inhibition was obtained at a propionate concentration of only 0.1 mmol/l. This propionate-induced inhibition was not affected by a 100-fold excess of unlabelled acetate. Human hepatocytes were much less susceptible in this respect: propionate concentrations of 10-20 mmol/l were required to obtain similar inhibitory effects in these cells, i.e. values greatly exceeding reported portal propionate concentrations in humans. The results suggest the existence of differences in the regulation of hepatic cholesterol (and triacylglycerol) synthesis between human and rat liver cells. These results do not support the hypothesis that the fibre-induced decrease in plasma cholesterol concentration in man is mediated by a direct effect of propionate on hepatic cholesterol synthesis.

Acetates↗

PubMed1994

Rapid desensitization of the histamine H2 receptor on the human monocytic cell line U937.

In the present study we have subjected the histamine H2 receptor on the monocytic cell line U937 to a thorough pharmacological characterization using a series of selective histamine H1, H2 and H3 receptor agonists and antagonists. Recent reports have demonstrated the existence of a histamine H2 receptor on HL-60 and HGT-1 cells with a pharmacological profile distinct from the commonly described histamine H2 receptor. U937 cells, however, seem to express classical histamine H2 receptors. Histamine and dimaprit dose dependently induce the formation of cAMP, whereas dimaprit's inactive analogues, nordimaprit and homodimaprit, show reduced potencies. Histamine H1 and H3 receptor agonists do not show histamine H2 receptor activity. Various histamine H2 receptor antagonists are able to block the histamine induced production of cAMP with an antagonistic profile comparable to that observed in the guinea-pig right atrium. Furthermore, endogenous histamine H2 receptors in U937 cells are found to be susceptible to receptor desensitization, a mechanism which may become apparent under pathophysiological conditions or during drug therapy. A 30-min pre-exposure of U937 cells to histamine (100 microM) results in a 50% attenuation of the production of cAMP to a subsequent application of an agonist. Desensitization of the histamine H2 receptor in U937 cells is found to be homologous as the beta-adrenoceptor mediated response remained unaffected.

Cells, Cultured↗

PubMed1994

Site-directed mutagenesis of the histamine H1-receptor reveals a selective interaction of asparagine207 with subclasses of H1-receptor agonists.

In this study we investigated the role of the threonine203 and the asparagine207 residues in the fifth transmembrane domain of the guinea-pig histamine H1-receptor by site-directed mutagenesis to non-functional alanines. Whereas the threonine203 residue is not important for the action of histamine, the asparagine207 residue appears to be involved in the binding of the N tau-nitrogen atom of histamine and its 2-methyl-analogue. For the 2-phenyl-analogue and non-imidazole H1-receptor agonists, this residue is, however, not essential for binding. On the basis of this study we conclude that different histamine H1-receptor agonists interact in different ways with the H1-receptor protein. Moreover, we speculate that the interaction with the N pi-nitrogen atom is essential for H1-receptor activation.

Animals↗

PubMed1994

Pharmacological characterization of the human histamine H2 receptor stably expressed in Chinese hamster ovary cells.

1. The gene for the human histamine H2 receptor was stably expressed in Chinese hamster ovary (CHO) cells and characterized by [125I]-iodoaminopotentidine binding studies. In addition, the coupling of the expressed receptor protein to a variety of signal transduction pathways was investigated. 2. After cotransfection of CHO cells with pCMVhumH2 and pUT626, a phleomycine-resistant clonal cell line (CHOhumH2) was isolated that expressed 565 +/- 35 fmol kg-1 protein binding sites with high affinity (0.21 +/- 0.02 nM) for the H2 antagonist, [125I]-iodoaminopotentidine. 3. Displacement studies with a variety of H2 antagonists indicated that the encoded protein was indistinguishable from the H2 receptor identified in human brain membranes and guinea-pig right atrium. The Ki-values observed in the various preparations correlated very well (r2 = 0.996-0.920). 4. Displacement studies with histamine showed that a limited fraction (32 +/- 6%) of the binding sites showed a high affinity for histamine (2 +/- 1.2 microM); the shallow displacement curves were reflected by a Hill-coefficient significantly different from unity (nH = 0.58 +/- 0.09). The addition of 100 microM Gpp(NH)p resulted in a steepening of the displacement curve (nH = 0.79 +/- 0.02) and a loss of high affinity sites for histamine. 5. Displacement studies with other agonists indicated that the recently developed specific H2 agonists, amthamine and amselamine, showed an approximately 4-5 fold higher affinity for the human H2 receptor than histamine. 6. Stimulation of CHOhumH2 cells with histamine resulted in a rapid rise of the intracellular cyclic AMP levels. After 10 min an approximately 10 fold increase in cyclic AMP could be measured. TheEC50 value for this response was 7 +/- 1 nM for histamine. This response was effectively blocked by tiotidine and cimetidine, resulting in Ki values of 8 +/- 1 nM and 0.56 +/- 0.24 MicroM respectively.7. Stimulation of CHOhumH2 cells with histamine neither inhibited the A23187-induced release of[3H]-arachidonic acid nor changed the intracellular IP3 levels.8. These results show that the cloned human gene encodes a histamine H2 receptor that is indistinguishable from the H2 receptor identified in human brain tissue. This receptor is functionally coupled to the adenylate cyclase in CHO cells, but does not influence the inositolphosphate turnover or arachidonic acid release.

Animals↗

PubMed1994

Dietary fish oil potentiates bile acid-induced cholesterol secretion into bile in rats.

Recently we demonstrated that dietary fish oil (FO) causes changes in intrahepatic cholesterol transport and hypersecretion of cholesterol into bile in rats (J. Clin. Invest. 88: 943-951, 1991). We have now investigated in more detail the relationship between cholesterol and bile acid secretion in rats with chronic bile diversion fed purified diets supplemented (9% wt/wt) with either FO or corn oil (CO) for 2 weeks. Effects of FO on biliary cholesterol secretion (+ 400% as compared to CO after 14 days) were much more pronounced than previously observed in rats with intact enterohepatic circulation (+50%). Biliary bile acid (+30%) and phospholipid (+120%) secretion were increased to a much lesser extent than that of cholesterol resulting in the formation of bile supersaturated with cholesterol. The biliary cholesterol/bile acid molar ratio was 0.069 and 0.032 in FO- and CO-fed rats, respectively, at noon of day 14. This ratio increased to 0.108 in FO-fed rats at midnight, when bile acid output was maximal, but remained unchanged in CO-fed rats during the day-night cycle. Intravenous administration of taurochenodeoxycholic acid (15 mumol/kg) resulted in a 2-fold increase in bile acid output and a simultaneous 1.6-fold stimulation of cholesterol secretion in both groups, implying that administration of the bile acid induced the secretion of 2-3 times as much cholesterol in FO- than in CO-fed rats. Likewise, administration of bilirubin ditaurate (30 mumol/kg), an inhibitor of bile acid-induced biliary lipid secretion, reduced cholesterol output in both groups by about 50% while bile acid output remained unchanged. It is concluded that, in rats, dietary fish oil increases the disposition of cholesterol into bile by potentiating bile acid-dependent cholesterol secretion, presumably by facilitating the recruitment of bile-destined cholesterol.

Animals↗

PubMed1994

Effects of dietary corn and olive oil versus coconut fat on biliary cholesterol secretion in rats.

We have studied the effects of dietary corn and olive oil versus coconut fat on bile formation and fluidity of hepatic plasma membranes in rats. After 4 weeks of feeding the purified diets containing 9% (w/w) of the test fats, there was no difference in plasma cholesterol concentration between the dietary groups. The amount of free and esterified cholesterol in the liver was significantly higher in rats fed either corn oil or olive oil as compared with coconut fat. In the rats fed olive oil, but not in those fed corn oil this was associated with lower rates of biliary phospholipid excretion. Bile flow was not differently influenced by the three dietary fats. Hepatic plasma membranes of the rats fed corn or olive oil contained more cholesterol and less phospholipids than those on coconut fat, which was, however, not accompanied by changes in fluidity of the membranes. These results indicate that in rats the type of dietary fat can induce considerable changes in hepatic cholesterol metabolism without affecting plasma cholesterol concentrations, and without consistent effects on biliary cholesterol secretion.

Animals↗

PubMed1993

Clofazimine and B669 inhibit the proliferative responses and Na+, K(+)-adenosine triphosphatase activity of human lymphocytes by a lysophospholipid-dependent mechanism.

The relationship between the phospholipase-stimulating and immunosuppressive properties of the riminophenazine anti-mycobacterial agent clofazimine and its experimental analogue, B669, has been investigated in vitro. At concentrations of 0.6 microM and upwards, both riminophenazines, particularly B669, caused dose-related inhibition of mitogen- and alloantigen-stimulated uptake of tritiated thymidine by human mononuclear leucocytes (MNL), while in short-term assays both agents increased the release of lysophosphatidylcholine (LPC) and arachidonic acid from these cells. Arachidonate per se at a concentration of 20 microM did not affect mitogen-activated lymphocyte proliferation, while cyclooxygenase and 5'-lipoxygenase inhibitors, as well as water- and lipid-soluble oxidant-scavengers and anti-oxidant enzymes, failed to protect the cells against the anti-proliferative effects of clofazimine and B669. However, LPC caused dose-related inhibition of lymphocyte proliferation. Moreover, co-incubation of NML with alpha-tocopherol (vitamin E), a lysophospholipid complex-forming agent, or with lysophospholipase, protected the cells against clofazimine and B669, as well as against LPC. Na+, K(+)-adenosine triphosphatase was identified as the primary target of riminophenazine/LPC-mediated inhibition of lymphocyte proliferation. Excessive release of anti-proliferative lysophospholipids during clofazimine or B669 treatment of mitogen- or antigen-activated lymphocytes is the probable biochemical mechanism of the immunosuppressive activity of these agents.

Anti-Infective Agents↗

PubMed1993

Extracellular ATP elevates cytoplasmatic free Ca2+ in HeLa cells by the interaction with a 5'-nucleotide receptor.

In the present study we have characterized the effects of ATP and several other nucleotides on the intracellular Ca2+ levels of HeLa cells. Using fura-2 microscopy fluorescence measurements, the ATP-mediated increase in intracellular Ca2+ was shown to consist of a rapid rise which decreased after a few seconds to a sustained elevated level. In the absence of extracellular Ca2+ or in the presence of 10 microM La3+, only a transient elevation of the Ca2+ concentration was observed. The ATP responses were not altered after treatment of HeLa cells with cholera toxin or pertussis toxin. Pharmacological analysis of this calcium response revealed that this effect was not mediated by the classical P2y purinoceptor but by a 5'-nucleotide receptor.

Adenosine Triphosphate↗

PubMed1993

Mouse P19 embryonal carcinoma cells express functional histamine H1-receptors.

In the present study, we have investigated the response in P19 embryonal carcinoma (EC) cells to histamine. We show that these cells, that resemble the pluripotent cells of an early mouse embryo, respond to histamine addition by a transient increase in intracellular Ca2+. The response is stereoselectively inhibited by the enantiomers of the H1-receptor antagonists chlorpheniramine and cicletanine. [3H]-mepyramine was found to bind with high affinity (Kd 4 nM) to a membrane preparation of P19 EC cells. The profile of these binding sites corresponded well with the results of the Ca2+ measurements. A high affinity [3H]-mepyramine binding site was also identified on intact cells. These data demonstrate that embryonal carcinoma cells express functional histamine H1-receptors and suggest that histamine might act as a regulatory factor in the early development of the mouse embryo.

Animals↗

PubMed1993

Dietary cholesterol-induced down-regulation of intestinal 3-hydroxy-3-methylglutaryl coenzyme A reductase activity is diminished in rabbits with hyperresponse of serum cholesterol to dietary cholesterol.

Key enzymes of cholesterol metabolism were studied in two inbred strains of rabbits with hyper- or hyporesponse of serum cholesterol to dietary cholesterol. Baseline 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase activity in liver was similar in hypo- and hyperresponders, but that in intestine was twofold higher in the hyporesponders. The addition of cholesterol (3 g/kg) to the diet caused similar depression of hepatic HMG-CoA reductase activity in the two strains, whereas intestinal HMG-CoA reductase activity was significantly reduced in hyporesponders but not in hyperresponders. Cholesterol feeding induced higher free cholesterol concentrations in hepatic and intestinal microsomes of both hypo- and hyperresponders and higher activity of hepatic acyl-CoA:cholesterol acyltransferase (ACAT). Hepatic ACAT activity was significantly lower in cholesterol-fed hyperresponders than in hyporesponders, which may have contributed to the observed higher free cholesterol concentrations in hepatic microsomes of cholesterol-fed hyperresponders. Intestinal ACAT activity was similar in hypo- and hyperresponders; cholesterol feeding tended (P = 0.11) to elevate the activity of this enzyme. Hepatic cholesterol 7 alpha-hydroxylase activity was significantly higher in cholesterol-fed hyperresponders than in hyporesponders; it was slightly depressed after cholesterol loading in both rabbit strains.

Animals↗

PubMed1993

Lysophospholipid-mediated inhibition of Na+,K(+)-adenosine triphosphatase is a possible mechanism of immunosuppressive activity of cyclosporin A.

The relationship between the phospholipase-stimulating and immunosuppressive properties of cyclosporin A (CsA) has been investigated in vitro. At concentrations of 0.025 microM and upwards, CsA caused dose-related inhibition of both mitogen- and alloantigen-stimulated uptake of tritiated thymidine by human mononuclear leukocytes (MNL), which was associated with a time- and dose-related enhancement of the generation of lysophosphatidylcholine (LPC), arachidonic acid, and prostaglandin E2 from mitogen-stimulated cells. Arachidonate alone, at concentrations of up to 20 microM, did not affect lymphocyte activation, whereas cyclooxygenase and 5'-lipoxygenase inhibitors failed to protect the cells against the antiproliferative effects of CsA. However, LPC caused dose-related inhibition of MNL proliferation. Moreover, coincubation of MNL with alpha-tocopherol, a lysophospholipid-complexing agent, or with lysophospholipase protected the cells against CsA, as well as against LPC. The Na+,K(+)-ATPase activity of mitogen-activated lymphocytes was also inhibited by CsA, whereas inclusion of alpha-tocopherol or lysophospholipase protected this enzyme. Excessive production of lysophospholipids and consequent inhibition of Na+,K(+)-ATPase during CsA treatment of mitogen- or antigen-activated lymphocytes is a possible biochemical mechanism of the immunosuppressive activity of this agent.

Adult↗

PubMed1993

Effects of dietary cholesterol on bile formation and hepatic processing of chylomicron remnant cholesterol in the rat.

We have studied the coupling between hepatic uptake of chylomicron remnant cholesteryl ester and biliary excretion of cholesterol and bile acids in rats, after feeding them a cholesterol-free (control) or a high-cholesterol diet (1% wt/wt) for 2 wk. We equipped rats with permanent catheters in the bile duct, duodenum and heart to allow experiments in unanesthetized, unrestrained animals. Cholesterol feeding induced a 20% increase in plasma cholesterol concentration, a threefold increase in hepatic bile acid synthesis and a 27% increase in bile acid pool size, whereas biliary excretion of cholesterol was decreased by 50%. The enlarged bile acid pool contained relatively less cholic acid and more chenodeoxycholic acid and muricholic acids. [3H]cholesteryl ester-labeled chylomicron remnants (150 micrograms protein per rat) were injected intracardially, and blood and bile were collected for a period of 22 hr. Plasma disappearance of remnants was significantly delayed by cholesterol feeding, probably caused by competition with diet-induced beta-very low density lipoproteins for hepatic uptake. In control rats biliary excretion of chylomicron remnant-derived radioactivity (50% in free cholesterol and 50% in bile acids) showed an initial peak 1 hr after injection (2.4% dose per hour). A second peak (90% in bile acids), amounting to 1.5% of the dose per hour, appeared 11 hr after injection. Total 22-hr excretion of 3H was 22% of the dose. In cholesterol-fed rats chylomicron remnant-derived radioactivity appeared more rapidly in bile, with a peak 1 hr after injection, amounting to 3.5% of the dose per hour. In this case radioactivity was mainly present as bile acid. Total excretion in 22 hr was 27% of the dose. We conclude that chylomicron remnant uptake by the liver is efficiently coupled to bile acid synthesis and biliary excretion, thus providing an efficient pathway for removal of intestine-derived cholesterol. After cholesterol feeding, chylomicron remnant cholesteryl ester is more efficiently converted to bile acids, a mechanism which may contribute to the resistance of rats to diet-induced elevation of plasma cholesterol. In contrast, biliary excretion in the form of free cholesterol, the second main excretory pathway, is significantly decreased by a high-cholesterol diet.

Animals↗

PubMed1992

Biochemical mechanisms of hydrogen peroxide- and hypochlorous acid-mediated inhibition of human mononuclear leukocyte functions in vitro: protection and reversal by anti-oxidants.

Both H2O2 (IC50 = 70 microM) and HOCl (IC50 = 8.5 microM) inhibited mitogen-induced MNL proliferation in a dose-dependent manner. This was found to be due to a depletion of intracellular ATP by at least two distinct mechanisms. HOCl and high concentrations (greater than 100 microM) of H2O2 inhibit ATP generation via sulfhydryl group oxidation on the active site of the glyceraldehyde-3-phosphate dehydrogenase (G3PDH) enzyme of the glycolytic pathway. On the other hand, low H2O2 concentrations cause ATP depletion by an activation of the DNA repair enzyme, poly(ADP-ribose)polymerase (pADPRP), leading to consumption of NAD+, an essential cofactor for G3PDH. The anti-oxidants ascorbate and cysteine protected MNL against the anti-proliferative effects of HOCl. Similar results were achieved with the HOCl-mediated inhibition of ATP production and G3PDH activity. However, ascorbate was unable to protect against H2O2-mediated inhibition of MNL functions, while cysteine protected against the inhibitory effects on ATP production and G3PDH activity, induced by this oxidant.

Adenosine Triphosphate↗

PubMed1992

Short-term desensitization of the histamine H1 receptor in human HeLa cells: involvement of protein kinase C dependent and independent pathways.

1. In this study we have investigated the effects of short-term exposure of cells to histamine on the subsequent H1 receptor responsiveness in HeLa cells, using Ca2+ fluorescence microscopy and video digital imaging. 2. In HeLa cells, histamine (100 microM) induces an immediate H1 receptor-mediated biphasic elevation of the intracellular Ca2+ concentration ([Ca2+]i) (basal [Ca2+]i: 81 +/- 30 nM, histamine-induced Ca2+ response: first phase: 1135 +/- 79 nM; second phase: 601 +/- 52 nM, n = 11). 3. The histamine H1 receptors on HeLa cells are readily susceptible to desensitization since repetitive exposure of the same group of cells to histamine (100 microM) markedly affected the release and influx component of the induced Ca2+ response (second application of histamine: first phase: 590 +/- 92 nM, second phase: 279 +/- 47 nM; third application of histamine: first phase: 454 +/- 127 nM, second phase: 240 +/- 45 nM, n = 6). Video digital imaging revealed an increase in the lag time between stimulation and monitoring of the Ca2+ response and a reduced increase in [Ca2+]i after desensitization with histamine. 4. Neither the release component of the ATP response (50 microM) nor the caffeine (3 mM)-induced Ca2+ release were found to be affected by desensitization with 100 microM histamine. However, the second phase of the ATP response was significantly reduced after desensitization with histamine (control cells: 516 +/- 33 nM; desensitized cells: 331 +/- 96 nM, n = 4, P < 0.05).5. Activation of protein kinase C (PKC) by phorbol-12-myristate-1 3-acetate was found to inhibit the histamine as well as ATP-induced Ca2" response in a dose-dependent manner.6. In PKC downregulated cells the second phase of the histamine-induced Ca2+ response was significantly elevated, indicating the involvement of PKC in the negative feedback on the Ca2+ influx(control cells: second phase: 601 +/- 52 nM (n = 11); PKC downregulated cells: second phase:890 +/- 90nM, n = I0, P<0.05).7. Homologous desensitization of H, receptor responsiveness was still observed in PKC downregulated cells, implying the rapid activation of a regulatory mechanism other than PKC.8. Based on our experimental data we suggest that short-term desensitization of the histamine H,receptor evolves from two different processes: a selective reduction of the histamine-induced Ca2+ release, mediated by a PKC-independent pathway, and a non-selective inhibition of the receptormediated Ca2+ influx activated by a PKC-dependent pathway.

Adenosine Triphosphate↗

PubMed1991

Homologous histamine H1 receptor desensitization results in reduction of H1 receptor agonist efficacy.

Prolonged exposure of the guinea-pig intestinal longitudinal smooth muscle to histamine caused homologous desensitization of the H1 receptor, which led to reduced H1 receptor-mediated production of [3H]inositol phosphates as well as to reduced H1 agonist-induced contractions. [3H]Mepyramine binding studies showed that desensitization affected neither the agonist affinity nor the number of H1 receptors. Combining the data from the binding studies and the contraction measurements it was found that desensitization results in a selective reduction of agonist efficacy.

Animals↗