Histamine H1-receptor-mediated calcium influx in DDT1MF-2 cells

On the role of histamine receptors in regulating pigmentary responses in Oreochromis mossambicus melanophores

PURPOSE

The present work was carried out to reveal the involvement of histamine receptors at the neuro-melanophore junction of teleost, Oreochromis mossambicus.

METHODS

The isolated scale melanophores were assayed using the mean melanophore size index and their responses were recorded in presence of various concentrations of histamine along with H(1) and H(2) receptor specific agonists and antagonist and potentiator compound 48/80.

RESULTS

Melanophores showed high sensitivity to histamine and its specific agonists. Histamine caused a dose-dependent pigment aggregation, whereas 2-(2-Pyridyl) ethylamine (PEA), a specific H(1)R agonist also caused aggregation in a similar manner. Conversely, amthamine, a specific H(2)R agonist resulted in pigment dispersion. The effects were antagonized by mepyramine; specific H(1)R antagonist and ranitidine a specific H(2)R antagonist.

CONCLUSION

It is concluded that O. mossambicus melanophores have both H(1) and H(2) receptors which mediate melanophore aggregation and dispersion respectively. Compound 48/80 augmented the melanin-aggregating and dispersing effects of PEA and amthamine. It is suggested that the effect of histamine is directly mediated through H1 and H2 receptors, whereas H1Rs may be predominantly involved in the aggregatory responses.

Stimulation of protein kinase B and p70 S6 kinase by the histamine H1 receptor in DDT1MF-2 smooth muscle cells

1. Previous studies have shown that the histamine H(1) receptor activates p42/p44 mitogen-activated protein kinases (MAPK) in DDT(1)MF-2 smooth muscle cells via a phosphatidylinositol 3-kinase (PI-3K)-dependent pathway. In this study the effect of histamine H(1) receptor stimulation on protein kinase B (PKB) and p70 S6 kinase, both of which are downstream targets of PI-3K, has been investigated. Increases in PKB and p70 S6 kinase activation were monitored by Western blotting using phospho-specific PKB (Ser(473)) and p70 S6 kinase (Thr(421)/Ser(424)) antibodies. 2. Histamine stimulated time and concentration-dependent increases in the phosphorylation of PKB and p70 S6 kinase in DDT(1)MF-2 cells. Both responses were completely inhibited by the histamine H(1) receptor antagonist mepyramine and following pre-treatment with pertussis toxin, to block G(i)/G(o) protein dependent pathways. 3. The PI-3K inhibitors wortmannin (IC(50) 5.9+/-0.5 nM) and LY 294002 (IC(50) 6.9+/-0.8 microM) attenuated the increase in PKB phosphorylation induced by histamine (100 microM) in a concentration-dependent manner. 4. Histamine-induced increases in p70 S6 kinase phosphorylation were partially sensitive to rapamycin (20 nM; 68% inhibition) but completely blocked by wortmannin (100 nM), LY 294002 (30 microM) and the MAPK kinase inhibitor PD 98059 (50 microM). 5. In summary, these data demonstrate that the histamine H(1) receptor stimulates PKB and p70 S6 kinase phosphorylation in DDT(1)MF-2 smooth muscle cells. However, functional studies revealed that histamine does not stimulate DDT(1)MF-2 cell proliferation or attenuate staurosporine-induced caspase-3 activity. The challenge for future research will be to link the stimulation of these kinase pathways with the physiological and pathophysiological roles of the histamine H(1) receptor.

Activation of the p38 and p42/p44 mitogen-activated protein kinase families by the histamine H(1) receptor in DDT(1)MF-2 cells

1. The mitogen-activated protein kinases (MAPKs) consist of the p42/p44 MAPKs and the stress-activated protein kinases, c-Jun N-terminal kinase (JNK) and p38 MAPK. In this study we have examined the effect of histamine H(1) receptor activation on MAPK pathway activation in the smooth muscle cell line DDT(1)MF-2. 2. Histamine stimulated time and concentration-dependent increases in p42/p44 MAPK activation in DDT(1)MF-2 cells. Responses to histamine were inhibited by the histamine H(1) receptor antagonist mepyramine (K(D) 3.5 nM) and following pre-treatment with pertussis toxin (PTX; 57% inhibition). 3. Histamine-induced increases in p42/p44 MAPK activation were blocked by inhibitors of MAPK kinase 1 (PD 98059), tyrosine kinase (genistein and tyrphostin A47), phosphatidylinositol 3-kinase (wortmannin and LY 294002) and protein kinase C (Ro 31-8220; 10 microM; 41% inhibition). Inhibitors of Src tyrosine kinase (PP2) and the epidermal growth factor tyrosine kinase (AG1478) were without effect. Removal of extracellular Ca(2+), chelation of intracellular Ca(2+) with BAPTA and inhibition of focal adhesion assembly (cytochalasin D) had no significant effect on histamine-induced p42/p44 MAPK activation. 4. Histamine stimulated time and concentration-dependent increases in p38 MAPK activation in DDT(1)MF-2 cells but had no effect on JNK activation. Histamine-induced p38 MAPK activation was inhibited by pertussis toxin (74% inhibition) and the p38 MAPK inhibitor SB 203580 (95% inhibition). 5. In summary, we have shown the histamine H(1) receptor activates p42/p44 MAPK and p38 MAPK signalling pathways in DDT(1)MF-2 smooth muscle cells. Interestingly, signalling to both pathways appears to involve histamine H(1) receptor coupling to G(i)/G(o)-proteins.

Study of calcium signaling in non-excitable cells

The fundamental importance of calcium signaling in the control of cellular physiology is widely recognized. A dramatic illustration of this is the fact that a Medline search for review articles containing the word "calcium" in the title reveals 4,629 hits, whereas the whole body of calcium signaling literature (approximately 2 x 10(6) pages) is more than enough to fill a decent-sized library. Most of this literature deals with calcium signaling in excitable cells types (mainly neurons and muscle cells), but non-excitable cell types are capable of calcium signaling as well. Although calcium fluxes in the latter cell types have attracted much less interest, the literature involved is still vast. Nevertheless, in this review article we hope to contribute some valuable insights to the field. First we shall discuss the experimental techniques available to the researcher interested in calcium signaling in non-excitable cell types with special attention to patch clamp electrophysiology. Subsequently, we shall review some of the results obtained with these techniques by focussing on the calcium-regulating mechanisms in non-excitable cells and discussing the importance of these mechanisms for physiology.

Neomycin inhibits histamine and thapsigargin mediated Ca2+ entry in DDT1 MF-2 cells independent of phospholipase C activation

The histamine H1 receptor mediated increase in cytoplasmic Ca2+ ([Ca2+]i) was measured in the presence of the known phospholipase C (PLC) inhibitor, neomycin. Neomycin (1 mM) inhibited the histamine (100 microM) induced rise in [Ca2+]i to the same extent as observed after blocking Ca2+ entry with LaCl3. Likewise, the increase in [Ca2+]i after re-addition of CaCl2 (2 mM) to extracellular Ca2+ deprived and histamine pretreated cells was strongly reduced by neomycin. However, neomycin did not inhibit the histamine induced formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) or the release of Ca2+ from internal stores. These results show that neomycin blocks histamine induced Ca2+ entry independent of phospholipase C activation. Inhibition of intracellular store Ca(2+)-ATPase by thapsigargin (1 microM), elicited an increase in [Ca2+]i due to a leakage from the stores, subsequently followed by store-dependent Ca2+ entry. Thapsigargin induced Ca2+ entry was also completely blocked by neomycin. These results indicate that neomycin inhibits histamine and thapsigargin induced Ca2+ entry. This inhibition is most likely exerted at the level of plasma membrane Ca2+ channels.

Adenosine A1 receptor-mediated changes in basal and histamine-stimulated levels of intracellular calcium in primary rat astrocytes

1. The effects of adenosine A1 receptor stimulation on basal and histamine-stimulated levels of intracellular free calcium ion concentration ([Ca2+]i) have been investigated in primary astrocyte cultures derived from neonatal rat forebrains. 2. Histamine (0.1 microM-1 mM) caused rapid, concentration-dependent increases in [Ca2+]i over basal levels in single type-2 astrocytes in the presence of extracellular calcium. A maximum mean increase of 1,468 +/- 94 nM over basal levels was recorded in 90% of type-2 cells treated with 1 mM histamine (n = 49). The percentage of type-2 cells exhibiting calcium increases in response to histamine appeared to vary in a concentration-dependent manner. However, the application of 1 mM histamine to type-1 astrocytes had less effect, eliciting a mean increase in [Ca2+]i of 805 +/- 197 nM over basal levels in only 30% of the cells observed (n = 24). 3. In the presence of extracellular calcium, the A1 receptor-selective agonist, N6-cyclopentyladenosine (CPA, 10 microM), caused a maximum mean increase in [Ca2+]i of 1,110 +/- 181 nM over basal levels in 30% of type-2 astrocytes observed (n = 53). The size of this response was concentration-dependent; however, the percentage of type-2 cells exhibiting calcium increases in response to CPA did not appear to vary in a concentration-dependent manner. A mean calcium increase of 605 +/- 89 nM over basal levels was also recorded in 23% of type-1 astrocytes treated with 10 microM CPA (n = 30). 4. In the absence of extracellular calcium, in medium containing 0.1 mM EGTA, a mean increase in [Ca2+]i of 504 +/- 67 nM over basal levels was recorded in 41% of type-2 astrocytes observed (n = 41) after stimulation with 1 microM CPA. However, in the presence of extracellular calcium, pretreatment with the A1 receptor-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine, for 5-10 min before stimulation with 1 microM CPA, completely antagonized the response in 100% of the cells observed. 5. In type-2 astrocytes, prestimulation with 10 nM CPA significantly increased the size of the calcium response produced by 0.1 microM histamine and the percentage of responding cells. Treatment with 0.1 microM histamine alone caused a mean calcium increase of 268 +/- 34 nM in 41% of the cells observed (n = 34). After treatment with 10 nM CPA, mean calcium increase of 543 +/- 97 nM was recorded in 100% of the cells observed (n = 33). 6. These data indicate that adenosine Al receptors couple to intracellular calcium mobilization and extracellular calcium influx in type-1 and type-2 astrocytes in primary culture. In addition, the simultaneous activation of adenosine Al receptors on type-2 astrocytes results in an augmentation of the calcium response to histamine H1 receptor stimulation.

Ca(2+)-dependent and -independent mechanism of cyclic-AMP reduction: mediation by bradykinin B2 receptors

1. Bradykinin caused a transient reduction of about 25% in the cyclic AMP level in forskolin prestimulated DDT1 MF-2 smooth muscle cells (IC50: 36.4 +/- 4.9 nM) and a pronounced, sustained inhibition (40%) of the isoprenaline-stimulated cyclic AMP level (IC50: 37.5 +/- 1.1 nM). 2. The Ca2+ ionophore, ionomycin, mimicked both the bradykinin-induced transient reduction in the forskolin-stimulated cyclic AMP level and the sustained reduction in the isoprenaline-stimulated cyclic AMP level. 3. The Ca(2+)-dependent effect on cyclic AMP induced by bradykinin was mediated solely by Ca2+ release from internal stores, since inhibition of Ca2+ entry with LaCl3 did not reduce the response to bradykinin. 4. The involvement of calmodulin-dependent enzyme activities, protein kinase C or an inhibitory GTP binding protein in the bradykinin-induced responses was excluded since a calmodulin inhibitor, calmidazolium, a PKC inhibitor, staurosporine and pertussis toxin, respectively did not affect the decline in the cyclic AMP level. 5. Bradykinin enhanced the rate of cyclic AMP breakdown in intact cells, which effect was not mimicked by ionomycin. This suggested a Ca(2+)-independent activation of phosphodiesterase activity by bradykinin in DDT1 MF-2 cells. 6. The bradykinin B1 receptor agonist, desArg9-bradykinin, did not affect cyclic AMP formation in isoprenaline prestimulated cells, while the bradykinin B2 receptor antagonists, Hoe 140 (D-Arg[Hyp3, Thi5, D-Tic7, Oic8]-BK) and D-Arg[Hyp3, Thi5,8, D-Phe7]-BK completely abolished the bradykinin response in both forskolin and isoprenaline prestimulated cells. 7. Bradykinin caused an increase in intracellular Ca2+, which was antagonized by the bradykinin B2 receptor antagonists, Hoe 140 and D-Arg[Hyp3, Thi5,8, D-Phe7]-BK. The bradykinin B2 receptor agonist,desArg9-bradykinin, did not evoke a rise in cytoplasmic Ca2 .8. It is concluded, that stimulation of bradykinin B2 receptors causes a reduction in cellular cyclic AMP in DDT1, MF-2 cells. This decline in cyclic AMP is partly mediated by a Ca2+/calmodulin independent activation of phosphodiesterase activity. The increase in [Ca2+], mediated by bradykinin B2 receptors inhibited forskolin- and isoprenaline-activated adenylyl cyclase differently, most likely by interfering with different components of the adenylyl cyclase signalling pathway.

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.

Increased affinity of histamine H1 binding to membranes of human myometrium at the end of pregnancy

1. The characterization of H1 binding sites in membrane preparations of human myometrium obtained from pregnant and non-pregnant women was performed by using 3H-mepyramine as the radioactive ligand. 2. Saturation curve analysis revealed that 3H-mepyramine is bound to a single class of binding sites. Changes in the H1 site binding parameters were observed at the end of pregnancy, resulting in an increased affinity relative to non-pregnant tissue (Kd: 131.0 +/- 8.8 (non-pregnant) and 72.5 +/- 7.5 (pregnant) nM, n = 6, P < 0.01). 3. A reduction in receptor concentration at the end of pregnancy was also observed, [Bmax: 565.2 +/- 43.7 (non-pregnant) and 309.6 +/- 25.9 (pregnant) fmol/mg prot, n = 6, P < 0.01]. It is possible that this reduction in Bmax could be attributed to a dilution factor due to the increase in membraneous proteins that occurs during gestation.

Selective potentiation of histamine H1-receptor stimulated calcium responses by 1,4-dithiothreitol in DDT1MF-2 cells

The effect of 1,4-dithiothreitol (DTT) on agonist-stimulated increases in intracellular free calcium concentration ([Ca2+]i) has been investigated in the smooth muscle cell line, DDT1MF-2, derived from hamster vas deferens. Pretreatment with DTT (1 mM) produced a large leftward parallel shift in concentration-response curve for histamine H1-receptor mediated increases in [Ca2+]i. The EC50 values for H1-receptor stimulated increases in [Ca2+]i in the absence and presence of DTT were 11.3 +/- 1.5 microM (N = 6) and 0.52 +/- 0.15 microM (N = 6), respectively. DTT had no significant effect on the maximum Ca2+ response elicited by histamine (100 microM). In the presence of DTT the partial H1-receptor agonist 2-pyridylethylamine (100 microM) increased [Ca2+]i from 112 +/- 14 nM to 237 +/- 24 nM (N = 10). In control cells 2-pyridylethylamine (100 microM) did not elicit a Ca2+ response. DTT had no significant effect on the maximum Ca2+ response elicited by 1 mM 2-pyridylethylamine. The enhancement of histamine H1-receptor Ca2+ responses by DTT was reversed by the sulphydryl oxidizing agent dithiobis-(2-nitrobenzoic acid). DTT had no significant effect on adenosine A1-, bradykinin and ATP-receptor stimulated increases in [Ca2+]i. [3H]mepyramine binding experiments confirmed that DTT increased agonist affinity. DTT produced a small, but significant, leftward shift in concentration-response curve for histamine displacement of [3H]mepyramine binding. These data suggest that DTT potentiates H1-receptor mediated Ca2+ responses by increasing agonist affinity.

Interactions between adenosine A1- and histamine H1-receptors

The interactions or "cross-talk" between adenosine A1-receptors and receptors coupled to phospholipase C (leading to the hydrolysis of inositol phospholipids) have been well documented in the literature. For example, activating the A1-receptor selectively potentiates the histamine H1-receptor stimulated hydrolysis of inositol phospholipids in guinea-pig cerebral slices. In contrast, when the adenosine receptor is activated in the cerebral cortex of mouse or man the histamine response is selectively inhibited. Our studies have focused on the smooth muscle cell line, DDT1 MF-2, derived from hamster vas deferens. These cells express A1-receptors which, in addition to the expected negative coupling to adenylate cyclase, also stimulate inositol phospholipid hydrolysis and Ca2+ mobilization. These A1-receptors also potentiate histamine H1-receptor responses, i.e. inositol phospholipid hydrolysis and Ca2+ mobilization. The mechanism(s) underlying the potentiation or inhibition of histamine H1-receptor responses by the adenosine A1-receptor remain to be unravelled. One mechanism may involve intracellular "cross-talk" at the G-protein level. This review will discuss how beta gamma subunits from G(i) proteins could be involved in augmenting responses to calcium mobilizing receptors.

Characteristics of [3H]mepyramine binding in DDT1MF-2 cells: evidence for high affinity binding to a functional histamine H1 receptor

The binding characteristics of [3H]mepyramine to histamine H1 receptors in the smooth muscle cell line, DDT1MF-2, have been investigated. Competition binding experiments produced dissociation constants (Ki) for mepyramine, (+)-chlorpheniramine, and promethazine of 3.4 nM, 2.6 nM and 0.66 nM, respectively. Saturation binding using [3H]mepyramine produced a Kd of 2.1 nM and a Bmax of 47 fmol/mg protein. These data suggest that a high-affinity [3H]mepyramine binding site can be detected with the characteristics of the "classical" histamine H1 receptor. The low-affinity [3H]mepyramine binding site reported previously [Mitsuhashi, M. and Payan, D.G. (1988) J. Cell. Physiol. 134, 367-375] is predominantly to a secondary [3H]mepyramine site. The "low affinity" or secondary [3H]mepyramine binding site on DDT1MF-2 cells is insensitive to quinine (10 microM) and is therefore distinct from the [3H]mepyramine binding protein found in rat liver.

Homologous and heterologous desensitization of histamine H1- and ATP-receptors in the smooth muscle cell line, DDT1MF-2: the role of protein kinase C

1. The possible role of protein kinase C (PKC) in homologous and heterologous desensitization of histamine H1- and ATP-receptors has been studied in monolayers of cultured vas deferens smooth muscle cells (DDT1MF-2). Cells were loaded with the calcium-sensitive fluorescent dye fura-2 and increases in intracellular free Ca2+ concentration ([Ca2+]i) monitored in response to histamine H1- or ATP-receptor activation. 2. Histamine and ATP stimulated the release of Ca2+ from intracellular Ca2+ stores and Ca2+ influx across the plasma membrane. Activation of PKC with the phorbol ester beta-phorbol-12,13 dibutyrate (PDBu; 1 microM) attenuated histamine (100 microM) and ATP (10 microM)-induced release of intracellular Ca2+ and Ca2+ influx. 3. The selective PKC inhibitor, Ro 31-8220 (10 microM), reversed the PDBu-induced attenuation of histamine (100 microM)-stimulated Ca2+ responses. 4. Histamine H1- and ATP-receptors are readily susceptible to homologous desensitization since short-term exposure to histamine or ATP (450 s) attenuated the Ca2+ responses elicited by a second application of the same agonist. Furthermore, H1-receptor activation-induced heterologous desensitization of ATP stimulated Ca2+ responses and vice versa. 5. Homologous and heterologous desensitization of histamine and ATP Ca2+ responses still occurred in the presence of the PKC inhibitor, Ro 31-8220 (10 microM). 6. These data suggest that PKC activation can attenuate histamine H1- and ATP-receptor mediated Ca2+ responses. However, based on our experimental data, PKC-independent mechanisms appear to be involved in the homologous and heterologous desensitization of histamine H1- and ATP-receptor mediated Ca2+ responses in DDT1MF-2 cells.

Intracellular cross-talk between receptors coupled to phospholipase C via pertussis toxin-sensitive and insensitive G-proteins in DDT1MF-2 cells

1. The effect on intracellular free calcium concentration ([Ca2+]i) of simultaneous activation of receptors coupled to phospholipase C via pertussis toxin (PTX)-sensitive and -insensitive G-proteins has been investigated in the hamster vas deferens smooth muscle cell line, DDT1MF-2. 2. In fura-2-loaded DDT1MF-2 cells, activation of adenosine A1-receptors (which are linked to PTX-sensitive G-proteins) with a maximal concentration of N6-cyclopentyladenosine (CPA; 300 nM) increased [Ca2+]i from 121 +/- 5 nM to 254 +/- 20 nM (n = 8). These experiments were performed in the presence of extracellular Ca2+. Stimulation of histamine H1-receptors (which are linked to PTX-insensitive G-proteins) with a low concentration of histamine (1 microM) increased [Ca2+]i from 128 +/- 8 nM to 150 +/- 13 nM (n = 8). When combined, CPA (300 nM) and histamine (1 microM) synergistically raised [Ca2+]i from 134 +/- 6 nM to 607 +/- 61 nM (n = 8). 3. Removal of extracellular Ca2+ (experiments performed in Ca(2+)-free buffer containing 0.1 mM EGTA) had no effect on the synergistic interaction between CPA (300 nM) and histamine (1 microM). 4. The addition of maximal concentrations of CPA (300 nM) and histamine (100 microM) resulted in a rise in [Ca2+]i which was additive when compared to the Ca2+ responses obtained with the two agonists alone. Low (30 nM) and subthreshold (3 nM) concentrations of CPA did not alter the Ca2+ response elicited by maximal concentrations of histamine (100 microM). 5. Subthreshold concentrations of CPA (3 nM) and low concentrations of histamine (1 microM) elicited synergistic rises in [Ca2+]i. 6 Synergistic Ca2+ responses were not observed between histamine Hl- and ATP-receptors when cells were simultaneously stimulated with either 1 microM or 10 microM of each agonist.7 These data suggest that adenosine A1-receptors linked to PTX-sensitive G-protein(s) and histamine H14-receptors linked to PTX-insensitive G-proteins interact synergistically to raise [Ca2+]i. In contrast,activation of ATP-receptors which are linked to PTX-insensitive G-protein(s) do not interact synergically with histamine H1-receptors.

The effects of elevated cyclic AMP levels on histamine-H1-receptor-stimulated inositol phospholipid hydrolysis and calcium mobilization in the smooth-muscle cell line DDT1MF-2

The effects of raising cyclic AMP levels, by forskolin stimulation, beta-adrenoceptor activation or cyclic AMP phosphodiesterase inhibition, on inositol phospholipid hydrolysis and increases in intracellular free [Ca2+] ([Ca2+]i) elicited by a range of agonists have been investigated in the hamster vas deferens smooth-muscle cell line DDT1MF-2. Isoprenaline (log [EC50 (M)] = -7.7 +/- 0.2), forskolin and the type IV cyclic AMP phosphodiesterase inhibitor rolipram elicited significant increases in the accumulation of cyclic [3H]AMP. Pretreatment with forskolin (10 microM) attenuated histamine (100 microM)- and N6-cyclopentyladenosine (CPA; 300 nM)-induced release of intracellular Ca2+, observed when cells are stimulated in Ca(2+)-free buffer containing 0.1 mM EGTA. Forskolin had no effect on ATP (100 microM)- or bradykinin (1 microM)-stimulated release of intracellular Ca2+. Histamine-induced intracellular Ca2+ release was also inhibited by pretreatment with rolipram (100 microM) or the membrane-permeant cyclic AMP analogue (Sp)-adenosine 3',5'-monophosphothioate (100 microM). Isoprenaline (1 microM) pretreatment (in the presence of 10 microM rolipram, a concentration which on its own did not decrease the histamine response) attenuated histamine-induced intracellular Ca2+ release. Forskolin inhibited histamine (100 microM)- and CPA (100 nM) stimulated accumulation of [3H]-inositol phosphates, but was without effect on ATP or bradykinin responses. Addition of forskolin (in the presence of 100 microM rolipram) after the cells had been stimulated with histamine (in experiments initiated in Ca(2+)-free buffer) inhibited the rise in [Ca2+]i observed when extracellular Ca2+ (2 mM) was re-applied (owing to receptor-mediated Ca2+ influx). Finally, the refilling of intracellular Ca2+ stores (after receptor-mediated Ca2+ influx is blocked by mepyramine) can be demonstrated in the presence of raised cyclic AMP levels.

Characteristics of histamine H1 receptors on HeLa cells

The affinities of antagonists at histamine H1 receptors on HeLa cells have been determined from inhibition of histamine-induced inositol phosphate formation in intact and from inhibition of [3H]mepyramine binding to HeLa cell membranes. The dissociation constants of mepyramine and (+)-chlorpheniramine were similar to values for binding to H1 receptors in other mammalian tissues, but much lower than the values reported in an earlier study with [3H]mepyramine and HeLa cell membranes. Evidence is presented that under conditions employed in the earlier study the binding of [3H]mepyramine is largely to secondary, non-H1 receptor sites.

Refilling the inositol 1,4,5-trisphosphate-sensitive Ca2+ store in neuroblastoma x glioma hybrid NG108-15 cells

Bradykinin-induced increases in the intracellular free Ca2+ concentration ([Ca2+]i) were recorded in single NG108-15 cells with indo-1-based dual-emission microfluorimetry (50% effective concentration, 16 nM). A 1-min exposure to 30 nM bradykinin completely depleted the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store; refilling the store required extracellular Ca2+ (half time, 2 min). Refilling the IP3-sensitive store was completely blocked by 1 microM La3+ and 10 microM nitrendipine, but not 10 microM verapamil, 10 microM flunarizine, 1 microM nitrendipine, or 0.1 microM La3+. Thapsigargin irreversibly depleted the Ca2+ store and prevented its refilling (half-maximal inhibitory concentration, 3 nM). Influx of Ca2+ across the plasma membrane did not increase after depletion of the IP3-sensitive store by exposure to bradykinin, although maintained presence of the agonist produced significant Ca2+ influx. Similarly, Mn2+ and Ba2+ influx, as measured by indo-1 quenching and spectral shifts, did not increase following depletion of IP3-sensitive store. In contrast to depletion of the IP3-sensitive Ca2+ store by bradykinin, thapsigargin (10 nM) treatment produced Ca2+ and Ba2+ influx. We conclude that after Ca2+ mobilization, the IP3-sensitive Ca2+ store in NG108-15 cells is refilled with cytoplasmic Ca2+ via a thapsigargin-sensitive Ca(2+)-Mg(2+)-ATPase. Cytoplasmic Ca2+ is replenished by a persistent leak of Ca2+ across the plasma membrane. This leak is not modulated by the status of the intracellular Ca2+ store. In NG108-15 cells, agonist and thapsigargin-evoked Ca2+ entry are mediated by activation of plasmalemmal Ca2+ channels independent of the status of the IP3-sensitive intracellular Ca2+ store.

Histamine H1-receptor-mediated inositol phospholipid hydrolysis in DDT1MF-2 cells: agonist and antagonist properties

1. The effect of histamine H1-receptor stimulation on inositol phospholipid hydrolysis has been investigated in the hamster vas deferens smooth muscle cell line, DDT1MF-2. 2. Histamine (EC50 = 27 microM) stimulated the accumulation of [3H]-inositol phosphates in DDT1MF-2 cells prelabelled with [3H]-myo-inositol. 2-Thiazolylethylamine (EC50 42 microM) produced a maximal response of similar magnitude to histamine while the maximal response obtained with N alpha-methylhistamine (EC50 = 72 microM) and 2-pyridylethylamine (EC50 = 85 microM) were much lower (circa 65%, histamine = 100%). 3. The H1-selective agonists 2-(3-fluorophenyl)-histamine (2-FPH) and 2-(3-chlorophenyl)-histamine (2-CPH) both appeared to act as partial H1-agonists in this system. Both compounds produced maximal responses of only 30% (with respect to histamine = 100) and were able to antagonize the inositol phosphate response to histamine (estimated Kp = 10.4 and 18.9 microM for 2-FPH and 2-CPH respectively). 4. The response to histamine was antagonized by the H1-antagonists, mepyramine (KD 0.4 nM), (+)-chlorpheniramine (KD 1.2 nM) and promethazine (KD 0.3 nM). Furthermore, the (-)-isomer of chlorpheniramine was approx. three orders of magnitude less potent than the corresponding (+)-isomer. 5. The response to histamine (0.1 mM) was not altered by prior treatment of cells with pertussis toxin (100 ng ml-1; 24 h) whereas the inositol phosphate response to adenosine A1-receptor stimulation in this cell line was significantly attenuated under these conditions. 6. These data indicate that histamine-stimulated inositol phospholipid hydrolysis in DDT1MF-2 cells is mediated via a classical H1-receptor. Furthermore, the results also suggest that histamine HI- and adenosine A,-receptors activate phospholipase C in DDTMF-2 cells via two different G-protein-coupled pathways.

Adenosine A1-receptor stimulated increases in intracellular calcium in the smooth muscle cell line, DDT1MF-2

1. The effect of a range of adenosine receptor agonists on intracellular free calcium concentration ([Ca2+]i) has been studied in the hamster vas deferens smooth muscle cell line DDT1MF-2. 2. Adenosine receptor agonists elicited a rapid and maintained increase in [Ca2+]i in fura-2 loaded DDT1MF-2 cells. The initial rise could be maintained in the absence of extracellular calcium, whereas the maintained or plateau phase was dependent upon the presence of extracellular calcium and appeared to be associated with calcium influx. The rank order of agonist potencies was N6-cyclopentyladenosine > 5'-N-ethylcarboxamidoadenosine > 2-chloroadenosine > adenosine. 3. The response to 2-chloroadenosine was antagonized by the antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, KD 0.14 nM) and 8-phenyltheophylline (KD 112 nM). 4. Pretreatment with the 5-lipoxygenase inhibitor AA861 (20 microM) produced only a small (14 +/- 2%) inhibition of the [Ca2+]i response elicted by N6-cyclopentyladenosine (300 nM), in nominally Ca(2+)-free buffer containing 0.1 mM EGTA. The cyclo-oxygenase inhibitor, indomethacin (2 microM) was without effect. 5. The Ca(2+)-influx associated with the plateau phase required the continued presence of agonist on the receptor. The antagonist DPCPX (100 nM) attenuated the rise in [Ca2+]i observed when extracellular Ca2+ was re-applied after the cells had been stimulated with N6-cyclopentyladenosine (CPA; 300 nM) in experiments initiated in nominally Ca(2+)-free buffer. 6. Pretreatment with pertussis toxin (200 ng ml-1 for 4 h) inhibited the CPA (100 nM) stimulated intracellular Ca2+ release and Ca2+ influx but was without effect on the response to histamine (100 microM). 7.These data suggest that adenosine A(1)-receptor activation in DDT(1)MF-2 cells stimulates release of Ca(2+) from intracellular stores and influx of extracellular Ca(2+) through Ca(2+) entry pathways in the plasma membrane which required the continued presence of agonist on the receptor.