Histamine-stimulated increases in intracellular calcium in the smooth muscle cell line, DDT1MF-2

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.

Pharmacology of [3H]-pyrilamine binding and of the histamine-induced inositol phosphates generation, intracellular Ca2+ -mobilization and cytokine release from human corneal epithelial cells

We recently reported on the successful generation of immortalized (CEPI-17-CL4) cells from primary human corneal epithelial (P-CEPI) cells which exhibited phenotypic, immunohistochemical and metabolic characteristics akin to the P-CEPI cells. The aims of the present studies were to investigate the ligand binding and functional coupling of the histamine receptors to various biochemical and physiological systems in the P-CEPI and CEPI-17-CL4 cells and to relate these findings to the normal and/or pathophysiological role of histamine on the human ocular surface. Specific [3H]-pyrilamine binding to CEPI-17-CL4 cell homogenates comprised >93% of the total binding and represented interaction with an apparent single population of high affinity (Kd=3.76+/-0.78 nM; n=4) and saturable (Bmax = 1582+/-161 fmol g(-1) tissue) number of histamine-1 (H1) receptor binding sites on CEPI-17-CL4 cell homogenates. The H1-receptor selective antagonists, pyrilamine (Ki=3.6+/-0.84 nM, n=4) and triprolidine (Ki = 7.7+/-2.6 nM, n=3), potently displaced [3H]-pyrilamine binding, while the H2- and H3-receptor selective antagonists, ranitidine and clobenpropit, were weak inhibitors (K(i)s>13 microM). Histamine induced phosphoinositide (PI) hydrolysis 2.7-4.4 fold above basal levels and with a potency of 14.9+/-4.9 microM (n=9) and 4.7+/-0.2 microM (n=9) in P-CEPI and CEPI-17-CL4 cells, respectively. Histamine-induced PI turnover was antagonized by H1-receptor selective antagonist, triprolidine, with a potency (Ki) of 3.2+/-0.66 nM (n=10) and 3.03+/-0.8 nM (n=4) in P-CEPI and CEPI-17-CL4 cells, respectively, but weakly effected by 10 microM cimetidine and clobenpropit, H2- and H3-receptor antagonists. The PI turnover response was attenuated by pre-treatment of the cells with the selective phospholipase C inhibitor, U73122 (1-(6-((17beta-3-methoxyestra- 1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione) (IC50=4.8+/-2.4 microM, n = 3). Histamine stimulated intracellular Ca2+ ([Ca2+]i) mobilization in CEPI-17-CL4 cells with a potency of 6.3+/-1.5 microM (n=4). The histamine-induced [Ca2+]i mobilization was reduced by about 28% following pre-incubation of the cells with 4 mM EGTA. While triprolidine completely inhibited histamine-induced [Ca2+]i mobilization, it did not influence the bradykinin-induced [Ca2+]i mobilization response. Histamine (EC50s = 1.28-2.77 microM, n=3-4) concentration-dependently stimulated the release of interleukin-6 (IL-6), IL-8 and granulocyte macrophage colony-stimulating factor, but it did not significantly alter release of tumour necrosis factor-alpha, PGE2 or collagenase-1 (matrix metalloproteinase-1; MMP-1) from CEPI cells. However, IL-1 (10 ng ml(-1)), foetal bovine serum (10%) and phorbol-12-myristate-13-acetate (3 microg ml(-1)) were effective positive control secretagogues of all the cytokines, PGE2 and MMP-1, respectively, from these cells. It is concluded that the CEPI cells express H1-histamine receptors which are positively coupled to PI turnover and [Ca2+]i mobilization which may be directly or indirectly responsible for the release of various cytokines from these cells at physiologically and/or pathologically relevant concentrations.

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.

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.

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.

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.

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

Undifferentiated monolayers of the hamster vas deferens smooth-muscle cell line, DDT1MF-2, were grown on glass coverslips and loaded with the Ca(2+)-sensitive fluorescent dye fura-2. Stimulation with histamine produced a rapid and maintained increase in intracellular free Ca2+ ([Ca2+]i), with an EC50 of 7.0 +/- 0.7 microM. The initial rise in [Ca2+]i can be attributed to Ca2+ release from intracellular stores, whereas the maintained or plateau phase is due to influx of extracellular Ca2+. The Ca2+ influx associated with the plateau phase required the continued presence of histamine on the receptor, since the H1-antagonist mepyramine (10 microM) attenuated the rise in [Ca2+]i observed when extracellular Ca2+ was re-applied after the cells had been stimulated with histamine, in experiments performed in nominally Ca(2+)-free buffer. Pretreatment with the inorganic Ca(2+)-channel blockers Ni2+ (1 mM) and Co2+ (1 mM) inhibited the influx component, whereas the organic voltage-operated Ca(2+)-channel antagonists nifedipine (10 microM) and PN-200-110 (10 microM) had no effect. These data suggest that histamine stimulates Ca2+ influx through an H1-receptor-activated Ca2+ channel. Experiments with Mn2+ indicated that the receptor-mediated Ca(2+)-influx pathway(s) is impermeable to Mn2+. Furthermore, the refilling of Ca2+ stores can occur independently of H1-receptor-mediated influx, since store refilling can be demonstrated even when the receptor-mediated Ca2+ entry is blocked by mepyramine. In conclusion, H1-receptor activation in the smooth-muscle cell line DDT1MF-2 stimulates both release of Ca2+ from intracellular stores [inositol 1,4,5-triphosphate (InsP3)-mediated] and Ca2+ influx through a receptor-activated Ca2+ channel. The subsequent refilling of the InsP3-sensitive intracellular Ca2+ store is independent of histamine H1-receptor stimulation (mepyramine-insensitive) and occurs without an observable rise in cytosolic free Ca2+.