Molecular and cellular analysis of histamine H1 receptors on cultured smooth muscle cells

Kinetics of nucleocytoplasmic Ca2+ transients in DDT1 MF-2 smooth muscle cells

The free calcium concentrations in the nucleus ([Ca2+]n) and in the cytoplasm ([Ca2+]c) of cultured DDT1 MF-2 smooth muscle cells were estimated using the fluorescent dye indo-1. With the use of confocal microscopy, line scans were made during the onset and the differential rise of the Ca2+ signal elicited by the agonists histamine and ATP. The results confirm our earlier findings that in these cells [Ca2+]n at rest was lower than [Ca2+]c. The present experiments show that this gradient over the nuclear envelope was also preserved in Ca(2+)-free solution containing 2 mM EGTA, underlining the selective barrier function of the nuclear envelope. During stimulation with histamine, an early Ca2+ rise in the vicinity of the nuclear envelope was found in contrast to the delayed Ca2+ rise 2 microns away on both sides of the envelope. This suggests the release of Ca2+ stored in the envelope and the perinuclear sarcoplasmic reticulum. The time course for reaching a uniform Ca2+ concentration ([Ca2+]u = [Ca2+]n = [Ca2+]c) in the nuclear and cytosolic compartment varied with the agonist used for stimulation and was dependent on the external Ca2+ concentration. The value of this uniform Ca2+ concentration itself was, however, independent of the type of stimulation. After reaching [Ca2+]u, a further rise occurred with [Ca2+]n becoming larger than [Ca2+]c. It is postulated that a critical Ca2+ concentration must be reached to induce this differential Ca2+ rise by releasing Ca2+ from an intranuclear Ca2+ store.

Functional diversity of histamine and histamine receptors

In order to analyze the mechanisms by which a single biogenic amine like histamine is capable of inducing a wide variety of both physiologic and pathologic functions in various tissues/cells, histamine responses were dissected in detail from a biochemical and pharmacologic point of view. Histamine is synthesized by multiple isozymes of histidine decarboxylase, and catabolized by either diamine oxidase or histamine-N-methyltransferase. Synthesized intracellular histamine may play a role in cell proliferation, whereas released histamine binds to at least three different histamine-specific receptors, then activates various intracellular components, such as Ca++, cAMP, protein kinase, and ion channels. These second messenger pathways interact differentially with each other in various tissues/cells. Moreover, histamine not only activates its own receptors, but also activates other related receptors such as the serotonin 1c receptor. Therefore, to understand the complex actions of histamine, new approaches should be established, in which multiple phenomena can be monitored simultaneously.

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

Suspensions of undifferentiated cultured vas deferens smooth muscle cells (DDT1MF-2) were loaded with the calcium-sensitive fluorescent dye fura-2. Exposure to histamine elicited a rapid and maintained increase in intracellular free calcium ([Ca2+] i) with an EC50 of 1.3 +/- 0.7 x 10(-5) M. The initial rise is a consequence of calcium release from intracellular stores, whereas the maintained or plateau phase, which is dependent upon the presence of extracellular calcium, is associated with calcium influx. Experiments in nominally Ca(2+)-free buffer attenuated the initial rise in [Ca2+]i (i.e. peak height) and virtually abolished the plateau phase. Re-addition of 2 mM Ca2+ (during experiments performed in nominally Ca(2+)-free buffer) resulted in a return of the plateau phase. Pretreatment with the H1-antagonist mepyramine (100 nM; Kd = 1.0 +/- 0.4 nM, N = 3) completely blocks the response to histamine, whereas tiotidine (2 microM; H2-antagonist) had no effect. In conclusion, the present data would suggest that functional H1-receptors found in hamster vas deferens smooth muscle cells are typical of the "classical" H1-receptor in both its control of intracellular Ca2+ and sensitivity to antagonism by mepyramine.

Histamine H1-receptor-like binding sites in the locust nervous tissue

The histamine H1-receptor-like binding sites in the nervous tissue of the locust Locusta migratoria were investigated with a conventional radio-receptor assay using [3H]mianserin as the radio ligand. Binding of [3H]mianserin to the binding site is sensitive to proteases and heat treatment. It shows the characteristics of ligand-receptor interactions. The single binding site has high affinity for mianserin (KD = 7.05 nM) and is present in high concentrations (Bmax = 1.53 pmol/mg) in the whole nervous tissue. All tested antihistamines have a high affinity for the binding site, which suggests that it represents an insect histamine receptor. Nevertheless, it shows its peculiarities distinguishing it from vertebrate histamine H1-receptors.

Neuro-endocrine interaction on lymphocytes. Testosterone-induced modulation of the lymphocyte substance P receptor

Human IM-9 B-lymphoblasts have been shown to express the receptor for the neuropeptide substance P (SP). The present study was undertaken to evaluate the effect of sex hormones on this receptor. Testosterone inhibited [125I]SP binding in a dose-dependent manner with an IC50 of approximately 100 nM, while both estradiol and progesterone failed to inhibit SP binding even at concentrations as high as 1 microM. Furthermore, Scatchard analysis indicated that the dissociation constant (Kd) of the SP receptor was markedly increased from 0.25 nM to 2.2 nM when cells were incubated with testosterone. These data indicate a unique neuro-endocrine interaction on lymphocytes involving the substance P receptor.