Thrombin attenuates the stimulatory effect of histamine on Ca2+ entry in confluent human umbilical vein endothelial cell cultures

Thrombospondin-1 supports blood pressure by limiting eNOS activation and endothelial-dependent vasorelaxation

AIMS

Thrombospondin-1 (TSP1), via its necessary receptor CD47, inhibits nitric oxide (NO)-stimulated soluble guanylate cyclase activation in vascular smooth muscle cells, and TSP1-null mice have increased shear-dependent blood flow compared with wild-type mice. Yet, the endothelial basement membrane should in theory function as a barrier to diffusion of soluble TSP1 into the arterial smooth muscle cell layer. These findings suggested that endothelial-dependent differences in blood flow in TSP1-null mice may be the result of direct modulation of endothelial NO synthase (eNOS) activation by circulating TSP1. Here we tested the hypothesis that TSP1 inhibits eNOS activation and endothelial-dependent arterial relaxation.

METHODS AND RESULTS

Acetylcholine (ACh)-stimulated activation of eNOS and agonist-driven calcium transients in endothelial cells were inhibited by TSP1. TSP1 also inhibited eNOS phosphorylation at serine(1177). TSP1 treatment of the endothelium of wild-type and TSP1-null but not CD47-null arteries inhibited ACh-stimulated relaxation. TSP1-null vessels demonstrated greater endothelial-dependent vasorelaxation compared with the wild type. Conversely, TSP1-null arteries demonstrated less vasoconstriction to phenylephrine compared with the wild type, which was corrected upon inhibition of eNOS. In TSP1-null mice, intravenous TSP1 blocked ACh-stimulated decreases in blood pressure, and both intravenous TSP1 and a CD47 agonist antibody acutely elevated blood pressure in mice.

CONCLUSION

TSP1, via CD47, inhibits eNOS activation and endothelial-dependent arterial relaxation and limits ACh-driven decreases in blood pressure. Conversely, intravenous TSP1 and a CD47 antibody increase blood pressure. These findings suggest that circulating TSP1, by limiting endogenous NO production, functions as a pressor agent supporting blood pressure.

Effects of hypercapnia and hypocapnia on [Ca2+]i mobilization in human pulmonary artery endothelial cells

The hydrogen ion is an important factor in the alteration of vascular tone in pulmonary circulation. Endothelial cells modulate vascular tone by producing vasoactive substances such as prostacyclin (PGI2) through a process depending on intracellular Ca2+ concentration ([Ca2+]i). We studied the influence of CO2-related pH changes on [Ca2+]i and PGI2 production in human pulmonary artery endothelial cells (HPAECs). Hypercapnic acidosis appreciably increased [Ca2+]i from 112 +/- 24 to 157 +/- 38 nmol/l. Intracellular acidification at a normal extracellular pH increased [Ca2+]i comparable to that observed during hypercapnic acidosis. The hypercapnia-induced increase in [Ca2+]i was unchanged by the removal of Ca2+ from the extracellular medium or by the depletion of thapsigargin-sensitive intracellular Ca2+ stores. Hypercapnic acidosis may thus release Ca2+ from pH-sensitive but thapsigargin-insensitive intracellular Ca2+ stores. Hypocapnic alkalosis caused a fivefold increase in [Ca2+]i compared with hypercapnic acidosis. Intracellular alkalinization at a normal extracellular pH did not affect [Ca2+]i. The hypocapnia-evoked increase in [Ca2+]i was decreased from 242 +/- 56 to 50 +/- 32 nmol/l by the removal of extracellular Ca2+. The main mechanism affecting the hypocapnia-dependent [Ca2+]i increase was thought to be the augmented influx of extracellular Ca2+ mediated by extracellular alkalosis. Hypercapnic acidosis caused little change in PGI2 production, but hypocapnic alkalosis increased it markedly. In conclusion, both hypercapnic acidosis and hypocapnic alkalosis increase [Ca2+]i in HPAECs, but the mechanisms and pathophysiological significance of these increases may differ qualitatively.

Dual effects of histamine and substance P on intracellular calcium levels in human U373 MG astrocytoma cells: role of protein kinase C

1. In human U373 MG astrocytoma cells agonist-induced increases in intracellular Ca2+ ([Ca2+]i) are rapidly returned towards prestimulated levels. Examination of the effect of histamine and substance P on [Ca2+]i in thapsigargin-treated cells has allowed a mechanism contributing to this effect to be characterized. 2. Histamine and substance P stimulated [3H]-inositol monophosphate ([3H]-IP1) accumulation in U373 MG cells. Concentration-response curves of [3H]-IP1 accumulation in suspensions of U373 MG cells in HEPES buffer containing 30 mM Li+ yielded best-fit EC50 values of 19.1+/-1.5 microM for histamine and 5.7+/-1.3 nM for substance P. 3. In confluent monolayers of fura-2 loaded U373 MG cells perfusion with 100 microM histamine resulted in a transient 597+/-50 nM increase in [Ca2+]i. The best-fit EC50 for histamine was 4.6+/-2.2 microM. The initial, transient, histamine response was often followed by further small transient increases in [Ca2+]i. 4. Treatment of U373 MG cells with 5 microM thapsigargin, followed by the readdition of 1.8 mM Ca2+ to the perfusion buffer, resulted in a steady-state level of [Ca2+]i 97+/-5 nM above pretreated levels (measured 400 s after readdition of Ca2+). Perfusion of histamine (100 microM, 100 s) caused a rapid decline in the thapsigargin-induced steady state level of [Ca2+]i. This effect of histamine was normally reversible upon washout. The best-fit EC50, for the histamine response was 0.8+/-0.2 microM. Substance P (10 nM, 100s) also caused a reduction in thapsigargin-induced steady-state levels of [Ca2+]i. 5. Neither 100 microM histamine nor 10 nM substance P inhibited the rate of quench of fura-2 fluorescence by Mn2+ in U373 MG cells pretreated with 5 microM thapsigargin, indicating that the depressant effect on steady-state raised [Ca2+]i was probably not due to a block of Ca2+ entry. 6. The depressant effect of histamine on [Ca2+]i was blocked by 1 microM mepyramine, and was partially reduced by pre-incubation with 1 microM staurosporine (61+/-7% reduction) and with Ro 31-8220 (24+/-10% and 50+/-6% reduction by 1 and 10 microM Ro 31-8220, respectively). Pre-incubation with H-89 did not alter the depressant effect of histamine. 7. Neither 1 microM staurosporine nor 10 microM KN-62 inhibited the binding of [3H]-mepyramine to guinea-pig cerebellar membranes, whereas it was reduced by 17+/-1% and 55+/-2% by 1 and 10 microM Ro 31-8220, respectively. However, [3H]-IP1 accumulation stimulated by histamine in U373 MG cells was not inhibited by 1 or 10 microM Ro 31-8220 and in 2 out of 3 experiments there was a significant potentiation of the response to histamine with both concentrations of Ro 31-8220. Staurosporine, 1 microM, similarly potentiated the response to 100 microM histamine in 3 out of 4 experiments. KN-62 (10 microM) did not stimulate histamine-induced [3H]-IP1 accumulation. 8. In HEPES buffer to which no Ca2+ had been added, histamine stimulated a transient 451+/-107 nM increase in [Ca2+]i. Pretreatment with 1 microM and 10 microM Ro 31-8220 did not significantly alter the initial peak response to histamine, but slowed the rate at which histamine-induced increases in [Ca2+]i were returned to prestimulated levels. Pretreatment with KN-62 had no significant effect on the response to histamine, but consistently inhibited the secondary slower phase of the decline in [Ca2+]i. H-89 did not alter the histamine response. 9. The effect of histamine in stimulating Ca2+ extrusion was not confined to U373 MG cells, since 100 microM histamine also caused a rapid decrease in steady-state levels of [Ca2+]i in thapsigargin-treated human HeLa cells. 10. The results indicate that agonists which increase [Ca2+]i via activation of phosphoinositide metabolism can also stimulate a homeostatic mechanism which acts to reduce [Ca2+]i. The balance of the evidence indicates that in U373 MG cells the latter effect most likely involves a PKC-mediated stimulation of a Ca2+-extrusion pump.

Effects of hypoxanthine-xanthine oxidase on Ca2+ stores and protein synthesis in human endothelial cells

We have investigated the effects of reactive O2 metabolites generated by the hypoxanthine-xanthine oxidase (HX-XO) system on intracellular Ca2+ and its relation with protein synthesis in human umbilical vein endothelial cells (HUVECs). Spectrofluorometry with fura 2 showed that the oxidative stress induced a rapid transient rise in cytosolic [Ca2+], followed by a sustained elevation above the baseline value. In the presence of La3+, which blocks Ca2+ influx from the extracellular medium, a transient [Ca2+] increase was still observed, but the sustained rise was suppressed. The HX-XO-related [Ca2+] changes were completely prevented by pretreatment with thapsigargin, which depletes intracellular Ca2+ stores. Hence, the effects of HX-XO on Ca2+ homeostasis were due to mobilization of Ca2+ from the intracellular stores with subsequent influx of extracellular Ca2+. HX-XO mobilized more of sequestered Ca2+ than did thrombin, a receptor agonist that depletes only a part of the intracellular Ca2+ stores (the hormone-sensitive stores). To determine the relevance of the HX-XO-related depletion of Ca2+ stores for cell function, we investigated the role of Ca2+ mobilization in the regulation of protein synthesis. Overall protein synthesis in HUVECs was markedly reduced by thapsigargin, which depletes both hormone-sensitive and -insensitive stores, but was not substantially affected by thrombin. Manipulation of the refilling of the Ca2+ stores via the availability of extracellular Ca2+ significantly influenced the thapsigargin-related and the HX-XO-related inhibition of overall protein synthesis. A corresponding effect of extracellular [Ca2+] was seen in polyribosome distribution profiles, which reflected an inhibition of translation initiation in both treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Histamine-modulated transdifferentiation of dermal microvascular endothelial cells

Homeostatic and inflammatory functions of skin microvessels are tightly regulated by vasoactive amines. Following stimulation with histamine, dermal microvascular endothelial cells (MEC) undergo a rapid change in phenotype (transdifferentiation) and subsequently exhibit an enhanced rate of growth. To elucidate mechanisms regulating MEC transdifferentiation, this study investigated the functional relationships among vimentin, Ca2+, and protein kinase C (PKC) in histamine-modulated dermal MEC in vitro. Distribution of vimentin and PKC in foreskin-derived MEC cultivated in a modified Iscove's medium was assessed with immunocytochemistry. Calcium ion kinetics in histamine-treated MEC were analyzed using the Ca2+ probe Fluo-3 in conjunction with interactive laser cytometry. Histamine, acting through H-1 receptors, produces a rapid (less than 100 ms) and differential elevation of free calcium in each of three cytological compartments defined by the vimentin cytoskeleton in epithelial MEC. A distinctive compartmentalized and nonuniform distribution of PKC precisely coincides with that observed for free-Ca2+ released in response to histamine. The studies reveal that histamine modulation of the MEC phenotype is associated with a rapid patterned reorganization of the vimentin skeleton. It is hypothesized that histamine induces vimentin post-translational modifications by activating a spatially localized interaction among cytoplasmic free Ca2+, PKC, and the vimentin matrix. The results further suggest that vimentin, in addition to its structural role, may participate in signal transduction and gene regulation processes in effecting MEC transdifferentiation.