Beta-adrenergic stimulation inhibits calcium efflux and alters the morphology of cultured arterial muscle cells

cAMP signaling inhibits dihydropyridine-sensitive Ca2+ influx in vascular smooth muscle cells

This study examines the role of the cAMP signaling pathway in the regulation of 45Ca influx in cultured vascular smooth muscle cells from the rat aorta. K+o-induced depolarization of smooth muscle cells increased the rate of 45Ca uptake by twofold to threefold. This effect was completely abolished by the dihydropyridine derivatives nifedipine and nicardipine, with a Ki of 3 and 10 nmol/L, respectively. Activators of cAMP signaling (isoproterenol, forskolin, cholera toxin) increased cAMP content by 50- to 100-fold and decreased voltage-dependent 45Ca uptake by 50% to 70%. Neither the dihydropyridines nor the cAMP activators affected basal 45Ca influx. Direct addition of the protein kinase inhibitor H-89 to the incubation medium in the 1- to 10-micromol/L range did not alter basal 45Ca uptake but completely abolished voltage-dependent Ca2+ transport. Preincubation of cells for 1 hour with 10 micromol/L H-89 did not modify basal and depolarization-induced 45Ca uptake in H-89-free medium but prevented forskolin-induced inhibition of voltage-dependent Ca2+ influx. The addition of cytoskeleton-active compounds reduced voltage-dependent Ca2+ transport and completely abolished its regulation by cAMP. Activation of cAMP signaling decreased the volume of smooth muscle cells by 12% to 15%. The same cell volume diminution in hyperosmotic medium did not alter voltage-dependent 45Ca uptake. Thus, data obtained in this study show that in contrast to cardiac and skeletal myocytes, in vascular smooth muscle cells, 45Ca influx, putatively due to L-type channels, is inhibited by cAMP. This regulatory pathway appears to be mediated via protein kinase A activation and cytoskeleton reorganization.

Activation of adenylyl cyclase downregulates sodium/calcium exchanger of arterial myocytes

Chronic elevation of adenosine 3',5'-cyclic monophosphate (cAMP) is known to inhibit the proliferation of cultured vascular smooth muscle cells. The present findings show that the activation of adenylyl cyclase with forskolin decreased Na+/Ca2+ exchanger (NCX) mRNA and activity. Fetal bovine serum restored NCX transcript and activity. The changes in NCX transcript preceded the changes in NCX activity. Incubation of low-passage immortalized myocytes with forskolin plus 3-isobutyl-1-methylxanthine (IBMX), which inhibits cAMP phosphodiesterase, decreased NCX mRNA by 60% in 6 h and 80% in 24 h. After a 6-h lag, forskolin plus IBMX decreased NCX activity almost linearly to 20% of control at 40 h. 1,9-Dideoxyforskolin, which does not activate adenylyl cyclase, had no effect on NCX mRNA or activity. Forskolin plus IBMX decreased the c-Myc transcript, an immediate-early gene whose expression correlates with cell proliferation, but had no effect on plasma membrane Ca(2+)-ATPase transcripts. Removal of forskolin plus IBMX and addition of fetal bovine serum increased NCX and c-Myc transcripts seven- to eightfold in 6 h and restored NCX activity in 24 h. Inhibition of protein or RNA synthesis by cycloheximide or actinomycin D, respectively, prevented the increase in NCX mRNA. In contrast to blocking NCX induction, cycloheximide potentiated c-Myc induction by serum. Transcription factors that regulate myocyte growth may mediate the opposing influences of serum and forskolin on NCX mRNA and activity.

cAMP and bFGF negatively regulate tropomyosin expression in rat cultured astroblasts

We have examined the expression of tropomyosin (TM) messenger RNAs (mRNAs) and protein isoforms in primary cultures of rat astroblasts during morphological changes. Three messenger RNA bands of 2.5, 1.8 and 1.2 kilobase pairs (kb) were detected by Northern blot. Using an antibody cross-reacting with all tropomyosin isoforms, we found that rat cerebellar neonatal astroblasts expressed three tropomyosin protein isoforms termed TM-As1, TM-As2 and TM-As3 (As for Astroblast) with respective molecular masses of 38,000, 33,000 and 31,000. Treatment of cells with agents which promote or mimick the action of cyclic AMP, or with growth factors, is known to induce astroblast morphological alteration from flat, polygonal epitheloid cells into star-shaped, process-bearing cells. In the presence of dibutyryl cAMP (dBcAMP), forskolin or basic fibroblast growth factor (bFGF), these morphological changes were found to be associated with dramatic decreases of the three mRNA transcripts and also of the three protein isoforms. This decrease was reversed upon removal of the drugs. The pattern of the tropomyosin protein isoforms in cultured astroblasts showed that TM-Asl, the most immunoreactive isoform recovered in the cytoskeletal insoluble cell fraction, had a developmental profile similar to that of F-actin. Therefore this isoform, which belongs to the high-molecular-mass family of proteins known to interact strongly with F-actin, could specifically be involved in the regulation/control of F-actin stability and thus be associated with the plasticity of astroblasts.

Growth properties and receptor expression in vascular smooth muscle cells from hypertensive rats

The objective of this study was to evaluate the growth properties and receptor expression in aorta-ring derived smooth muscle cells (SMCs) cultured from control (WKY) and spontaneously hypertensive rats (SHR). SHR-SMCs exhibited a 3-4 day lag period before migrating. In addition, SHR-SMCs had a significantly higher growth rate, shorter population doubling time and higher saturation density level characteristics that were retained at higher passage levels. beta-adrenergic and angiotensin (All) receptors were measured using iodocyanopindolol (ICYP) and [3H]-All, respectively. All receptor expression was similar in both WKY and SHR-SMC cultures. WKY-SMCs exhibited little ICYP binding (Bmax 8.27 +/- 2.0 fmol/mg) while SHR-SMC binding capacity was 8 fold higher (Bmax 65 +/- 9.2 fmol/mg). In addition, the responsiveness of the beta-receptor, as assessed by adenylyl cyclase stimulation, was similar for WKY and SHR-SMCs. These data suggest that factors regulating SMC receptor expression in vitro are selective since All and adrenergic receptor densities exhibit different responses to hypertension.

Serotonin produces a configurational change of cultured smooth muscle cells that is associated with elevation of intracellular cAMP

Early passaged bovine pulmonary artery smooth muscle cells (SMC) respond to serotonin (5-HT) by developing a reversible change in configuration. (Lee et al. J. Cell. Physiol. 138:145, 1989). This configurational change does not occur in pulmonary artery endothelial cells (EC) subjected to 5-HT and is adenosine triphosphate (ATP) dependent, lost with passage of SMC, and inhibited by various agents that block high-affinity 5-HT uptake. We now report a second configurational change (also dendritic formation) of SMC produced by 5-HT only in the presence of isobutylmethylxanthine (IBMX), an inhibitor of phosphodiesterase. This configurational change was also ATP dependent, but unlike the first response, (Lee et al., 1989), it occurred in both first and later passaged SMC and was not inhibited by blockade of 5-HT uptake. Also, unlike the response with 5-HT alone that failed to elevate cAMP, this one was associated with a large elevation of cAMP (eight fold above control values), similar to the response to the beta-agonist isoproterenol, plus IBMX. The second response was not blocked by a variety of 5-HT receptor antagonists but was reproduced by (+/-)-8-hydroxy-DPAT HBr (8-OH-DPAT), a reputed 5-HT1A agonist. The response was not dependent upon Ca2+ and was blocked by 1-2 mM n-phenylanthranilic acid or anthracene-9-carboxylic acid, electrically conductive Cl- channel inhibitors. Hence, 5-HT in the presence of IBMX causes a marked elevation of cAMP of SMC and this elevation in cAMP likely results in a cellular configurational change through a Cl- channel-dependent mechanism similar to that we previously described for EC in the presence of beta-adrenergic agonist stimulation (Ueda et al. Circ. Res. 66:951, 1990). EC do not show a similar response to 5-HT possibly because cAMP is not adequately elevated, even in the presence of IBMX, to enhance Cl- channel activity. We propose that our observations indicate the presence of two sites of action of 5-HT on the smooth muscle cell, one intracellularly and another at a cell surface receptor.

Stellate cells of aortic intima: II. Arborization of intimal cells in culture

The present study analyzed effects of different cAMP-elevators on cell morphology in primary culture of human intimal and medial cells from grossly normal and atherosclerotic areas. In primary culture of human aortic cells adenylate cyclase activator forskolin and other cAMP elevators induced arborization of cells, i.e. they reversibly changed the shape of cells. This resulted in the formation of thin branching processes and in the concentration of cytoplasm around the nucleus. In the culture, the shape of the arborized cells resembled that of stellate ones detected in the aortic intima in situ. The arborization of cells was accompanied by destruction of myofilaments. Due to cAMP elevators' effect, most of the arborized cells were exhibited in the cultures isolated from the elastic-hyperplastic layer of the intima. The number of arborized cells was significantly less in the cultures isolated from the musculo-elastic layer and still lesser in those isolated from media. We failed to reveal any significant difference in the number of arborized cells cultured from fatty streaks, atherosclerotic plaques and grossly normal aortic areas. Obtained results suggest that the previously revealed polymorphism of human aortic intimal cells may be accounted for by the cell shape transformations underlined by the mechanism similar to that of arborization in vitro.

Sodium-calcium exchange in membrane vesicles from aortic myocytes: stimulation by endogenous proteolysis masks inactivation during vesicle preparation

Plasma membrane vesicles were purified from rat aortic myocytes by centrifugation in a discontinuous sucrose gradient. Vesicles were prepared in the presence or absence of five proteinase inhibitors (aprotinin, benzamidine, leupeptin, pepstatin A and phenylmethylsulfonyl fluoride). The proteinase inhibitors decreased the Vmax by 3.4-fold and had no effect on the Km for Ca2+ of Na+ gradient-dependent 45Ca2+ influx. The proteinase inhibitors had no direct effect on exchange activity, and they had no effect on membrane purity as indicated by 5'-nucleotidase activity. Removing the proteinase inhibitors or adding trypsin or chymotrypsin increased exchange activity approx. 2-fold. The Vmax of exchange activity in intact aortic myocytes is approx. 10-fold higher than the Vmax in plasma membrane vesicles prepared in the presence of proteinase inhibitors. Exchange activity in plasma membrane vesicles is only a sixtieth of the expected value, because the vesicles have approx. 7-fold higher 5'-nucleotidase activity and approx. 6-fold higher specific exchange activity than the crude homogenate. The large loss of exchange activity may be caused by a change in a regulatory domain of the exchanger because endogenous proteolysis restores some of the activity lost during vesicle preparation.

Dual effect of serotonin on growth of bovine pulmonary artery smooth muscle cells in culture

We have previously reported that serotonin (5-hydroxytryptamine [5HT]) alters cultured bovine pulmonary artery smooth muscle cell (SMC) configuration through two different regulatory mechanisms. We now report that 5HT also regulates SMC growth through these same two mechanisms--a stimulatory event initiated intracellularly and inhibition of growth resulting from a cell surface action. 5HT (1 microM) plus 0.1 mM iproniazid (a 5HT metabolic inhibitor) produced a severalfold stimulation of DNA synthesis (as measured by [3H]thymidine incorporation) of SMCs after a 17-24-hour incubation with only a slight elevation of cellular cAMP. This stimulatory effect responded synergistically with other growth factors including platelet-derived growth factor, fibroblast growth factor, and epidermal growth factor and was effectively reversed by 5HT uptake inhibition. It was not produced by 5-hydroxyindoleacetic acid, a metabolite of 5HT. In the presence of 1 microM 5HT plus 0.1 mM isobutylmethylxanthine (IBMX), cAMP was elevated eightfold, dendritic formation occurred, and [3H]thymidine labeling of SMCs was inhibited. Inhibition of labeling by [3H]thymidine was mimicked by other agents that elevated cellular cAMP (10 microM histamine, 1 microM isoproterenol plus 0.1 mM IBMX, and 10 microM forskolin) and by 1 mM dibutyryl cAMP. This inhibitory effect was not blocked by either inhibition of 5HT uptake or 5HT-receptor antagonists ketanserin (5HT2); methiothepin, spiperone, and mianserin (5HT1/5HT2); and 3-tropanyl-indole-3-carboxylate and 3-tropanyl-3,5-dichlorobenzoate (5HT3). However, similar to 5HT, the 5HT1A agonist, (+/-)-8-hydroxy-(+/-)-2-dipropylamino-8-hydroxy-1,2,3, 4-tetrahydronaphthalenehydrobromide, in association with IBMX, produced an elevation in cAMP and inhibition of labeling by [3H]thymidine. 5HT, in the presence of either iproniazid or IBMX, did not alter [Ca2+]i, indicating that [Ca2+]i was not a signal for either of these actions.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular localization of group II phospholipase A2 in rat vascular smooth muscle cells and its possible relationship to eicosanoid formation

We investigated the localization of group II phospholipase A2 (PLA2-II) in rat vascular smooth muscle cells (VSMCs) by applying immunofluorescence and immunoelectron microscopy with its polyclonal antibody. In unstimulated cells, no immunolabelling was detected in the cells. On the other hand, in the cells stimulated with tumor necrosis factor (TNF) and/or forskolin (FK), intense fluorescence was detected in the cytoplasm. The immunoperoxidase reactions were detected in the cisternae of rough endoplasmic reticulum (rER), trans-cisternae of Golgi apparatus, and small vesicles beneath the plasma membrane. Western blot analysis showed VSMCs secrete PLA2-II after stimulation. Secreted PLA2-II was associated with the plasma membrane and extracellular matrix. Colchicine inhibited PLA2-II synthesis and its secretion to the extracellular space. These observations indicate that in VSMCs PLA2-II is synthesized at rER. transported to Golgi apparatus, discharged into extracellular space via the small vesicles, and microtubules may concern with its process. Furthermore, in VSMCs treated with TNF or TNF + FK, prostaglandin E2 formation was also increased. Actinomycin D and cycloheximide inhibited the potentiation of the prostaglandin E2 formation induced by TNF or TNF + FK, indicating that both RNA and protein synthesis are required for the potentiation. These results suggest an involvement of PLA2-II in the prostaglandin formation.

Inhibition of sodium-calcium and sodium-proton exchangers by amiloride congeners in arterial muscle cells

The inhibitory potencies of several amiloride congeners towards Na(+)-Ca2+ and Na(+)-H+ exchange were compared in rat aortic myocytes. N-(2,4-Dimethylbenzyl)amiloride (DMB) was 10 times more potent towards Na(+)-Ca2+ than Na(+)-H+ exchange. Amiloride and ethylisopropylamiloride were about 5,000 and 10,000 times more potent toward Na(+)-H+ than Na(+)-Ca2+ exchange respectively. N-(3,4-Dichlorobenzyl)amiloride was almost equipotent towards both exchangers. About 40 nM ethylisopropylamiloride inhibited Na(+)-H+ exchange by 50%. Ethylisopropylamiloride (10 microM) had no effect on basal or angiotensin-evoked 45Ca2+ efflux or net Ca2+ efflux. In contrast to ethylisopropylamiloride, 25-50 microM DMB, which strongly inhibits Na(+)-Ca2+ exchange, markedly decreased both 45Ca2+ efflux and net Ca2+ efflux produced by angiotensin. Replacing extracellular Na+ with N-methyl-D-glucamine inhibited angiotensin-evoked 45Ca2+ efflux similarly to DMB. Neither DMB nor Na+ placement had any effect on basal or angiotensin-evoked production of [3H]inositol phosphates. These findings suggest that Na(+)-H+ exchange has no major influence on short-term Ca2+ regulation and provide evidence that Na(+)-Ca2+ exchange is a major pathway of rapid Ca2+ efflux in stimulated arterial muscle cells.

Energy dependence of sodium-calcium exchange in vascular smooth muscle cells

Three different types of mitochondrial poisons (oligomycin, antimycin A, and dinitrophenol) strongly inhibited Na(+)-Ca2+ exchange in aortic myocytes. Exchange activity was assayed as 45Ca2+ uptake that depended on inverting the Na+ gradient and was inhibited by 25 microM dimethylbenzamil. Glucose markedly decreased the inhibition of exchange activity by these three poisons. Glucose also prevented rotenone from inhibiting exchange and depleting cellular ATP. In the absence of glucose, rotenone decreased ATP and exchange activity with half-times of 0.8 and 0.9 min, respectively. Almost eliminating cellular ATP with rotenone maximally inhibited exchange by 80%. Repletion of ATP with glucose substantially restored Na(+)-Ca2+ exchange activity. Ca2+ uptake by organelles, subsequent to entry via exchange for Na+, does not appear to contribute significantly to exchange activity as assayed in intact myocytes. The specific activity of Na(+)-Ca2+ exchange was approximately 30 nmol.min-1.mg protein-1. These findings suggest that ATP modulates exchange activity and that there are approximately 150,000 Na(+)-Ca2+ exchangers per cell, assuming that the turnover number is 1,000 s-1.

Effect of centrifugal force and catecholamines on glycosaminoglycans synthesis of vascular smooth muscle cells in culture

To evaluate the effect of hypertension on glycosaminoglycan (GAG) synthesis, cultured vascular smooth muscle cells (CVSMCs) from the aorta of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were exposed to centrifugal forces and catecholamines. GAG synthesis of CVSMCs was measured by the incorporation of [3H]glucosamine into GAGs which were secreted into the culture medium for 24 h. Basal level of GAG synthesis was much higher in SHR than in WKY, when expressed in terms of DNA contents. When exposed to centrifugal force, CVSMCs from rats of both strains synthesized more GAGs. GAG synthesis was enhanced by both noradrenaline (NA) and adrenaline (Ad) in WKY. The enhanced GAG synthesis in WKY by NA or Ad was prevented by pretreatment with propranolol, but not prazosin. In SHR, NA and Ad did not enhance GAG synthesis at this concentration of catecholamines. However, the effects of propranolol or prazosin on GAG synthesis in SHR, when incubated with either NA or Ad, were compatible with the phenomena observed in WKY. Adding dibutyryl cyclic AMP to the culture medium enhanced GAG synthesis in rats of both strains. These data suggest that not only the mechanical stress of high intra-arterial pressure but also beta receptor stimulation, via increasing cyclic AMP, enhance GAG synthesis of vascular smooth muscle cells in hypertension.

Thrombin reverts the beta-adrenergic agonist-induced morphological response in rat glioma C6 cells

Treatment of rat glioma C6 cells with a beta-adrenergic agonist leads to a rise in cAMP level and a subsequent change in cell morphology from an epithelial to an astrocyte type of appearance. This morphological change is reverted by the addition of thrombin. In 10-15 min the cells acquire their normal epithelial morphology. The reversion by thrombin is inhibited by hirudin, but not by antithrombin III (an inhibitor of the proteolytic action of thrombin). Using the intracellular Ca2(+)-indicator fura-2, we observed that the addition of thrombin to the glioma cells generated a Ca2(+)-signal which was inhibited by pretreatment of the cells with hirudin or with 1 mM neomycin. These results suggest that thrombin uses the phospholipid-inositol pathway to counteract the morphological response, which was induced by activation of the cAMP pathway.

Taurine-induced neuronal differentiation: the influence of calcium and the ganglioside GM1

Taurine-induced differentiation was examined in the murine neuroblastoma Neuro-2a cell line in the presence or absence of the monosialoganglioside GM1 and under conditions in which Ca2+ levels were manipulated. Taurine (4 mM), GM1 (200 micrograms/ml), or taurine with GM1 were applied to culture media that contained either various concentrations of Ca2+ or the Ca2+ ionophore A23187. Taurine or GM1 and taurine with GM1 increased the number of cells emitting neurites above that found for controls. A significant interaction was found between treatment (taurine, GM1 or taurine + GM1) and the manipulations of Ca2+ levels, affecting the number of neurites and producing changes on the neuritic and perikaryal surfaces. Treatment with both taurine and taurine + GM1 and the various concentrations of Ca2+ resulted in a significant increase in neurite elongation. The Ca2+ ionophore A23187 in the presence of taurine or taurine + GM1 caused neurites to grow longer than observed in media containing Ca2+, either in a low concentration (about 125 microM) or at 1-2 mM. Taurine-treated cultures in the presence of extracellular Ca2+ or A23187 were characterized by surfaces with numerous microvillar, spine-like projections. This effect was enhanced with GM1 and was less pronounced in the medium containing low levels of Ca2+. Transmission electron microscopy of the taurine-stimulated neurons revealed an excessive number of clear-core vesicles (40-200 nm in diameter) in perikarya, neurites and neuritic varicosities and growth cones. In addition, numerous aggregates of intermediate filaments were seen. They were most abundant in the taurine + GM1 treated cultures. The taurine + A23187 cultures also exhibited numerous microtubules within the elongated processes. The different neuritic patterns induced by taurine under conditions in which Ca2+ levels were manipulated and/or when cells were exposed to exogenous GM1 suggest that taurine's actions depend in part on Ca2+ flux.

Ionomycin releases calcium from the sarcoplasmic reticulum and activates Na+/Ca2+ exchange in vascular smooth muscle cells

Ionomycin (1 microM) produced a large spike in cytosolic free Ca2+ [( Ca2+]i). The ionophore had no effect on [Ca2+]i if the sarcoplasmic reticulum had previously been Ca2+ depleted by stimulating neurohormone receptors. Ionomycin markedly increased 45Ca2+ efflux and decreased total cell Ca2+ by 60 to 70% in 1 min. Replacing extracellular Na+ [( Na+]o) with choline or N-methyl-D-glucamine strongly inhibited the effects of ionomycin on 45Ca2+ efflux and total Ca2+. Ionomycin caused similar peak increases in [Ca2+]i in the presence and absence of [Na+]o, but the exponential fall from the peak was faster in the presence of [Na+]o. Dimethylbenzamil, a potent blocker of Na+/Ca2+ exchange in these cells, strongly inhibited the effects of ionomycin on 45Ca2+ efflux and total cell Ca2+. We conclude that the increase in cytosolic free Ca2+ produced by ionomycin may be sufficient to activate the plasma membrane Na+/Ca2+ exchanger which removes Ca2+ from the cytosol and helps restore basal [Ca2+]i.

Cyclic AMP- and cytochalasin B-induced arborization in cultured aortic smooth muscle cells: its cytopharmacological characterization

The present study analyzed effects of dibutyryl cyclic AMP (DB-cAMP) and cytochalasin B (CB) on the morphology of cultured aortic smooth muscle cells (SMC) from rat using phase-contrast microscopy, scanning electron microscopy, and fluorescence staining of actin filaments by the NBD-phallacidin method. The exposure of SMC to each of these agents led to rapid, extensive, and reversible (within 1-2 h of drug withdrawal) changes in their morphology including cytoplasmic arborization (stellation). The latter was preceded by (i) marginal membrane ruffles (DB-cAMP) and (ii) increased zeiotic activity (CB), which were visible within 20 min of the exposure, followed (30-90 min incubation) by a centripetal retraction of the cytoplasm and progressive development of complete or partial arborization. Further, the effects of substances interfering with the assembly-disassembly of microtubules (colchicine, taxol, lidocaine) on DB-cAMP- and CB-induced arborization were studied. None of these agents antagonized CB-induced morphological changes. Colchicine, but not lumicolchicine, taxol, or lidocaine (in a short-term study) prevented DB-cAMP-induced arborization. Taxol added to cell cultures for 24 h promoted DB-cAMP-induced arborization. Both DB-cAMP and CB resulted in the disintegration of actin filaments. The present data suggest that the arborization of cultured aortic SMC is a cytoskeleton-based process involving stabilization of microtubules and disintegration of actin filaments. Our study also suggests that the SMC arborization may represent an in vitro case of SMC stellation found in situ.

Relationship between cytosolic free Ca2+ and Na+-Ca2+ exchange in aortic muscle cells

We examined the relationship between extracellular Na+ ([Na+]o) and cytosolic free Ca2+ ([Ca2+]i) in primary and passaged cultures of aortic muscle cells. Removing [Na+]o increased [Ca2+]i by approximately 10-fold in cells that were Na+ loaded. Decreasing [Na+]o from 140 to 32 mM caused the half-maximal increase in [Ca2+]i. [Ca2+]i exhibited a sigmoidal dependence on [Na+]o. Mg2+, a competitive inhibitor of Na2+-Ca2+ antiport in these cells, antagonized the increase in [Ca2+]i produced by lowering [Na+]o. High K+ decreased the potency of Mg2+ 6.5-fold as previously reported for Na+ gradient-dependent 45Ca2+ influx. In contrast to the Na+-loaded cells, removing [Na+]o caused no detectable change in [Ca2+]i in cells with normal Na+ even though the calculated electrochemical driving force for Na+-Ca2+ exchange was large enough to almost maximally increase [Ca2+]i in the Na+-loaded cells. We conclude that Na+-Ca2+ antiport activity is latent in the unstimulated cell at basal intracellular Na+ and Ca2+.

Phorbol 12-myristate 13-acetate prevents isoproterenol-induced morphological change in cultured vascular smooth muscle cells

The effect of phorbol 12-myristate 13-acetate (PMA) on isoproterenol (ISO)- and dibutyryl cAMP (dBcAMP)-induced morphological change and cytoskeletal reorganization was studied in cultured vascular smooth muscle cells (VSMC) using the fluorescence staining of actin and microtubules. The treatment of VSMC with 1.0 microM of ISO or with 1.0 mM of dBcAMP for 90 min induced the disruption of actin-containing stress fibers followed by cytoplasmic arborization. The addition of 100 or 10 nM of PMA prevented both the destruction of actin fibers and cell arborization induced either by ISO or by dBcAMP. However, PMA rather enhanced cAMP production stimulated by ISO. 1-Oleoyl-2-acetyl-sn-glycerol (100 micrograms/ml) mimicked this inhibitory effect of PMA whereas 4 alpha-phorbol 12,13-didecanoate (100 nM) failed to block the arborization. These results indicated that the inhibition of arborization by PMA was mediated through the activation of protein kinase C. Colchicine at 5.0 microM also had an inhibitory effect on ISO- and dBcAMP-induced cell arborization. However, immunofluorescence studies revealed that colchicine but not PMA elicited the reorganization of microtubules, suggesting that the effect of PMA was mediated through a mechanism different from that of colchicine. These observations indicated that the morphology of VSMC was regulated through the alteration of cytoskeletal organization induced by cAMP-mediated and by protein kinase C-dependent systems.

Angiotensin decreases cyclic GMP accumulation produced by atrial natriuretic factor

Atrial natriuretic factor (ANF) produced rapid increases in cyclic GMP (cGMP) in cultured aortic smooth muscle cells. Angiotensin II (ANG II) markedly decreased the accumulation of cGMP that was evoked by ANF. Arginine vasopressin and ATP, which evoke transient increases in free Ca2+ similarly to ANG II, also inhibited cGMP accumulation. The effect of the calcium mobilizing neurohormones was mimicked by the divalent cation ionophore, A23187. The cyclic nucleotide phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, prevented ANG II from inhibiting ANF-evoked cGMP accumulation. ANG II also inhibited cGMP accumulation induced by nitroprusside, a compound that activates cytosolic guanylate cyclase. These findings support the hypothesis that ANG II decreases cGMP accumulation by stimulating cGMP hydrolysis, apparently via a Ca2+-activated cGMP phosphodiesterase.

Na+/K+/Cl- cotransport in cultured vascular smooth muscle cells: stimulation by angiotensin II and calcium ionophores, inhibition by cyclic AMP and calmodulin antagonists

The specific activity of the Na+/K+/Cl cotransporter was assayed by measuring the initial rates of furosemide-inhibitable 86Rb+ influx and efflux. The presence of all three ions in the external medium was essential for cotransport activity. In cultured smooth muscle cells furosemide and bumetanide inhibited influx by 50% at 5 and 0.2 microM, respectively. The dependence of furosemide-inhibitable 86Rb+ influx on external Na+ and K+ was hyperbolic with apparent Km values of 46 and 4 mM, respectively. The dependence on Cl was sigmoidal. Assuming a stoichiometry of 1:1:2 for Na+/K+/Cl-, a Km of 78 mM was obtained for Cl. In quiescent smooth muscle cells cotransport activity was approximately equal to Na+ pump activity with each pathway accounting for 30% of total 86Rb+ influx. Growing muscle cells had approximately 3 times higher cotransport activity than quiescent ones. Na+ pump activity was not significantly different in the growing and quiescent cultures. Angiotensin II (ANG) stimulated cotransport activity as did two calcium-transporting ionophores. A23187 and ionomycin. The removal of external Ca2+ prevented A23187, but not ANG, from stimulating the cotransporter. Calmodulin antagonists selectively inhibited 86Rb+ influx via the cotransporter. Beta-adrenoreceptor stimulation with isoproterenol, like other treatments which increase cAMP, inhibited cotransport activity. Cultured porcine endothelial cells had 3 times higher cotransport activity than growing muscle cells. Calmodulin antagonists inhibited cotransport activity, but agents which increase cAMP or calcium had no effect on cotransport activity in the endothelial cells.

Morphological alterations in endothelial cells from human aorta and umbilical vein induced by forskolin and phorbol 12-myristate 13-acetate: a synergistic action of adenylate cyclase and protein kinase C activators

The morphological effects on human endothelial cells of phorbol 12-myristate 13-acetate (PMA) and of agents that increase intracellular cAMP concentration were studied. The adenylate cyclase activator forskolin (10 microM), the cyclic nucleotide phosphodiesterase inhibitor methylisobutylxanthine (100 microM), dibutyryl-cAMP (10 microM), histamine (10 microM), and PMA (0.1 microM) significantly altered the morphology of human aortic and umbilical vein endothelial cells in primary cultures. These effects reached a maximum 40-80 min after the effector addition and became negligible 30-60 min after its removal. PMA and forskolin were strongly synergistic in altering endothelial cell morphology. All the effects of cAMP-elevating compounds and of PMA were abolished completely by 1 microM colchicine. In explants taken from human adult or child aortas, forskolin and PMA produced alterations in endothelial morphology qualitatively identical to those observed in endothelial cell cultures. Endothelium in these preparations closely resembled that found in zones of expected altered hemodynamic stresses of human aorta. Our data suggest that the morphology of endothelium in vivo may be regulated by separate or synergistic action of hormone-dependent adenylate cyclase and of inositol phospholipid turnover systems and might be important for maintenance of endothelial monolayer integrity under normal physiological and pathological conditions.

Extracellular calcium-induced neuroblastoma cell differentiation: involvement of phosphatidylinositol turnover

The rat CNS neuroblastoma B50 cell line is known to differentiate on addition of 1 mM dibutyryl cyclic AMP or on withdrawal of serum. In this report it is shown that high levels of extracellular calcium (10-25 mM) cause neurite extension, an important component of morphological differentiation. Stimulation of calcium influx with the ionophore A 23187 or blockade of calcium efflux with lanthanum are less efficient than extracellular calcium in stimulating neurite extension. These data suggest that intracellular calcium is not sufficient to cause full expression of a calcium-dependent differentiated state. Furthermore, phosphatidylinositol turnover is sharply altered as early as 1 h after addition of calcium to the medium while cyclic nucleotide levels remain unaffected. This suggests that activation of the phosphatidylinositol second-messenger system by calcium at the level of the cell membrane is the initial step in the cascade of events leading to neurite extension. Later events include a decrease in DNA synthesis (6-10 h after addition of calcium), and increase in intracellular calcium levels (12-24 h after calcium addition) concurrent with neurite extension. The intracellular increase in calcium levels is facilitated by synergistic action of 1 mM dibutyryl cyclic AMP with high external calcium (10-25 mM). This combined treatment results in a more complex pattern of neurite formation characterized by many synaptic-like junctions; this pattern is not obtained when either dibutyryl cyclic AMP or calcium is used as sole inducer.