Novel Ca2+ signalling mechanisms in vascular myocytes: symposium overview

This commentary presents the proceedings of the symposium sponsored by Cardiovascular Section of American Physiological Society in San Diego, CA on 12 April 2003. The major focus of this symposium was on the actions and physiological relevance of several novel Ca2+ signalling mechanisms in vascular smooth muscle (VSM) cells. Five important topics were presented in this symposium including the discovery and roles of cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) in mediating Ca2+ release, Ca2+ sparks and activation of plasma membrane KCa channels in VSM cells, the role of cADPR-mediated activation of ryanodine receptors in the control of vascular tone, the role of [Ca2+]i in mechanotransduction in the arterioles, and interactions of mitochondrial Ca2+ release and SR Ca2+ mobilization. The purpose of this symposium was to promote discussions and exchange of ideas between scientists with interests in Ca2+ signalling mechanisms and those with interests in vascular physiology and pharmacology. The cross-fertilization of ideas is expected to greatly advance our understanding of the physiological and pharmacological relevance of these new Ca2+ signalling mechanisms.

Pressure-dependent myogenic constriction of cerebral arteries occurs independently of voltage-dependent activation

Pressure-induced decreases in arterial diameter are accompanied by membrane depolarization and Ca(2+) entry via voltage-gated Ca(2+) channels. Recent evidence also suggests the involvement of Ca(2+) sensitization of the contractile proteins. Both PKC and Rho kinase are candidate second messengers for the mediation of the sensitization process. We investigated the signaling pathways of pressure-induced decreases in rat cerebral artery diameter in vessels that were depolarized with a 60 mM potassium-physiological salt solution (KPSS). Arteries were mounted on a pressure myograph, and pressure-induced constrictions were recorded. In some experiments simultaneous changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) were recorded by using fura 2 fluorescence photometry. Pressure increases induced constriction with significant changes in [Ca(2+)](i) at high pressures (60-100 mmHg). The ratio of the change in diameter to change in [Ca(2+)](i) was greater for pressure-induced constriction compared with constriction produced by depolarization with 60 mM KPSS, suggesting that in addition to increases in [Ca(2+)](i), enhanced myofilament Ca(2+) sensitivity occurs during pressure-induced decreases in arterial diameter. Depolarizing the membrane with 60 mM KPSS increased [Ca(2+)](i) via a Ca(2+) influx pathway insensitive to PKC inhibition. Cerebral arteries were able to maintain their diameters in the continued presence of 60 mM KPSS. Pressure-induced constriction under these conditions was not associated with further increases in Ca(2+) but was abolished by selective inhibitors of PLC, PKC, and Rho kinase. We report for the first time that in rat cerebral arteries, pressure-induced decreases in arterial diameter are not only due to increases in voltage-gated Ca(2+) influx but also to accompanying increases in myofilament sensitivity to Ca(2+) mediated by PKC/Rho kinase activation.

Age-related changes in the calcium homeostasis of adherent neutrophils

The elderly are susceptible to infections and show a decline in neutrophil (PMN) functions that are regulated by cytosolic calcium [Ca2+]i. This study measures [Ca2+]i in suspended and adherent PMN of young and elderly individuals by using flow cytometry, confocal microscopy, the bacterial peptide fMLP, and the fluorescent Ca2+ indicator fluor-3/acettoxymethyl ester. PMN from both age groups show a steep and transient fMLP-induced Ca2+ increase. This increase is independent of external divalent cations and is desensitized by a subsequent exposure to the same agonist. Adherent PMN of the elderly express elevated [Ca2+]i before (basal) and after fMLP activation but show reduced ability to mobilize Ca2+ into and from the cytosol. PMN of the elderly take longer (13.7 +/- 3 s) to attain the maximal response compared to those of young adults (5.7 +/- 0.8 s). PMN from both age groups show heterogeneity in the time and magnitude of this response. However, PMN of the elderly show a decrease in the proportion of cells with prompt and effective reaction and an increase in the representation of a cell subpopulation manifesting delayed response. We conclude that age-related delayed and reduced PMN response to a bacterial peptide could hamper functional activities that are essential in host protection against infections.

Agonist-stimulated calcium entry in primary cultures of human cerebral microvascular endothelial cells

Primary cultures of human cerebral microvascular endothelial cells (HCMEC) were loaded with fura-2. The intracellular free Ca2+ concentration ([Ca2+]i) was measured by digital imaging microscopy. Agonists ATP (100 micro), thrombin (10 units/ml), and histamine (25 microM) induced a transient [Ca2+]i increase. Histamine (100 microM) induced a biphasic [Ca2+]i increase with an initial [Ca2+]i peak followed by a [Ca2+]i plateau. The [Ca2+]i plateau was blocked by the receptor-operated Ca2+ channel (ROC) blockers SK&F 96365 and NCDC, indicating a contribution by Ca2+ influx through ROC to the [Ca2+]i plateau. However, this [Ca2+]i plateau was not blocked by the voltage-gated Ca2+ channel (VGC) blocker diltiazem (DTZ). Depolarization with 80K+ or application of the VGC agonist BAY K 8644 did not alter the resting [Ca2+]i; but 80K+ reduced the histamine (100 microM) induced [Ca2+]i plateau. These results show that HCMEC are devoid of functional VGC. Thus the membrane potential (Em) regulates Ca2+ entry mainly by enhancing the electrochemical Ca2+ gradient, such that hyperpolarization increases while depolarization decreases [Ca2+]i. Blockade of sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) by CPA increased [Ca2+]i. This effect was dependent on extracellular Ca2+ and reduced by iberiotoxin (IBTX) blockade of Ca2+-activated K+ channels (Kca), suggesting a role for Kca in regulating Ca2+ influx. Ca2+ is the principal activator of endothelial nitric oxide synthase (eNOS), which stimulates cyclic GMP production. The final result that the eNOS inhibitor L-NAME enhanced the histamine (100 microM) induced [Ca2+]i plateau suggests a negative feedback loop (via cGMP) of endothelial NO on its own synthesis in the regulation of endothelial [Ca2+]i signal.

Influence of group B streptococci on piglet pulmonary artery response to bradykinin

To study whether a sepsis-induced increase in des-Arg9-bradykinin (des-Arg9-BK) and bradykinin (BK) B1-receptor activity participates in the observed increase in pulmonary vascular resistance in neonatal group B streptococcal sepsis (GBS), isometric force bioassays of pulmonary artery (PA) rings were studied, after 4-h exposure to either Krebs or GBS, by using the following protocols: 1) BK dose-response curve, 2) vascular response to BK with NG-nitro-L-arginine methyl ester (L-NAME), and 3) response to des-Arg9-BK (BK metabolite and B1 agonist). PA rings exposed to BK resulted in contraction in the GBS group and a decrease in resting tension in the Control group (P = 0.034) at a concentration of 10(-5) M. GBS-treated PA rings contracted more to des-Arg9-BK than did Controls (P < 0.001). BK (10(-6) M) relaxed preconstricted PA rings incubated in GBS less than BK relaxed Controls (P < 0.001), and preincubation with L-NAME decreased relaxation in both. These results suggest that GBS decreased endothelium-dependent BK relaxation and increased contractile response to des-Arg9-BK. We speculate that this occurs secondary to upregulation of B1 receptors reflected by B1-agonist-mediated PA contraction.

A role for the sarcoplasmic reticulum in Ca2+ extrusion from rabbit inferior vena cava smooth muscle

Ca2+ extrusion from rabbit inferior vena cava smooth muscle was studied using ratiometric fura 2 fluorimetry. Concomitant blockade of the plasma membrane Ca(2+)-adenosinetriphosphatase (ATPase; PCMA), Na(+)-Ca2+ exchanger, and sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) completely prevented the decline in intracellular Ca2+ concentration ([Ca2+)]i) normally observed when Ca2+ is removed from the extracellular space (ECS) after stimulated Ca2+ influx. Blockade of the Na(+)-Ca2+ exchanger by removal of external Na+ reduced the rate of [Ca2+]i decline by 47%. Blockade of SERCA with cyclopiazonic acid reduced it by 23%, and this was not additive to the effects of Na+ removal. Exposure to nominally Ca(2+)-free solution prevented the sarcoplasmic reticulum (SR) from reloading only if the Na(+)-Ca2+ exchanger was operational. Our results can be explained by an SR contribution to Ca2+ extrusion in which SERCA is arranged in series with Na(+)-Ca2+ exchange.

Estrogen augments cyclopiazonic acid-mediated, endothelium-dependent vasodilation

The modulatory effects of chronic estrogen treatment on the responses to cyclopiazonic acid, an endoplasmic reticulum Ca2(+)-ATPase inhibitor, were studied in rings of aorta and the isolated perfused kidney of the rat. Rings of aorta were obtained from the following groups of age-matched rats (i) male, (ii) female, and two groups of rats implanted with a subcutaneous pellet (iii) ovariectomized, placebo-treated, (iv) ovariectomized, 17beta-estradiol-treated (0.5 mg/pellet/21 days). In phenylephrine (2 microM) pre-contracted rings with intact endothelium, cyclopiazonic acid (10(-7) to 3 x 10(-5) M) produced endothelium-dependent relaxations in a concentration-dependent manner. The cyclopiazonic acid dilation as a percentage loss of phenylephrine tone was greater in aortic rings from female (72.9 +/- 2.4%) and estrogen-treated rats (65.5 +/- 4.8%) compared to those from male (51.5 +/- 3.4%) or ovariectomized rats (40.8 +/- 3.9%) (P < 0.05, one-way analysis of variance (ANOVA)). These relaxation responses of cyclopiazonic acid were converted to contractions by pre-treatment with an inhibitor of nitric oxide (NO) synthase, N(omega)-nitro-L-arginine methyl ester (L-NAME, 200 microM; 30 min). There were no differences in cyclopiazonic acid-induced contractions of aortas excised from either estrogen-treated or untreated-ovariectomized rats. In perfused kidneys, cyclopiazonic acid (10(-5) M) caused a larger decrease in perfusion pressure in kidneys from female rats (110 +/- 0.4 mmHg) than it did in kidneys from male rats (80 +/- 0.6 mmHg). These results demonstrate that cyclopiazonic acid causes a greater endothelium-dependent dilation in estrogen-treated ovariectomized and control female rats, possibly due to unmasking of estrogen-enhanced Ca2+ entry into the endothelial cells.

Aluminum fluoride induces a reversible Ca2+ sensitization in alpha-toxin-permeabilized vascular smooth muscle

The mechanism of aluminumfloride (AlF)-induced Ca2+ sensitization was explored in alpha-toxin-permeabilized rabbit mesenteric artery. In the presence of 0.18 microM Ca2+ and deferoxamine, a strong chelator of aluminum (Al3+), fluoride (F-; applied in the form of NaF) induced very slow tension development, while in the presence of tracer levels of Al3+ tension developed rapidly possibly due to formation of Al-F complexes (especially AlF4-). As a result, AlF significantly shifted the relationship between tension development and free Ca2+ concentration in the Ca(2+)-EGTA buffer (pCa-tension curve) to the left. The rate of the tension development also depended on the EGTA concentration: increasing the EGTA concentration from 0.5 to 10 mM markedly decreased the maximal rate of contraction ((dT/dt)max), probably due to chelation of Al3+ by EGTA, without effect on the maximal tension (delta Tmax). The AlF-induced Ca2+ sensitization could be reversed by extensive washing with relaxing solution (pCa greater than 8), in contrast to the contractions induced by guanosine 5'-[gamma-thio]triphosphate (GTP gamma s; a non-hydrolyzable GTP analogue) or phorbol 12,13-dibutyrate (PDBu) which were irreversible. However, the action of all the compounds appeared to be mediated through a H-7 (1-[5-isoquinolinesulfonyl]-2-methylpiperazine dihydrochloride)-sensitive pathway, and no additive effects among them were observed. In addition, GDP increased (dT/dt)max due to AlF without changing delta Tmax, whereas guanosine 5'-[beta-thio]diphosphate (GDP beta s; a non-hydolyzable GDP analogue) decreased both parameters. These findings suggest that AlF acts on G-proteins to enhance Ca2+ sensitivity of contractile elements through a H-7-sensitive pathway.

Ion channels and regulation of intracellular calcium in vascular endothelial cells

Endothelial cells in vivo form an interface between flowing blood and vascular tissue, responding to humoral and physical stimuli to secrete relaxing and contracting factors that contribute to vascular homeostasis and tone. The activation of endothelial cell-surface receptors by vasoactive agents is coupled to an elevation in cytosolic Ca2+, which is caused by Ca2+ entry via ion channels in the plasma membrane and by Ca2+ release from intracellular stores. Ca2+ entry may occur via four different mechanisms: 1) a receptor-mediated channel coupled to second messengers; 2) a Ca2+ leak channel dependent on the electrochemical gradient for Ca2+; 3) a stretch-activated nonselective cation channel; and 4) internal Na+-dependent Ca2+ entry (Na+-Ca2+ exchange). The rate of Ca2+ entry through these ion pathways can be modulated by the resting membrane potential. Membrane potential may be regulated by at least two types of K channels: inwardly rectifying K channels activated upon hyperpolarization or shear stress; and a Ca2+-activated K channel activated upon depolarization, which may function to repolarize the agonist-stimulated endothelial cell. After agonist stimulation, cytosolic Ca2+ increases in a biphasic manner, with an initial peak due to inositol 1,4,5-trisphosphate-mediated Ca2+ release from intracellular stores, followed by a sustained plateau that is dependent on the presence of [Ca2+]o and on membrane potential. The delay in agonist-activated Ca2+ influx is consistent with the coupling of receptor activation to Ca2+ entry via a second messenger. Oscillations in [Ca2+]i, which may involve both Ca2+ entry and release, have been observed in isolated and confluent endothelial cell monolayers stimulated by histamine and bradykinin. Receptor-mediated Ca2+ entry, release, and refilling of intracellular stores follows a cycle that involves the plasma membrane.

A Ca2+-activated K+ channel from rabbit aorta: modulation by cromakalim

A large conductance Ca2+-activated K+ channel from rabbit aorta was incorporated into planar lipid bilayers. This channel had a conductance of 337 +/- 7 pS in symmetrical 250 mM KCl solutions and had a Na+/K+ permeability ratio of less than 0.04. In asymmetrical solutions containing 300 mM KCl cis (intracellular), 100 mM KCl trans (extracellular) or 100 mM KCl cis 500 mM KCl trans, the reversal potentials for the channel were -30 and +46 mV, respectively. This channel possessed voltage-dependent activation and cis (intracellular) Ca2+ sensitivity. Cromakalim (50 nM) added to the trans side of the bilayer significantly increased the Popen by 56% from 0.09 +/- 0.01 to 0.14 +/- 0.01 (P less than 0.01) at -40 mV without altering the single channel conductance. This effect was dose-dependent, increasing at higher cromakalim concentrations. The primary effect of cromakalim was to decrease the tau slow of the channel closed state from 266 +/- 32 to 147 +/- 17 ms which is sufficient to account for the increase in Popen of the channel in the presence of cromakalim.

Bradykinin and inositol 1,4,5-trisphosphate-stimulated calcium release from intracellular stores in cultured bovine endothelial cells

The relative importance of intracellular and extracellular Ca2+ in the release of endothelium-derived relaxing factor (EDRF) and the mechanisms involved in the release of intracellular Ca2+ were investigated in cultured bovine endothelial cells. The release of EDRF by bradykinin, determined by bioassay, was dose-dependent showing an EC50 of 4 x 10(-10) M. The bradykinin-induced EDRF release from endothelial cells was maintained in the presence of extracellular Ca2+. However, in the absence of external Ca2+, bradykinin-induced EDRF release was both attenuated and transient. In cells loaded to isotopic equilibrium with 45Ca, bradykinin increased the 45Ca efflux into both calcium-containing and calcium-free solutions, with an EC50 for the increase in 45Ca efflux induced by bradykinin of 1.3 x 10(-9) M. The involvement of an intracellular Ca2+ store and the participation of a second messenger in its release were investigated in saponin-permeabilized endothelial cells. In saponin-permeabilized cells, ATP-sensitive calcium uptake was Ca2+,Mg2+ -ATPase-dependent. The ATP-sensitive uptake of calcium at different free Ca2+ concentrations showed at least two compartments involved in the uptake of Ca2+. The 45Ca uptake into the compartment with the lowest affinity and highest capacity could be inhibited by sodium azide, suggesting that this uptake was into mitochondria. The majority of the 45Ca uptake into the azide-insensitive store could be released by inositol-1,4,5-trisphosphate (IP3). The IP3-induced release was not affected by apyrase or exogenous GTP. The EC50 for the release of Ca2+ by IP3 was 1.0 microM and was unaffected by an inhibitor of IP3 breakdown (2,3-diphosphoglyceric acid).(ABSTRACT TRUNCATED AT 250 WORDS)

The intracellular calcium stores in the rabbit trachealis

We investigated the role of the intracellular Ca2+ stores in the regulation of the rabbit tracheal smooth muscle contraction. Carbachol (10 microM)- and 80K-induced contractions were reduced by preincubating tissues in Ca2+-free (EGTA-PSS) solution. Contractile amplitude plotted as a function of the duration of EGTA-PSS preexposure was described by a biexponential for carbachol and a monoexponential for 80K. In EGTA-PSS, a prior caffeine (50 mM)-induced contraction prevented any subsequent phasic carbachol response; the converse was also true. In contrast, prior exposure to 80K increased the amplitude of a subsequent carbachol or caffeine contraction measured in EGTA-PSS. Repletion of Ca2+ plus either 80K or a low concentration of carbachol (0.3 microM) resulted in delayed tension development. Preincubation in forskolin (10(-5) M) in PSS also delayed tension development. We propose that the internal stores, most likely the sarcoplasmic reticulum in the airway muscle function both to supply and remove Ca2+ from the cytoplasm.

Effects of isosorbide dinitrate and diltiazem on Ca2+ flux and contraction in artery

We studied the effects of isosorbide dinitrate and diltiazem on histamine-stimulated 45Ca fluxes and contractions of isolated porcine coronary artery. Isosorbide dinitrate was slightly more potent as an inhibitor of intracellular compared to extracellular calcium-dependent contraction. Isosorbide dinitrate inhibited histamine-stimulated calcium efflux and intracellular calcium-dependent contraction over similar concentration ranges. Isosorbide dinitrate partially inhibited histamine-stimulated calcium influx, but this effect was significant only at high concentration and correlated weakly with inhibition of contraction that was dependent on extracellular calcium. Diltiazem more potently inhibited extracellular vs. intracellular calcium-dependent contraction. Diltiazem partially inhibited histamine-stimulated calcium efflux and intracellular calcium-dependent contraction to similar extents (55-60%) and produced similar concentration-response relationships for inhibition of histamine-stimulated calcium influx and extracellular calcium-dependent contraction. The data suggest that alterations of cellular calcium metabolism are major mechanisms of vascular smooth muscle relaxation by isosorbide dinitrate and diltiazem, but that the specific alterations differ for the two drugs. Isosorbide dinitrate may inhibit contraction primarily by enhancing intracellular calcium sequestration, but possibly also by inhibiting agonist-stimulated calcium influx at high isosorbide dinitrate concentrations. Diltiazem primarily inhibits stimulated calcium influx, but may also inhibit intracellular calcium release.

Human airway smooth muscle in culture

We describe a method for culturing human airway smooth muscle. Cells were enzymatically and mechanically dispersed from strips of smooth muscle harvested from surgically removed lobar bronchi, and were seeded on to dishes containing Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum. After 14-21 days confluent monolayers of cells formed, which were subcultured and identified as smooth muscle by positive immunocytochemical staining for actin and myosin. The retention of functional plasmalemmal receptors and of intracellular signal transduction pathways in cell culture was demonstrated in 45Ca-labelled monolayers by the stimulation of efflux of intracellularly stored 45Ca in response to extracellularly applied 10 microM carbachol or 10 microM histamine. Human airway smooth muscle in cell culture provides a novel preparation for investigating the physiology and pathophysiology of the human airways.

Effect of dB-c-AMP and forskolin on the 45Ca influx, net Ca uptake and tension in rabbit aortic smooth muscle

The effects of dibutyril-adenosine-3',5'-cyclic monophosphate (dB-c-AMP) and forskolin on aortic tension and 45Ca influx were measured. dB-c-AMP reduced both the rate of force development and the maximal tension achieved in solutions containing various K+ concentrations. Stimulated 45Ca influx was also reduced, however, to a lesser extent than was the tension. Forskolin showed more marked effects of a similar nature. Thus, both these agents which increase intracellular c-AMP caused a rightward shift in the curve expressing force (ordinate) as a function of Ca2+ influx (abscissa). Consequently, we found that dB-c-AMP stimulated more net Ca to be taken up by sarcoplasmic reticulum (SR) at the same influx rate. The conclusion that c-AMP produced these effects by stimulating Ca uptake into the superficial SR was supported by the finding that dB-c-AMP increased the amount of Ca taken up into a caffeine releasable fraction.

Calcium entry through receptor-operated channels in bovine pulmonary artery endothelial cells

The activation of endothelial cells by endothelium-dependent vasodilators has been investigated using bioassay, patch clamp and 45Ca flux methods. Cultured pulmonary artery endothelial cells have been demonstrated to release EDRF in response to thrombin, bradykinin, ATP and the calcium ionophore A23187. The resting membrane potential of the endothelial cells was -56 mV and the cells were depolarized by increasing extracellular K+ or by the addition of (0.1-1.0 mM)Ba2+ to the bathing solution. The electrophysiological properties of the cultured endothelial cells suggest that the membrane potential is maintained by an inward rectifying K+ channel with a mean single channel conductance of 35.6 pS. The absence of a depolarization-activated inward current and the reduction of 45Ca influx with high K+ solution suggests that there are no functional voltage-dependent calcium or sodium channels. Thrombin and bradykinin were shown to evoke not only an inward current (carried by Na+ and Ca2+) but also an increase in 45Ca influx suggesting that the increase in intracellular calcium necessary for EDRF release is mediated by an opening of a receptor operated channel. High doses of thrombin and bradykinin induced intracellular calcium release, however, at low doses of thrombin no intracellular calcium release was observed. We propose that the increased cytosolic calcium concentration in endothelial cells induced by endothelium dependent vasodilators is due to the influx of Ca2+ through a receptor operated ion channel and to a lesser degree to intracellular release of calcium from a yet undefined intracellular store.

Ca2+ regulation of vascular smooth muscle

Regulation of intracellular free Ca2+ concentrations in vascular smooth muscle is accomplished mainly by Ca2+ channels and ATP-dependent Ca2+ pumps in the plasmalemma and sarcoplasmic reticulum (SR). Ca2+ entry through the plasmalemma is apparently mediated by four different pathways: leak; receptor-operated Ca2+ channels; potential sensitive Ca2+ channels; and stretch-activated channels. The agonist releasable intracellular Ca2+ store appears to be identical with the SR. Evidence for the involvement of Ca2+-induced Ca2+ release and inositol-1,4,5-trisphosphate in the release of SR Ca2+ is discussed. Smooth muscle contractions induced by certain agonists may be further enhanced by inhibition of Ca2+ uptake by the SR and of active Ca2+ extrusion across the plasmalemma. At the moment it is not clear from a consideration of the Ca2+ regulatory mechanisms present in vascular smooth muscle how dietary Ca2+ affects vascular tone. The increased Ca2+ permeation through smooth muscle cell membranes of resistance arteries taken from spontaneously hypertensive rats may be relevant to this problem.

Ca2+ sources mobilized by alpha 1-receptor activation in vascular smooth muscle

We propose the following model of Ca2+ mobilization by noradrenaline in vascular smooth muscle. Upon receptor occupation Ca2+ from a labile small intracellular store on the inner plasmalemma is released. This Ca2+ does not function as activator Ca2+ but triggers Ca2+ release from the sarcoplasmic reticulum (Ca2+-induced Ca2+ release). Simultaneously Ca2+ from an extracellularly bound store (on the external surface of the plasmalemma) is dislodged, which enters the cell through receptor linked channels. These processes are responsible for the early 'phasic' component of the noradrenaline contraction. In addition, Ca2+ from the free extracellular Ca2+ pool enters through receptor operated channels, supporting the maintained tension development.

A theoretic consideration on the use of calcium antagonists in the treatment of hypertension

A cellular basis for the reduction of contractile tension in vascular smooth muscle by calcium antagonists is described. A better understanding of the mechanisms and pathways that supply activating calcium ions to the cytoplasm will permit investigation of specific functional abnormalities that lead to hypertension. If the "leak channel" is found to be the responsible site, then antihypertensive drugs even more specific than those currently available can be designed.

Cyclic AMP modulation of adrenoreceptor-mediated arterial smooth muscle contraction

We examined the effects of cyclic AMP (cAMP) on the intracellular Ca2+ release in both the intact and skinned arterial smooth muscle. The amount of Ca2+ in the sarcoplasmic reticulum (SR) was estimated indirectly by caffeine-induced contraction of the skinned preparation and directly by caffeine-stimulated 45Ca efflux from the previously labeled skinned preparation. The norepinephrine-induced release contraction was markedly enhanced by dibutyryl cAMP (dbcAMP) and reduced by propranolol. The stimulatory effect of dbcAMP was best observed when the muscle was exposed to 10(-5) M dbcAMP and 2 X 10(-6) M norepinephrine was used to induce the release contraction. 10(-5) M cAMP had no effect on the Ca2+-induced contraction or on the pCa-tension relationship in the skinned preparation. This concentration of cAMP increased Ca2+ uptake into the SR of the skinned preparation when the Ca2+ in the SR was first depleted. 10(-5) M cAMP stimulated Ca2+-induced Ca2+ release from the SR after optimal Ca2+ accumulation by the SR. The results indicate that the stimulatory effect of cAMP on the norepinephrine-induced release contraction could be due to enhancement of the Ca2+-induced Ca2+ release from the SR in arterial smooth muscle.

Bay K8644 differentiates between potential and receptor operated Ca2+ channels

Bay K8644 increased unidirectional Ca2+ influx and produced tension development in rabbit aorta. Both responses could be evoked in the tissue maximally stimulated with norepinephrine. When the arterial rings were maximally activated by high K+ depolarization, Bay K8644 was without effect. The tension evoked by high K+ and Bay K8644 was more sensitive to the dihydropyridine Ca2+ antagonist PY108-068 than norepinephrine induced tension. These results indicate that Bay K8644 activates only potential operated Ca2+ channels which are opened by high K+ depolarization.

Effects of Na readmission on cellular 45Ca fluxes in Na-depleted guinea pig taenia coli

The removal of Na from the medium causes a cellular Ca uptake in the smooth muscle of the guinea pig taenia coli which is rapidly reversed if medium Na is readmitted. This net extrusion was characterized in tissues which were first Na-depleted in a zero-Na (sucrose) solution. Li was able to substitute for Na in mediating this effect. K was also able to mimic Na in this respect if the depolarization-mediated Ca influx caused by the isotonic K solution was blocked with 10(-5) M D -600. The net Ca extrusion upon Na readmission was due to a small decrease in Ca influx, as well as a marked increase in the transmembrane Ca efflux rate, as revealed by 45Ca washout experiments. The increased 45Ca efflux upon Na readmission could be mimicked by Li, K, choline and tris. We conclude that the Na/Ca-exchange hypothesis is insufficient to explain these data, in that both Ca extrusion and 45Ca efflux can be stimulated in the absence of a Na gradient, or in the absence of any monovalent cationic gradient. These observations are discussed in terms of a possible intracellular competition of Ca and monovalent cations for anionic binding sites, as well as with regard to a possible direct stimulation of a plasmalemmal CaATPase by monovalent cations.

The effects of amrinone on contractility, Ca2+ uptake and cAMP in smooth muscle

Amrinone, a known positive inotropic agent in the heart, was found to cause a dose-dependent (10--100 micrograms/ml) inhibition of norepinephrine (NE) or high-K+-induced contractions of rabbit aorta. Amrinone also inhibited carbachol or high-K+-induced contractions of guinea-pig taenia coli. Neither total tissue 45Ca uptake nor the rate of 45Ca uptake induced by 80 mM K+ in rabbit aorta was altered by pretreatment with amrinone. On the other hand, a similar pretreatment with amrinone inhibited NE (10(-6) or 10(-5) M) induced tissue 45Ca uptake. Amrinone (100 micrograms/ml) caused about a 70% increase in cAMP concentration over resting levels. It is concluded that amrinone causes a nonspecific inhibition of smooth muscle contractility by acting probably at multiple sites to decrease the availability of Ca2+ required for activation. One or more of these mechanisms may involve cAMP.

Calcium requirement for activation of intact aortic smooth muscle

1. Elevation of [K(+)](e) induced a contraction of rabbit aorta. If 10 mM-La(3+) was applied to rabbit aortae prior to [K(+)](e) elevation no contraction occurred. When 10 mM-La(3+) was applied simultaneously with, or at short time periods after, elevation of [K(+)](e) graded contractions were obtained whose magnitudes were higher if La(3+) was added at a later stage.2. Net cellular Ca fluxes associated with these graded contractions were measured using (45)Ca as well as atomic absorption. Correlation of the graded contractions with the net inward Ca movements yielded a Ca activation curve for intact arterial smooth muscle cells. The curve was S-shaped with half-maximal activation at a net Ca uptake of 41 mumole/kg wet wt. of aorta and full activation at 64 mumole/kg. If corrections were made for the extracellular space, full activation occurred at a net Ca uptake of 102 mumole/kg smooth muscle cells or roughly 25 mumole in excess of the value calculated to bind to the activation sites on arterial smooth muscle myosin.3. The contractile force of the smooth muscle cells not only depended on the magnitude, but also on the rate of the net Ca uptake.4. Elevation of tissue c-AMP through inhibition of phosphodiesterase greatly reduced both the rate and magnitude of contraction without affecting the size of the high K(+) stimulated Ca influx. It was concluded that high K(+)-induced depolarization activates the rabbit aorta by stimulating a net Ca uptake, and that the degree of activation is modified by c-AMP sensitive Ca accumulation by intracellular organelles.

Active and passive Ca2+ fluxes across cell membranes of the guinea-pig taenia coli

The exchange of Ca between the extracellular fluid and the cellular compartment has been investigated in smooth muscle cells of taenia coli. It was found that during the initial phase of metabolic depletion by DNP + IAA, the net inwards flux of Ca amounts to 0.02 pmol cm(-2)-sec(-1). This increase might be proportional to the physiological calcium leak. The study of the relation between the inwardly directed Na gradient and the cellular Ca content has revealed that this Na gradient exerts no effect during prolonged exposure to K-free solution and a very limited effect during exposure to Na-deficient solutions. The cellular 45Ca release induced by metabolic inhibition is not affected by substituting Li or choline for Na. The supplementary calcium which enters the cells during exposure to a solution at low temperature is extruded on returning to a solution at 35 degrees C, even if the Na gradient is reversed. This finding and the effects of metabolic inhibition indicate that Ca extrusion in smooth muscle cells is a process which depends on metabolism and which is not affected by the inwardly directed Na gradient.

Stimulation of 45Ca efflux from smooth muscle cells by metabolic inhibition and high K depolarization

The characteristics of the extracellular and cellular calcium exchange in taenia coli have been studied by efflux experiments under different experimental conditions. The exchange of extracellularly bound calcium is accelerated by the presence of calcium in the external solution. If a Ca-free solution is used as washing solution, the slowly exchanging extracellular calcium also contributes appreciably to the later phase of the Ca efflux and obscures the changes of the cellular calcium exchange. There is no evidence for a Ca-Ca exchange diffusion. Most of the 45Ca bound at extracellular binding sites can be released by a 10 min exposure to 2 mM EGTA or to 10 mM La3+. This La concentration moreover largely inhibits the release of 45Ca from the cellular compartment by metabolic depletion. A release of cellular 45Ca can be induced by metabolic depletion or by K depolarization. Both procedures probably act at the same sequestering sites. However, while DNP + IAAa cts in the absence of external Ca, it is observed that K depolarization can only cause a Ca release if external Ca can enter the cells.

Excitation-contraction coupling in rabbit aorta studied by the lanthanum method for measuring cellular calcium influx

Lanthanum inhibits 45 Ca efflux from internally injected squid axons. Evidence for La 3+ blockade of Ca 2+ fluxes across the cell membrane was also obtained for aortic smooth muscle: 1mM La 3+ completely inhibited contractions which otherwise resulted from adding Ca 2+ to calcium-free depolarizing solutions, and 2 mM La 3+ caused a 50% inhibition of the late 45 Ca efflux. Ca 2+ bound extracellularly could be displaced by La 3+ , and the binding sites preferred La 3+ over Ca 2+ . We could thus use La 3+ to eliminate extracellular bound Ca 2+ from our calcium-influx measurements as follows: after exposure of the aortic strips to experimental solutions labeled with 45 Ca, the extracellular Ca 2+ label was displaced by putting the strips in a calcium-free solution containing 2 mM La 3+ for 60 minutes. The loss of intracellular Ca 2+ was minimized during this hour by the La 3+ blockade of Ca 2+ membrane fluxes. Tissue weight, 45 Ca, and total Ca 2+ were then measured using standard techniques. Studies employing this new method showed that depolarization by high K + , Na + replacement, and high pH activate smooth muscle contraction by stimulating Ca 2+ influx. Low pH and La 3+ block Ca 2+ influx. Norepinephrine initiates aortic contractions by release of intracellularly sequestered Ca 2+ . Angiotensin and histamine appear to release Ca 2+ from this same fraction.

Calcium distribution and exchange in the rat uterus

The calcium content and distribution of the rat uterus were determined employing flame photometry and Ca⁴⁵ determinations. The total uterine calcium concentration was found to be 2.25 millimoles (mmoles) per kilogram wet weight, 0.45 of which was inexchangeable. The exchangeable Ca could be divided into 0.8 mmole/kg wet weight extracellular and 1.0 mmole/kg wet weight intracellular. The concentration of ionic Ca in rat serum was obtained by equilibrium dialysis as 1.5 mM or 53 % of the total serum Ca. The observed Ca distribution required that its active transport be postulated, since the membrane was shown to be permeable to Ca and the internal Ca concentration was far below its electrochemical equilibrium value. Metabolic inhibition by iodoacetate or dinitrophenol caused a net Ca uptake, but cooling to 4°C and ouabain did not. Iodoacetate did not affect the Ca⁴⁵ efflux, but did increase the influx, suggesting that active Ca transport is accomplished by an exclusion mechanism. In experiments with varied external sodium concentrations, no evidence was obtained that sodium competes with calcium for inward transport. Cellular Ca binding was measured under conditions of prolonged metabolic inhibition, which abolished both active transport and the membrane potential. The association constants obtained were compatible with intracellular Ca binding to proteins, but insufficient to account for the low level of intracellular ionic Ca believed essential for relaxation. Hence metabolically dependent intracellular Ca binding was postulated. The Ca⁴⁵ efflux was slowed down by Ca-free efflux media. The presence of Sr or EDTA could completely prevent this decrease in efflux rate, and Ba could partly prevent it. Changes in Mg and Na concentration did not affect the rate of Ca⁴⁵ efflux. A model to explain Ca exchange across smooth muscle membranes has been proposed.

The influence of high potassium depolarization and acetylcholine on calcium exchange in the rat uterus

Net and radioactive calcium movements were studied in the rat uterus during stimulation with acetylcholine and high potassium solutions. High potassium did not affect the efflux of intracellular Ca⁴⁵, but was able to release Ca⁴⁵ from a small parallel Ca fraction which was believed to be located in the cell membranes. High potassium did markedly slow the influx of Ca⁴⁵ and caused a net calcium efflux. Acetylcholine had no effect on calcium movements in polarized myometrium, but it increased the Ca⁴⁵ influx in depolarized uteri. Ca⁴⁵ taken up during contraction exchanged more slowly during subsequent efflux than Ca⁴⁵ taken up at rest. The results were interpreted as supporting the hypothesis that myometrial contraction is induced by a release of calcium from the inside of the cell membrane and the endoplasmic reticulum, and relaxation follows the removal of ionic cytoplasmic calcium by these same structures.