Phosphorylation of smooth myosin light chain kinase by smooth muscle Ca2+/calmodulin-dependent multifunctional protein kinase

A temporal Ca2+-desensitization of myosin light chain kinase in phasic smooth muscles induced by CaMKKß/PP2A pathways

Cell signaling pathways regulating myosin regulatory light chain (LC20) phosphorylation contribute to determining contractile responses in smooth muscles. Following excitation and contraction, phasic smooth muscles, such as digestive tract and urinary bladder, undergo a relaxation due to a decline of cellular [Ca²⁺] and a decreased Ca²⁺ sensitivity of LC20 phosphorylation, named Ca²⁺ desensitization. Here, we determined mechanisms underlying the temporal Ca²⁺ desensitization of LC20 phosphorylation in phasic smooth muscles using permeabilized strips of mouse ileum and urinary bladder. Upon the stimulation with pCa6.0 at 20°C, the contraction and the LC20 phosphorylation peaked within 30 sec and then declined to about 50% of the peak force at 2 min after stimulation. During the relaxation phase after the contraction, the LC20 kinase (MLCK) was inactivated, but no fluctuation in the LC20 phosphatase activity occurred, suggesting that the MLCK inactivation is a cause of the Ca²⁺-induced Ca²⁺-desensitization of LC20 phosphorylation. The MLCK inactivation was associated with phosphorylation at the calmodulin binding domain of the kinase. Treatment with antagonists for CaMKKß (STO-609 and TIM-063) attenuated both the phasic response of the contraction and MLCK phosphorylation, whereas neither CaMKII, AMPK nor PAK induced the MLCK inactivation in phasic smooth muscles. Conversely, PP2A inhibition amplified the phasic response. Signaling pathways through CaMKKß and PP2A may contribute to regulating the Ca²⁺ sensitivity of MLCK and the contractile response of phasic smooth muscles.

CaMKII in addition to MLCK contributes to phosphorylation of regulatory light chain in cardiomyocytes

The aim was to identify kinase activities involved in the phosphorylation of regulatory light chain (RLC) in situ in cardiomyocytes. In electrically stimulated rat cardiomyocytes, phosphatase inhibition by calyculin A unmasked kinase activities evoking an increase of phosphorylated RLC (P-RLC) from about 16% to about 80% after 80 min. The phosphorylation rate in cardiomyocytes was reduced by about 40% by the myosin light chain kinase (MLCK) inhibitor, ML-7. In rat ventricular muscle strips, calyculin A induced a positive inotropic effect that correlated with P-RLC levels. The inotropic effect and P-RLC elevation were abolished by ML-7 treatment. The kinase activities phosphorylating RLC in cardiomyocytes were reduced by about 60% by the non-selective kinase inhibitor staurosporine and by about 50% by the calmodulin antagonist W7. W7 eliminated the inhibitory effect of ML-7, suggesting that the cardiac MLCK is Ca(2+)/calmodulin (CaM)-dependent. The CaM-dependent kinase II (CaMKII) inhibitor KN-93 attenuated the calyculin A-induced RLC phosphorylation by about 40%, indicating a contribution from CaMKII. The residual phosphorylation in the presence of W7 indicated that also CaM-independent kinase activities might contribute. RLC phosphorylation was insensitive to protein kinase C inhibition. In conclusion, in addition to MLCK, CaMKII phosphorylates RLC in cardiomyocytes. Involvement of other kinases cannot be excluded.

Regulation of myosin light chain kinase during insulin-stimulated glucose uptake in 3T3-L1 adipocytes

Myosin II (MyoII) is required for insulin-responsive glucose transporter 4 (GLUT4)-mediated glucose uptake in 3T3-L1 adipocytes. Our previous studies have shown that insulin signaling stimulates phosphorylation of the regulatory light chain (RLC) of MyoIIA via myosin light chain kinase (MLCK). The experiments described here delineate upstream regulators of MLCK during insulin-stimulated glucose uptake. Since 3T3-L1 adipocytes express two MyoII isoforms, we wanted to determine which isoform was required for insulin-stimulated glucose uptake. Using a siRNA approach, we demonstrate that a 60% decrease in MyoIIA protein expression resulted in a 40% inhibition of insulin-stimulated glucose uptake. We also show that insulin signaling stimulates the phosphorylation of MLCK. We further show that MLCK can be activated by calcium as well as signaling pathways. We demonstrate that adipocytes treated with the calcium chelating agent, 1,2-b (iso-aminophenoxy) ethane-N,N,N',N'-tetra acetic acid, (BAPTA) (in the presence of insulin) impaired the insulin-induced phosphorylation of MLCK by 52% and the RLC of MyoIIA by 45% as well as impairing the recruitment of MyoIIA to the plasma membrane when compared to cells treated with insulin alone. We further show that the calcium ionophore, A23187 alone stimulated the phosphorylation of MLCK and the RLC associated with MyoIIA to the same extent as insulin. To identify signaling pathways that might regulate MLCK, we examined ERK and CaMKII. Inhibition of ERK2 impaired phosphorylation of MLCK and insulin-stimulated glucose uptake. In contrast, while inhibition of CaMKII did inhibit phosphorylation of the RLC associated with MyoIIA, inhibition of CAMKIIδ did not impair MLCK phosphorylation or translocation to the plasma membrane or glucose uptake. Collectively, our results are the first to delineate a role for calcium and ERK in the activation of MLCK and thus MyoIIA during insulin-stimulated glucose uptake in 3T3-L1 adipocytes.

Differential control of calcium homeostasis and vascular reactivity by Ca2+/calmodulin-dependent kinase II

The multifunctional Ca(2+)/calmodulin-dependent kinase II (CaMKII) is activated by vasoconstrictors in vascular smooth muscle cells (VSMC), but its impact on vasoconstriction remains unknown. We hypothesized that CaMKII inhibition in VSMC decreases vasoconstriction. Using novel transgenic mice that express the inhibitor peptide CaMKIIN in smooth muscle (TG SM-CaMKIIN), we investigated the effect of CaMKII inhibition on L-type Ca(2+) channel current (ICa), cytoplasmic and sarcoplasmic reticulum Ca(2+), and vasoconstriction in mesenteric arteries. In mesenteric VSMC, CaMKII inhibition significantly reduced action potential duration and the residual ICa 50 ms after peak amplitude, indicative of loss of L-type Ca(2+) channel-dependent ICa facilitation. Treatment with angiotensin II or phenylephrine increased the intracellular Ca(2+) concentration in wild-type but not TG SM-CaMKIIN VSMC. The difference in intracellular Ca(2+) concentration was abolished by pretreatment with nifedipine, an L-type Ca(2+) channel antagonist. In TG SM-CaMKIIN VSMC, the total sarcoplasmic reticulum Ca(2+) content was reduced as a result of diminished sarcoplasmic reticulum Ca(2+) ATPase activity via impaired derepression of the sarcoplasmic reticulum Ca(2+) ATPase inhibitor phospholamban. Despite the differences in intracellular Ca(2+) concentration, CaMKII inhibition did not alter myogenic tone or vasoconstriction of mesenteric arteries in response to KCl, angiotensin II, and phenylephrine. However, it increased myosin light chain kinase activity. These data suggest that CaMKII activity maintains intracellular calcium homeostasis but is not required for vasoconstriction of mesenteric arteries.

Smooth muscle signalling pathways in health and disease

Smooth muscle contractile activity is a major regulator of function of the vascular system, respiratory system, gastrointestinal system and the genitourinary systems. Malfunction of contractility in these systems leads to a host of clinical disorders, and yet, we still have major gaps in our understanding of the molecular mechanisms by which contractility of the differentiated smooth muscle cell is regulated. This review will summarize recent advances in the molecular understanding of the regulation of smooth muscle myosin activity via phosphorylation/dephosphorylation of myosin, the regulation of the accessibility of actin to myosin via the actin-binding proteins calponin and caldesmon, and the remodelling of the actin cytoskeleton. Understanding of the molecular 'players' should identify target molecules that could point the way to novel drug discovery programs for the treatment of smooth muscle disorders such as cardiovascular disease, asthma, functional bowel disease and pre-term labour.

CaM-kinaseII-dependent commitment to microcystin-induced apoptosis is coupled to cell budding, but not to shrinkage or chromatin hypercondensation

The protein phosphatase inhibitor microcystin-LR (MC) induced hepatocyte apoptosis mediated by the calcium-calmodulin-dependent multifunctional protein kinase II (CaMKII). CaMKII antagonists were added at various times after MC to define for how long the cells depended on CaMKII activity to be committed to execute the various parameters of death. Shrinkage and nonpolarized budding were reversible and not coupled to commitment. A critical commitment step was observed 15-20 min after MC (0.5 microM) addition. After this, CaMKII inhibitors no longer protected against polarized budding, DNA fragmentation, lost protein synthesis capability, and cell disruption. Commitment to chromatin hypercondensation occurred 40 min after MC addition. In conclusion, irreversible death commitment was coupled to polarized budding, but not to shrinkage or chromatin condensation. Antioxidant prevented chromatin condensation when given after the CaMKII-dependent commitment point, suggesting that CaMKII had mediated the accumulation of a second messenger of reactive oxygen species nature.

Effects of ML-7 and Y-27632 on carbachol- and endothelin-1-induced contraction of bovine trabecular meshwork

The trabecular meshwork is considered a smooth muscle like tissue contributing to aqueous outflow regulation and thus to regulation of intraocular pressure. An elevation in intraocular pressure is one of the greatest risk factors for most forms of glaucoma. We assume that contraction of trabecular meshwork reduces aqueous humor outflow and thus enhances intraocular pressure, whereas relaxation exerts the opposite effect. The present paper supports the hypothesis of the trabecular meshwork being a smooth muscle-like tissue. We perform measurements of isometric force in isolated bovine trabecular meshwork strips. Contractility of this tissue is induced by carbachol or endothelin-1. The contractile force is successfully inhibited by ML-7, a highly specific inhibitor of myosin light chain kinase. The contraction is also reduced in the presence of the RhoA kinase inhibitor Y-27632. We further describe the protein expression of smooth muscle myosin and its regulatory kinase, the myosin light chain kinase, in human and bovine trabecular meshwork cells. Additionally, the serine phosphorylation of myosin light chain kinase is shown. These data indicate that the trabecular meshwork expresses major contractility regulating proteins which are involved in tissue function. Inhibition of the signaling pathways which lead to myosin phosphorylation causes inhibition of contractile force in trabecular meshwork. According to our concept of aqueous humor outflow regulation, trabecular meshwork relaxing substances appear to be ideal antiglaucomatous drugs, leading to increased outflow facility.

Theoretical model of the interactions between Ca2+, calmodulin and myosin light chain kinase

Active Ca2+/calmodulin (CaM)-dependent myosin light chain kinase (MLCK) plays an important role in the process of MLC phosphorylation and consecutive smooth muscle contraction. Here, we propose a mathematical model of a detailed kinetic scheme describing interactions among Ca2+, CaM and MLCK and taking into account eight different aggregates. The main model result is the prediction of the Ca2+ dependent active form of MLCK, which is in the model taken as proportional to the concentration of Ca4CaM.MLCK complex. Wegscheider's condition is additionally applied as a constraint enabling the prediction of some parameter values that have not yet been obtained by experiments.

A signal transduction pathway model prototype II: Application to Ca2+-calmodulin signaling and myosin light chain phosphorylation

An agonist-initiated Ca(2+) signaling model for calmodulin (CaM) coupled to the phosphorylation of myosin light chains was created using a computer-assisted simulation environment. Calmodulin buffering was introduced as a module for directing sequestered CaM to myosin light chain kinase (MLCK) through Ca(2+)-dependent release from a buffering protein. Using differing simulation conditions, it was discovered that CaM buffering allowed transient production of more Ca(2+)-CaM-MLCK complex, resulting in elevated myosin light chain phosphorylation compared to nonbuffered control. Second messenger signaling also impacts myosin light chain phosphorylation through the regulation of myosin light chain phosphatase (MLCP). A model for MLCP regulation via its regulatory MYPT1 subunit and interaction of the CPI-17 inhibitor protein was assembled that incorporated several protein kinase subsystems including Rho-kinase, protein kinase C (PKC), and constitutive MYPT1 phosphorylation activities. The effects of the different routes of MLCP regulation depend upon the relative concentrations of MLCP compared to CPI-17, and the specific activities of protein kinases such as Rho and PKC. Phosphorylated CPI-17 (CPI-17P) was found to dynamically control activity during agonist stimulation, with the assumption that inhibition by CPI-17P (resulting from PKC activation) is faster than agonist-induced phosphorylation of MYPT1. Simulation results are in accord with literature measurements of MLCP and CPI-17 phosphorylation states during agonist stimulation, validating the predictive capabilities of the system.

A signal transduction pathway model prototype I: From agonist to cellular endpoint

The postgenomic era is providing a wealth of information about the genes involved in many cellular processes. However, the ability to apply this information to understanding cellular signal transduction is limited by the lack of tools that quantitatively describe cellular signaling processes. The objective of the current studies is to provide a framework for modeling cellular signaling processes beginning at a plasma membrane receptor and ending with a measurable endpoint in the signaling process. Agonist-induced Ca(2+) mobilization coupled to down stream phosphorylation events was modeled using knowledge of in vitro and in vivo process parameters. The simulation process includes several modules that describe cellular processes involving receptor activation phosphoinositide metabolism, Ca(2+)-release, and activation of a calmodulin-dependent protein kinase. A Virtual Cell-based simulation was formulated using available literature data and compared to new and existing experimental results. The model provides a new approach to facilitate hypothesis-driven investigation and experimental design based upon simulation results. These investigations may be directed at the timing of multiple phosphorylation/dephosphorylation events affecting key enzymatic activities in the signaling pathway.

The long myosin light chain kinase is differentially phosphorylated during interphase and mitosis

We have shown previously that the activity of the long myosin light chain kinase (MLCK) is cell cycle regulated with a decrease in specific activity during mitosis that can be restored following treatment with alkaline phosphatase. To better understand the role and significance of phosphorylation in regulating MLCK function during mitosis, we examined the phosphorylation state of in vivo derived MLCK. Phosphoamino acid analysis and phosphopeptide mapping demonstrate that the long MLCK is differentially phosphorylated on serine residues during interphase and mitosis with the majority of the phosphorylation sites located within the N-terminal IgG domain. Biochemical assays show that Aurora B binds and phosphorylates the IgG domain of the long MLCK. In addition, phosphopeptide maps of the endogenous full-length MLCK from mitotic cells and in vitro phosphorylated IgG domain demonstrate that Aurora B phosphorylates the same sites as those observed in vivo. Altogether, these studies suggest that the long MLCK may be a cellular target for Aurora B during mitosis.

A fluorescent resonant energy transfer-based biosensor reveals transient and regional myosin light chain kinase activation in lamella and cleavage furrows

Approaches with high spatial and temporal resolution are required to understand the regulation of nonmuscle myosin II in vivo. Using fluorescence resonance energy transfer we have produced a novel biosensor allowing simultaneous determination of myosin light chain kinase (MLCK) localization and its [Ca2+]4/calmodulin-binding state in living cells. We observe transient recruitment of diffuse MLCK to stress fibers and its in situ activation before contraction. MLCK is highly active in the lamella of migrating cells, but not at the retracting tail. This unexpected result highlights a potential role for MLCK-mediated myosin contractility in the lamella as a driving force for migration. During cytokinesis, MLCK was enriched at the spindle equator during late metaphase, and was maximally activated just before cleavage furrow constriction. As furrow contraction was completed, active MLCK was redistributed to the poles of the daughter cells. These results show MLCK is a myosin regulator in the lamella and contractile ring, and pinpoints sites where myosin function may be mediated by other kinases.

Ca2+/calmodulin-dependent protein kinase II is required for microcystin-induced apoptosis

The potent natural toxins microcystin, nodularin, and okadaic acid act rapidly to induce apoptotic cell death. Here we show that the apoptosis correlates with protein phosphorylation events and can be blocked by protein kinase inhibitors directed against the multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). The inhibitors used comprised a battery of cell-permeable protein kinase antagonists and CaMKII-directed peptide inhibitors introduced by microinjection or enforced expression. Furthermore, apoptosis could be induced by enforced expression of active forms of CaMKII but not with inactive CaMKII. It is concluded that the apoptogenic toxins, presumably through their known ability to inhibit serine/threonine protein phosphatases, can cause CaMKII-dependent phosphorylation events leading to cell death.

Localization and activity of myosin light chain kinase isoforms during the cell cycle

Phosphorylation on Ser 19 of the myosin II regulatory light chain by myosin light chain kinase (MLCK) regulates actomyosin contractility in smooth muscle and vertebrate nonmuscle cells. The smooth/nonmuscle MLCK gene locus produces two kinases, a high molecular weight isoform (long MLCK) and a low molecular weight isoform (short MLCK), that are differentially expressed in smooth and nonmuscle tissues. To study the relative localization of the MLCK isoforms in cultured nonmuscle cells and to determine the spatial and temporal dynamics of MLCK localization during mitosis, we constructed green fluorescent protein fusions of the long and short MLCKs. In interphase cells, localization of the long MLCK to stress fibers is mediated by five DXRXXL motifs, which span the junction of the NH(2)-terminal extension and the short MLCK. In contrast, localization of the long MLCK to the cleavage furrow in dividing cells requires the five DXRXXL motifs as well as additional amino acid sequences present in the NH(2)-terminal extension. Thus, it appears that nonmuscle cells utilize different mechanisms for targeting the long MLCK to actomyosin structures during interphase and mitosis. Further studies have shown that the long MLCK has twofold lower kinase activity in early mitosis than in interphase or in the early stages of postmitotic spreading. These findings suggest a model in which MLCK and the myosin II phosphatase (Totsukawa, G., Y. Yamakita, S. Yamashiro, H. Hosoya, D.J. Hartshorne, and F. Matsumura. 1999. J. Cell Biol. 144:735-744) act cooperatively to regulate the level of Ser 19-phosphorylated myosin II during mitosis and initiate cytokinesis through the activation of myosin II motor activity.

Ca(2+) movement from leaky sarcoplasmic reticulum during contraction of rat arterial smooth muscles

To examine the Ca(2+) buffering function of the sarcoplasmic reticulum during arterial contraction, we studied Ca(2+) movement during stimulation with K(+) or norepinephrine in arteries with a leaky sarcoplasmic reticulum. Responses were compared in endothelium-denuded strips of femoral, mesenteric and carotid arteries of the rat. To make the sarcoplasmic reticulum leaky to Ca(2+), Ca(2+)-induced Ca(2+) release channels of the sarcoplasmic reticulum were locked open by treatment with ryanodine plus caffeine. After ryanodine treatment, the contractile responses to K(+) (3-20 mM) were augmented when compared with control responses in femoral and mesenteric arteries, but were inhibited in the carotid artery. Similar results were obtained when the contractile responses to norepinephrine were determined. The inhibition by ryanodine of the K(+)- or norepinephrine-contractions seen in the carotid artery was reversed by pretreatment with cyclopiazonic acid (10 microM), an inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, but was not by charybdotoxin (100 nM), a blocker of Ca(2+)-activated K(+) channels. We conclude that (1) after ryanodine treatment, Ca(2+) entering from the extracellular space during stimulation with K(+) or norepinephrine is first taken up into the leaky sarcoplasmic reticulum and then reaches the myofilaments in femoral and mesenteric arteries, while in the carotid artery, Ca(2+) leaked from the sarcoplasmic reticulum reaches mainly the plasma membrane from where it is extruded into the extracellular space, and (2) the different movement of Ca(2+) may be due to the relative location of the sarcoplasmic reticulum in the smooth muscle cell of each artery.

Inhibition of CaM kinase II activation and force maintenance by KN-93 in arterial smooth muscle

Ca(+)/calmodulin-dependent protein kinase II (CaM kinase II) has been implicated in the regulation of smooth muscle contractility. The goals of this study were to determine: 1) to what extent CaM kinase II is activated by contractile stimuli in intact arterial smooth muscle, and 2) the effect of a CaM kinase II inhibitor (KN-93) on CaM kinase II activation, phosphorylation of myosin regulatory light chains (MLC(20)), and force. Both histamine (1 microM) and KCl depolarization activated CaM kinase II with a time course preceding maximal force development, and suprabasal CaM kinase II activation was sustained during tonic contractions. CaM kinase II activation was inhibited by KN-93 pretreatment (IC(50) approximately 1 microM). KN-93 inhibited histamine-induced tonic force maintenance, whereas early force development and MLC(20) phosphorylation responses during the entire time course were unaffected. Both force development and maintenance in response to KCl were inhibited by KN-93. Rapid increases in KCl-induced MLC(20) phosphorylation were also inhibited by KN-93, whereas steady-state MLC(20) phosphorylation responses were unaffected. In contrast, phorbol 12,13-dibutyrate (PDBu) did not activate CaM kinase II and PDBu-stimulated force development was unaffected by KN-93. Thus KN-93 appears to target a step(s) essential for force maintenance in response to physiological stimuli, suggesting a role for CaM kinase II in regulating tonic contractile responses in arterial smooth muscle. Pharmacological activation of protein kinase C bypasses the KN-93 sensitive step.

PKC-dependent regulation of transepithelial resistance: roles of MLC and MLC kinase

The mechanisms by which protein kinase C (PKC) activation results in increased transepithelial resistance (TER) are unknown [G. Hecht, B. Robinson, and A. Koutsouris. Am. J. Physiol. 266 (Gastrointest. Liver Physiol. 29): G214-G221, 1994]. We have previously shown that phosphorylation of the regulatory light chain of myosin II (MLC) is associated with decreases in TER and have suggested that contraction of the perijunctional actomyosin ring (PAMR) increases tight junction (TJ) permeability [J. R. Turner, B. K. Rill, S. L. Carlson, D. Carnes, R. Kerner, R. J. Mrsny, and J. L. Madara. Am. J. Physiol. 273 (Cell Physiol. 42): C1378-C1385, 1997]. We therefore hypothesized that PKC activation alters TER via relaxation of the PAMR. Activation of PKC by the phorbol ester phorbol 12-myristate 13-acetate (PMA) resulted in a progressive dose-dependent increase in TER that was apparent within 15 min (111% of controls) and maximal within 2 h (142% of controls). Similar increases were induced by a diacylglycerol analog, and the effects of both PMA and the diacylglycerol analog were prevented by the PKC inhibitor bisindolylmaleimide I. PMA treatment caused progressive decreases in MLC phosphorylation, by 12% at 15 min and 41% at 2 h. Phosphorylation of MLC kinase (MLCK) increased by 64% within 15 min of PMA treatment and was stable over 2 h (51% greater than that of controls). Thus increases in MLCK phosphorylation preceded decreases in MLC phosphorylation. These data suggest that PKC regulates TER via decreased phosphorylation of MLC, possibly due to inhibitory phosphorylation of MLCK. The decreased phosphorylation of MLC likely reduces PAMR tension, leading to decreased TJ permeability.

Inhibitory effect of phosphorylated myosin light chain kinase on the ATP-dependent actin-myosin interaction

Myosin light chain kinase (MLCK) phosphorylates the regulatory light chain of myosin in the presence of Ca(2+) and calmodulin (Ca(2+)-CaM) so that myosin can interact with actin filaments. MLCK has another activity that is not attributable to this kinase activity, i.e., it inhibits the ATP-dependent movement of actin filaments on a myosin-coated glass surface. MLCK itself can be phosphorylated at site A and site B with a few kinases. The phosphorylation at site A reduces kinase activity. However, we have no knowledge as to how phosphorylation of MLCK affects the inhibitory activity of MLCK. When MLCK was phosphorylated at site B, it exerted an inhibitory effect on the movement in much lower concentrations. When Ca(2+)-CaM or ML-9 was present, the inhibition was reduced. The reduction was less when the movement was arrested by the MLCK phosphorylated at site B. This observation was explained by the increase in the affinity of MLCK to myosin upon the phosphorylation at site B.

Signaling mechanisms underlying the vascular myogenic response

The vascular myogenic response refers to the acute reaction of a blood vessel to a change in transmural pressure. This response is critically important for the development of resting vascular tone, upon which other control mechanisms exert vasodilator and vasoconstrictor influences. The purpose of this review is to summarize and synthesize information regarding the cellular mechanism(s) underlying the myogenic response in blood vessels, with particular emphasis on arterioles. When necessary, experiments performed on larger blood vessels, visceral smooth muscle, and even striated muscle are cited. Mechanical aspects of myogenic behavior are discussed first, followed by electromechanical coupling mechanisms. Next, mechanotransduction by membrane-bound enzymes and involvement of second messengers, including calcium, are discussed. After this, the roles of the extracellular matrix, integrins, and the smooth muscle cytoskeleton are reviewed, with emphasis on short-term signaling mechanisms. Finally, suggestions are offered for possible future studies.

Calculation of a Gap restoration in the membrane skeleton of the red blood cell: possible role for myosin II in local repair

Human red blood cells contain all of the elements involved in the formation of nonmuscle actomyosin II complexes (V. M. Fowler. 1986. J. Cell. Biochem. 31:1-9; 1996. Curr. Opin. Cell Biol. 8:86-96). No clear function has yet been attributed to these complexes. Using a mathematical model for the structure of the red blood cell spectrin skeleton (M. J. Saxton. 1992. J. Theor. Biol. 155:517-536), we have explored a possible role for myosin II bipolar minifilaments in the restoration of the membrane skeleton, which may be locally damaged by major mechanical or chemical stress. We propose that the establishment of stable links between distant antiparallel actin protofilaments after a local myosin II activation may initiate the repair of the disrupted area. We show that it is possible to define conditions in which the calculated number of myosin II minifilaments bound to actin protofilaments is consistent with the estimated number of myosin II minifilaments present in the red blood cells. A clear restoration effect can be observed when more than 50% of the spectrin polymers of a defined area are disrupted. It corresponds to a significant increase in the spectrin density in the protein free region of the membrane. This may be involved in a more complex repair process of the red blood cell membrane, which includes the vesiculation of the bilayer and the compaction of the disassembled spectrin network.

A two-dimensional peptide gel electrophoresis system for phosphopeptide mapping and amino acid sequencing

A novel two-dimensional electrophoresis system to be carried out on polyacrylamide gels under nondenaturing conditions was developed to efficiently fractionate the peptides resulting from endoproteinase digestion of 32P-labeled proteins. In particular, nondenaturing gel isoelectric focusing was combined with alkaline 40% polyacrylamide gel electrophoresis to generate phosphopeptide maps with high reproducibility, thus allowing both protein fingerprinting and comparative analysis of different samples. The potential application of this method for subsequent amino acid sequencing of the isolated phosphopeptides was further demonstrated by successful manual and automated Edman sequencing. Taken together these data show that such a simple and precise approach is suitable for both analytical and preparative aims.

Biochemical regulation of the nonmuscle myosin light chain kinase isoform in bovine endothelium

Specific models of vascular permeability are critically dependent on myosin light chain phosphorylation, a reaction catalyzed by a novel high molecular-weight (214 kD) Ca2+/calmodulin (CaM)-dependent myosin light chain kinase (MLCK) isoform recently cloned in human endothelium (Am. J. Respir. Cell Mol. Biol., 1997;16:489-494). To evaluate mechanisms of endothelial cell (EC) barrier dysfunction evoked by the serine protease thrombin, we studied the regulation of the 214-kD EC MLCK isoform expressed in bovine endothelium. The EC MLCK isoform bound biotinylated CaM in a Ca2+-dependent manner and co-immunoprecipitated in a functional complex with myosin, actin, and CaM. Thrombin rapidly increased MLCK activity in concert with time-dependent translocation of the enzyme to the actin cytoskeleton. To evaluate whether EC MLCK activity was regulated by direct phosphorylation, amino acid sequence analysis identified multiple potential EC MLCK sites for Ser/Thr phosphorylation, including highly conserved phosphorylation sites for cyclic adenosine monophosphate-dependent protein kinase A (PKA) adjacent to the CaM-binding region. EC MLCK activity was attenuated by either PKA-mediated MLCK phosphorylation or inhibition of Ser/Thr phosphatase activity (fluoride or calyculin), which significantly increased MLCK phosphorylation while decreasing MLCK activity (3- to 4-fold decrease). In summary, although the EC MLCK isoform exhibits multiple features intrinsic to this family of kinases, thrombin-mediated EC contraction and barrier dysfunction requires increased EC MLCK-actin interaction and MLCK translocation to the cytoskeleton. EC MLCK activity appears to be highly dependent upon the phosphorylation status of this key contractile effector.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Definition of optimal substrate recognition motifs of Ca2+-calmodulin-dependent protein kinases IV and II reveals shared and distinctive features

The substrate recognition determinants of Ca2+-calmodulin-dependent protein kinase (CaMK) IV and CaMKIIalpha were investigated using peptide substrates modeled on the amino acid sequence encompassing Ser-9 of synapsin I. For both kinases, hydrophobic residues (Leu or Phe) at the -5 position, are well tolerated, whereas non-hydrophobic residues (Arg, Ala, or Asp) decrease Vmax/Km by 55- to >4000-fold. At the -3 position, substitution of Ala for Arg leads to decreases of 99- and 343- fold in Vmax/Km for CaMKIV and CaMKIIalpha, respectively. For both kinases, the nature of the residues occupying the -4, -1, and + 4 positions exerts relatively little influence on phosphorylation kinetics. CaMKIV and CaMKIIalpha respond differently to substitutions at the -2 and +1 positions. Substitution of Arg at the -2 position with non-basic residues (Gln or Ala) leads to 6-fold decreases in Vmax/Km for CaMKIV, but 17-28-fold increases for CaMKIIalpha. Additionally, peptides containing Leu, Asp, or Ala at the +1 position are phosphorylated with similar efficiencies by CaMKIV, whereas the Leu-substituted peptide is preferred by CaMKIIalpha (by a factor of 5.8-9.7-fold). Thus, CaMKIV and CaMKIIalpha preferentially phosphorylate substrates with the motifs: Hyd-X-Arg-X-X-Ser*/Thr*, and Hyd-X-Arg-NB-X-Ser*/Thr*-Hyd, respectively, where Hyd represents a hydrophobic, X any, and NB a non-basic amino acid residue. The different specificities of the two kinases may contribute to their targeting to distinct physiological substrates during Ca2+-dependent cellular events.

Role of vascular nitric oxide in physiological and pathological conditions

This review describes the ability of certain diseases, such as essential hypertension, atherosclerosis, angina, and vasospasm, to reduce vascular nitric oxide (NO) formation or to increase its metabolism. In contrast, others, such as hypotension, sepsis, stroke, myocardial depression, and inflammatory responses, increase NO synthesis. The mechanism implicated in the changes in the formation and metabolism of NO are described. To prevent or treat these pathological processes, in which a deficiency in vascular NO formation plays a causative role, NO may be provided through methods such as direct NO administration or indirect NO supply through either NO donors or L-arginine, which facilitates NO formation.

Nitric oxide and its role in the cardiovascular system

Nitric oxide (NO) is a ubiquitous, naturally occurring molecule found in a variety of cell types and organ systems. In the cardiovascular system, NO is an important determinant of basal vascular tone, prevents platelet activation, limits leukocyte adhesion to the endothelium, and regulates myocardial contractility. NO may also play a role in the pathogenesis of common cardiovascular disorders, including hypotension accompanying shock states, essential hypertension, and atherosclerosis. In this review, we discuss the biochemistry of NO and focus on its biology and pathophysiology in the cardiovascular system.

Phosphorylation of eukaryotic protein synthesis initiation factor 4E at Ser-209

Initiation factor 4E (eIF-4E) binds to the m7GTP-containing cap of eukaryotic mRNA and facilitates the entry of mRNA into the initiation cycle of protein synthesis. eIF-4E is a phosphoprotein, and the phosphorylated form binds to mRNA caps 3-4-fold more tightly than the nonphosphorylated form. A previous study indicated that the major phosphorylation site was Ser-53 (Rychlik, W., Russ, M. A., and Rhoads, R. E. (1987) J. Biol. Chem. 262, 10434-10437). In the present study, we synthesized the phosphopeptide expected to result from tryptic digestion of eIF-4E, O-phosphoseryllysine. Surprisingly, the tryptic and synthetic phosphopeptides did not comigrate electrophoretically. Accordingly, we redetermined the phosphorylation site by isolating a chymotryptic phosphopeptide on reverse phase high performance liquid chromatography. The peptide was sequenced by Edman degradation and corresponded to 198QSHADTATKSGSTTKNRF215. The site of phosphorylation was determined to be Ser-209 by four methods: the increase in the ratio of dehydroalanine to serine derivatives during Edman degradation, the release of 32P, the further digestion of the chymotryptic phosphopeptide with trypsin, Glu-C, and Asp-N, and site-directed mutagenesis of eIF-4E cDNA. The S209A variant was not phosphorylated in a rabbit reticulocyte lysate system, whereas the wild-type, S53A, and S207A variants were. This site falls within the consensus sequence for phosphorylation by protein kinase C.

Autophosphorylation of smooth muscle myosin light chain kinase at its regulatory domain

Autophosphorylation of smooth muscle myosin light chain kinase was initially reported by Foyt et al. [Foyt, H. L., & Means, A. R. (1985) J. Cyclic Nucleotide Protein Phosphorylation Res. 260, 8978-8983], however, the effects of autophosphorylation on the kinase activity as well as the location of the sites have not been elucidated. Here we demonstrate that MLCK is autophosphorylated at three sites, Thr 803, Ser 815, and Ser 823, and this phosphorylation alters MLCK activity. Two phosphorylation sites are located in the regulatory domain of the kinase, the threonine site toward the autoinhibitory region and the serine site (Ser 815) in close proximity to the calmodulin anchoring site. The autophosphorylation was significantly inhibited by the binding of calmodulin. The autophosphorylation at Thr 803 is an intramolecular process, and the alignment of the basic amino acid residues nearby Thr 803 was highly homologous to the phosphorylation site of myosin light chain, suggesting that the regulatory site is in close proximity to the catalytic site in the three-dimensional structure. The phosphorylation at the threonine site activated the calmodulin-independent activity while the phosphorylation at the serine site inhibited the calmodulin-dependent activity due to a decrease in the affinity for calmodulin. This finding shows another example of the activation of calmodulin-dependent kinases by autophosphorylation at its autoinhibitory region and provides a new clue for understanding the calmodulin/MLCK signalling pathway.

Plasmodium falciparum calcium-dependent protein kinase phosphorylates proteins of the host erythrocytic membrane

The unusual Ca(2+)-dependent protein kinase from Plasmodium falciparum (PfCPK) [1], whose gene structure and expression in bacteria have been reported [1], was purified to homogeneity. The purified recombinant kinase has a native molecular mass of 62,000, is activated by Ca2+ (K0.5 = 15 microM) in the presence of Mg2+ or Mn2+, and can associate with 45Ca2+. The activation by Ca2+ could be partially replaced by Mn2+, but not by Zn2+ or Mg2+. PfCPK preferentially phosphorylated casein and histone H1. The Km and Vmax for Mg2+ ATP were 26 microM and 70 nmol min-1 mg-1, respectively, with casein as substrate; and 34 microM and 143 nmol min-1 mg-1, respectively, with histone H1 as substrate. The kinase undergoes autophosphorylation on both serine and threonine residues. Calmodulin antagonists (calmidazolium, trifluoperazine, N-[6-aminohexyl]-5-chloro-1-napthalene-sulfonamide, and ophiobolin A) could inhibit the kinase activation, but much higher concentrations of the antagonists are needed than was required to inhibit calmodulin-mediated effects. PfCPK preferentially phosphorylates proteins of the host erythrocytic membrane in vitro but phosphorylates parasitic proteins only to a minor extent. The selectivity of the phosphorylation may be partially controlled by phosphatidylserine which is bound to some of the erythrocytic membrane proteins. Using a rabbit polyclonal antiserum against the recombinant protein, the kinase was found to be mainly expressed in the ring and schizont stages, and mainly localized in the parasitic membrane-organelle fraction and partially localized on the erythrocytic membrane.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

Nitric oxide and the cardiovascular system

Nitric oxide is a simple diatomic molecule that functions as a cellular messenger in a number of organ systems, included among which is the cardiovascular system. Serving several physiologic roles in the cardiovascular system, nitric oxide is an important determinant of basal vascular tone, regulates myocardial contractility, and modulates platelet-vessel wall interactions. From the perspective of cardiovascular pathophysiology, nitric oxide has been implicated in the pathogenesis of essential hypertension, atherosclerosis, and the hypotension associated with shock states. This review will focus in detail on the chemistry, biology, and pathobiology of nitric oxide as it relates to cardiovascular function.

Phosphorylation of myosin light chain kinase: a cellular mechanism for Ca2+ desensitization

Phosphorylation of the regulatory light chain of myosin by the Ca2+/calmodulin-dependent myosin light chain kinase plays an important role in smooth muscle contraction, nonmuscle cell shape changes, platelet contraction, secretion, and other cellular processes. Smooth muscle myosin light chain kinase is also phosphorylated, and recent results from experiments designed to satisfy the criteria of Krebs and Beavo for establishing the physiological significance of enzyme phosphorylation have provided insights into the cellular regulation and function of this phosphorylation in smooth muscle. The multifunctional Ca2+/calmodulin-dependent protein kinase II phosphorylates myosin light chain kinase at a regulatory site near the calmodulin-binding domain. This phosphorylation increases the concentration of Ca2+/calmodulin required for activation and hence increases the Ca2+ concentrations required for myosin light chain kinase activity in cells. However, the concentration of cytosolic Ca2+ required to effect myosin light chain kinase phosphorylation is greater than that required for myosin light chain phosphorylation. Phosphorylation of myosin light chain kinase is only one of a number of mechanisms used by the cell to down regulate the Ca2+ signal in smooth muscle. Since both smooth and nonmuscle cells express the same form of myosin light chain kinase, this phosphorylation may play a regulatory role in cellular processes that are dependent on myosin light chain phosphorylation.

Effects of calyculin A on tension and myosin phosphorylation in skinned smooth muscle of the rabbit mesenteric artery

1. Using beta-escin and ionomycin-treated skinned smooth muscle strips of the rabbit mesenteric artery, the effects of calyculin A (CL-A, an inhibitor of type 1 and 2A phosphatases) on mechanical activities, phosphorylation of myosin light chain (MLC) and the relationship between the two were studied in Ca(2+)-free solution containing 4 mM EGTA and these effects were compared with those evoked by Ca2+. 2. The threshold concentration of Ca2+ required to increase either tension or MLC-phosphorylation was 0.1 microM and maximum effects were obtained at 10 microM. MLC was mainly monophosphorylated, rather than diphosphorylated, in the presence of Ca2+. ED50 value for Ca2+ was 0.54 microM for either tension or MLC-phosphorylation. The relationship between tension and MLC-phosphorylation is linear in the pCa range 7-5.5. 3. In Ca(2+)-free solution (containing either 20 mM EGTA or 4 mM EGTA with or without 4 mM BAPTA), 3 microM CL-A produced a contraction, the maximum amplitude of which was similar to that evoked by 10 microM Ca2+. CL-A (0.03-3 microM) concentration-dependently increased both tension and MLC-phosphorylation in Ca(2+)-free solution containing 4 mM EGTA. The threshold concentration of CL-A required for the increase in either tension or MLC-phosphorylation was 0.03 microM and maximum effects were obtained at 3 microM. In the presence of CL-A, MLC was not only monophosphorylated but also diphosphorylated. ED50 values for CL-A were 0.39 microM for tension, 0.44 microM for the monophosphorylated form of MLC and 0.54 microM for all phosphorylated (mono + di) forms. The relationship between tension and the monophosphorylated form of MLC was linear over the concentration range studied and was similar to that for Ca2+. 4. H-7 (3 microM, an inhibitor of protein kinase C) inhibited neither the tension nor phosphorylation of MLC induced by 10 microM Ca2+ or 3 microM CL-A. At a high concentration (30 microM), H-7 slightly inhibited both the tension and phosphorylation of MLC induced by either stimulant without a change in the tension-MLC-phosphorylation relationship. KN-62, an inhibitor of Ca(2+)-calmodulin-dependent protein kinase II, did not modify either the tension or the phosphorylation of MLC induced by 10 microM Ca2+ or 3 microM CL-A. CK-II, another inhibitor of Ca(2+)-calmodulin-dependent protein kinase II, did not inhibit the contraction induced by 3 microM CL-A. 5. SM-1 (0.03-0.3 mM) and ML-9 (0.1 and 0.3 mM), inhibitors of MLC-kinase, each lowered the resting level of MLC-phosphorylation in Ca2+-free solution and also inhibited both the tension and MLC-phosphorylation induced by 10 microM Ca2+ or 3 microM CL-A, in a concentration-dependent manner.Neither SM-1 nor ML-9 modified the relationship between tension and either monophosphorylated or all phosphorylated (mono + di) forms of MLC in the presence of Ca2+ or CL-A.6. In a solution containing MgITP (the substrate for myosin ATPase but not for MLC-kinase) with no MgATP, 10 microM Ca2+ failed to produce contraction. Under these conditions, the amplitude of the contraction induced by 3 microM CL-A was greatly diminished in comparison with that induced in the presence of MgATP.7. The present results suggest that in smooth muscle cells of the rabbit mesenteric artery, CL-A in Ca2+-free solution, produces a maximum contraction through an indirect activation of Ca2+-calmodulin independent(constitutively active) MLC-kinase via its inhibitory action on MLC-phosphatases. Based on this evidence, it is hypothesized that, in these cells, a constitutively active MLC-kinase may be present, though its action may be concealed by that of endogenous MLC-phosphatase.

Multifunctional Ca2+/calmodulin-dependent protein kinase

Multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) is a prominent mediator of neurotransmitters which elevate Ca2+. It coordinates cellular responses to external stimuli by phosphorylating proteins involved in neurotransmitter synthesis, neurotransmitter release, carbohydrate metabolism, ion flux and neuronal plasticity. Structure/function studies of CaM kinase have provided insights into how it decodes Ca2+ signals. The kinase is kept relatively inactive in its basal state by the presence of an autoinhibitory domain. Binding of Ca2+/calmodulin eliminates this inhibitory constraint and allows the kinase to phosphorylate its substrates, as well as itself. This autophosphorylation significantly slows dissociation of calmodulin, thereby trapping calmodulin even when Ca2+ levels are subthreshold. The kinase may respond particularly well to multiple Ca2+ spikes since trapping may enable a spike frequency-dependent recruitment of calmodulin with each successive Ca2+ spike leading to increased activation of the kinase. Once calmodulin dissociates, CaM kinase remains partially active until it is dephosphorylated, providing for an additional period in which its response to brief Ca2+ transients is potentiated.

Molecular cloning of the chicken gizzard telokin gene and cDNA

Telokin is a protein which consiste of the C-terminal portion of smooth muscle myosin light chain kinase (MLCK) (M. Ito, R. Dabrowska, V. Guerriero, Jr., and D. J. Hartshone (1989) J. Biol. Chem. 264, 13971-13974). In this study, the chicken gizzard telokin cDNA and gene were cloned and analyzed. The telokin cDNA coded 157 amino acid residues which were completely identical to the C-terminal portion of the amino acid sequence of chicken gizzard MLCK. The telokin gene was coded in a 6.3-kb EcoRI genomic fragment and it consisted of three exons. The 5'-leader sequence of the telokin cDNA and genomic sequence revealed that the telokin gene was included in the MLCK gene and the transcription started in the intronic sequence of the MLCK gene. The analysis of the telokin gene suggests that the telokin expression was under the control of an independent promotor. Northern blotting and the reverse transcriptase-polymerase chain reaction methods revealed that telokin was expressed not only in chicken gizzard but also in chicken heart, lung, intestine, and skeletal muscle although the levels of the expression in the latter were much less than that in the gizzard.

Regulation of contraction and relaxation in arterial smooth muscle

Intracellular calcium concentration ([Ca2+]i)-dependent activation of myosin light chain kinase and its phosphorylation of the 20-kd light chain of myosin is generally considered the primary mechanism responsible for regulation of contractile force in arterial smooth muscle. However, recent data suggest that the relation between [Ca2+]i and myosin light chain phosphorylation is variable and depends on the form of stimulation. The dependence of myosin phosphorylation on [Ca2+]i has been termed the "[Ca2+]i sensitivity of phosphorylation." The [Ca2+]i sensitivity of phosphorylation is "high" when relatively small increases in [Ca2+]i induce a large increase in myosin phosphorylation. Conversely, the [Ca2+]i sensitivity of phosphorylation is "low" when relatively large increases in [Ca2+]i are required to induce a small increase in myosin phosphorylation. There are two proposed mechanisms for changes in the [Ca2+]i sensitivity of phosphorylation: Ca(2+)-dependent decreases in the [Ca2+]i sensitivity of phosphorylation induced by phosphorylation of myosin light chain kinase by Ca(2+)-calmodulin protein kinase II and agonist-dependent increases in the [Ca2+]i sensitivity of phosphorylation by inhibition of a myosin light chain phosphatase. I will review the proposed mechanisms responsible for the regulation of [Ca2+]i and the [Ca2+]i sensitivity of phosphorylation in arterial smooth muscle.

Na(+)-Ca2+ exchange, myoplasmic Ca2+ concentration, and contraction of arterial smooth muscle

Na(+)-Ca2+ exchange is proposed to be an important regulator of myoplasmic intracellular Ca2+ concentration ([Ca2+]i) and contraction in vascular smooth muscle. We investigated the role of Na(+)-Ca2+ exchange in regulating [Ca2+]i in swine carotid arterial tissues that were loaded with aequorin to allow simultaneous measurement of [Ca2+]i and force. Reversal of Na(+)-Ca2+ exchange, by reduction of extracellular Na+ concentration ([Na+]o) to 1.2 mM, induced a large increase in aequorin-estimated [Ca2+]i and a low [Ca2+]i sensitivity. The contraction induced by 1.2 mM [Na+]o was partially caused by depolarization and opening of L-type Ca2+ channels because 10 microM diltiazem partially attenuated the 1.2 mM [Na+]o-induced increases in [Ca2+]i. High dose ouabain (10 microM), a putative endogenous Na+,K(+)-ATPase inhibitor, increased both [Ca2+]i and force. However, the increases in [Ca2+]i and force were mostly blocked by 10 microM phentolamine, suggesting the predominant effect of ouabain was to increase norepinephrine release from nerve terminals. In the presence of 10 microM phentolamine, 10 microM ouabain slightly accentuated 1 microM histamine-induced increases in [Ca2+]i and force. The ouabain dose necessary to induce contraction in the absence of phentolamine was significantly less than the ouabain dose necessary to accentuate histamine-induced contractions in the presence of phentolamine. These results suggest that Na(+)-Ca2+ exchange exists in swine arterial smooth muscle. These data also suggest that ouabain (which should increase [Na+]i and inhibit Na(+)-Ca2+ exchange) primarily enhances contractile function in the swine carotid artery by releasing catecholamines from nerve terminals; direct action of Na+,K(+)-ATPase inhibitors on smooth muscle appears to occur only with very high doses.

Cytokinesis is more rapid in Ha-T24-ras transfected rat embryo fibroblasts than in non-transfected control cells

It has long been known that neoplastic cells are characterized by increases in cell motility. Earlier studies from this laboratory indicated that mitotic events were also altered in many tumor and experimentally transformed cells and that this included increases in metaphase duration and a reduction in the duration of cytokinesis. The studies presented in this paper were done to determine whether or not transfection of normal rat embryo fibroblasts by the Ha-T24-ras oncogene could also produce such alterations in mitotic events. The results obtained with the use of time lapse video microscopy indicate that neither the duration of metaphase nor the rate of chromosome movement during anaphase was altered but that the rate of furrow progression during cytokinesis occurred at a significantly more rapid rate. Thus, the cellular alterations induced by transfection with Ha-T24-ras accelerate microfilament-dependent cytokinetic furrowing without significant effects on microtubule-dependent mitotic events. One of several possible mechanisms that could account for these observations involves a down regulation of protein kinase C which has been reported to occur in many neoplastic cells including those transformed by ras. Such a hypothesis could also have broader implications because it may be applicable to the increase in motility and metastatic activity generally observed in transformed cells.