The integration of receptor-regulated intracellular calcium release and calcium entry across the plasma membrane

Vascular Smooth Muscle Cell Signaling Mechanisms for Contraction to Angiotensin II and Endothelin-1

Vasoactive peptides, such as endothelin-1 and angiotensin II are recognized by specific receptor proteins located in the cell membrane of target cells. Following receptor recognition, the specificity of the cellular response is achieved by G-protein coupling of ligand binding to the regulation of intracellular effectors. These intracellular effectors will be the subject of this brief review on contractile activity initiated by endothelin-1 and angiotensin II.Activation of receptors by endothelin-1 and angiotensin II in smooth muscle cells results in phopholipase C (PLC) activation leading to the generation of the second messengers insitol trisphosphate (IP(3)) and diacylglycerol (DAG). IP(3) stimulates intracellular Ca(2+) release from the sarcoplasmic reticulum and DAG causes protein kinase C (PKC) activation. Additionally, different Ca(2+) entry channels, such as voltage-operated (VOC), receptor-operated (ROC), and store-operated (SOC) Ca(2+) channels, as well as Ca(2+)-permeable nonselective cation channels (NSCC), are involved in the elevation of intracellular Ca(2+) concentration. The elevation in intracellular Ca(2+) is transient and initiates contractile activity by a Ca(2+)-calmodulin interaction, stimulating myosin light chain (MLC) phosphorylation. When the Ca(2+) concentration begins to decline, Ca(2+)-sensitization of the contractile proteins is signaled by the RhoA/Rho-kinase pathway to inhibit the dephosphorylation of MLC phosphatase (MLCP) thereby maintaining force generation. Removal of Ca(2+) from the cytosol and stimulation of MLCP initiates the process of smooth muscle relaxation. In pathological conditions such as hypertension, alterations in these cellular signaling components can lead to an over stimulated state causing maintained vasoconstriction and blood pressure elevation.

Evolving concepts on the age-related changes in "muscle quality"

The deterioration of skeletal muscle with advancing age has long been anecdotally recognized and has been of scientific interest for more than 150 years. Over the past several decades, the scientific and medical communities have recognized that skeletal muscle dysfunction (e.g., muscle weakness, poor muscle coordination, etc.) is a debilitating and life-threatening condition in the elderly. For example, the age-associated loss of muscle strength is highly associated with both mortality and physical disability. It is well-accepted that voluntary muscle force production is not solely dependent upon muscle size, but rather results from a combination of neurologic and skeletal muscle factors, and that biologic properties of both of these systems are altered with aging. Accordingly, numerous scientists and clinicians have used the term "muscle quality" to describe the relationship between voluntary muscle strength and muscle size. In this review article, we discuss the age-associated changes in the neuromuscular system-starting at the level of the brain and proceeding down to the subcellular level of individual muscle fibers-that are potentially influential in the etiology of dynapenia (age-related loss of muscle strength and power).

Store-operated Ca2+ entry modulates sarcoplasmic reticulum Ca2+ loading in neonatal rabbit cardiac ventricular myocytes

Store-operated Ca2+ entry (SOCE), which is Ca2+ entry triggered by the depletion of intracellular Ca2+ stores, has been observed in many cell types, but only recently has it been suggested to occur in cardiomyocytes. In the present study, we have demonstrated SOCE-dependent sarcoplasmic reticulum (SR) Ca2+ loading (load(SR)) that was not altered by inhibition of L-type Ca2+ channels, reverse mode Na+/Ca2+ exchange (NCX), or nonselective cation channels. In contrast, lowering the extracellular [Ca2+] to 0 mM or adding either 0.5 mM Zn2+ or the putative store-operated channel (SOC) inhibitor SKF-96365 (100 microM) inhibited load(SR) at rest. Interestingly, inhibition of forward mode NCX with 30 microM KB-R7943 stimulated SOCE significantly and resulted in enhanced load(SR). In addition, manipulation of the extracellular and intracellular Na+ concentrations further demonstrated the modulatory role of NCX in SOCE-mediated SR Ca2+ loading. Although there is little knowledge of SOCE in cardiomyocytes, the present results suggest that this mechanism, together with NCX, may play an important role in SR Ca2+ homeostasis. The data reported herein also imply the presence of microdomains unique to the neonatal cardiomyocyte. These findings may be of particular importance during open heart surgery in neonates, in which uncontrolled SOCE could lead to SR Ca2+ overload and arrhythmogenesis.

Molecular properties and physiological roles of ion channels in the immune system

The discovery of a diverse and unique set of ion channels in T lymphocytes has led to a rapidly growing body of knowledge about their functional roles in the immune system. Here we review the biophysical and molecular characterization of K+, Ca2+, and Cl- channels in T lymphocytes. Potent and specific blockers, especially of K+ channels, have provided molecular tools to elucidate the involvement of voltage- and calcium-activated potassium channels in T-cell activation and cell-volume regulation. Their unique and differential expression makes lymphocyte K+ channels excellent pharmaceutical targets for modulating immune system function. This review surveys recent progress at the biophysical, molecular, and functional roles of the ion channels found in T lymphocytes.

Regulation of the type III InsP(3) receptor by InsP(3) and ATP

Many hormones and neurotransmitters raise intracellular calcium (Ca(2+)) by generating InsP(3) and activating the inositol 1,4, 5-trisphosphate receptor (InsP(3)R). Multiple isoforms with distinct InsP(3) binding properties () have been identified (). The type III InsP(3)R lacks Ca(2+)-dependent inhibition, a property that makes it ideal for signal initiation (). Regulation of the type III InsP(3)R by InsP(3) and ATP was explored in detail using planar lipid bilayers. In comparison to the type I InsP(3)R, the type III InsP(3)R required a higher concentration of InsP(3) to reach maximal channel activity (EC(50) of 3.2 microM versus 0.5 microM for the types III and I InsP(3)R, respectively). However, the type III InsP(3)R did reach a 2.5-fold higher level of activity. Although activation by InsP(3) was isoform-specific, regulation by ATP was similar for both isoforms. In the presence of 2 microM InsP(3), low ATP concentrations (<6 mM) increased the open probability and mean open time. High ATP concentrations (>6 mM) decreased channel activity. These results illustrate the complex nature of type III InsP(3)R regulation. Enhanced channel activity in the presence of high InsP(3) may be important during periods of prolonged stimulation, whereas allosteric modulation by ATP may help to modulate intracellular Ca(2+) signaling.

Impairment of Ca(2+) mobilization in circular muscle cells of the inflamed colon

This study investigated whether inflammation modulates the mobilization of Ca(2+) in canine colonic circular muscle cells. The contractile response of single cells from the inflamed colon was significantly suppressed in response to ACh, KCl, and BAY K8644. Methoxyverapamil and reduction in extracellular Ca(2+) concentration dose-dependently blocked the response in both normal and inflamed cells. The increase in intracellular Ca(2+) concentration in response to ACh and KCl was significantly reduced in the inflamed cells. However, Ca(2+) efflux from the ryanodine- and inositol 1,4, 5-trisphosphate (IP(3))-sensitive stores, as well as the decrease of cell length in response to ryanodine and IP(3), were not affected. Heparin significantly blocked Ca(2+) efflux and contraction in response to ACh in both conditions. ACh-stimulated accumulation of IP(3) and the binding of [(3)H]ryanodine to its receptors were not altered by inflammation. Ruthenium red partially inhibited the response to ACh in normal and inflamed states. We conclude that the canine colonic circular muscle cells utilize Ca(2+) influx through L-type channels as well as Ca(2+) release from the ryanodine- and IP(3)-sensitive stores to contract. Inflammation impairs Ca(2+) influx through L-type channels, but it may not affect intracellular Ca(2+) release. The impairment of Ca(2+) influx may contribute to the suppression of circular muscle contractility in the inflamed state.

Regulation of type 1 inositol 1,4,5-trisphosphate-gated calcium channels by InsP3 and calcium: Simulation of single channel kinetics based on ligand binding and electrophysiological analysis

Cytosolic calcium acts as both a coagonist and an inhibitor of the type 1 inositol 1,4,5-trisphosphate (InsP3)-gated Ca channel, resulting in a bell-shaped Ca dependence of channel activity (Bezprozvanny, I., J. Watras, and B.E. Ehrlich. 1991. Nature. 351:751-754; Finch, E.A., T.J. Turner, and S.M. Goldin. 1991. Science. 252: 443-446; Iino, M. 1990. J. Gen. Physiol. 95:1103-1122). The ability of Ca to inhibit channel activity, however, varies dramatically depending on InsP3 concentration (Combettes, L., Z. Hannaert-Merah, J.F. Coquil, C. Rousseau, M. Claret, S. Swillens, and P. Champeil. 1994. J. Biol. Chem. 269:17561-17571; Kaftan, E.J., B.E. Ehrlich, and J. Watras. 1997. J. Gen. Physiol. 110:529-538). In the present report, we have extended the characterization of the effect of cytosolic Ca on both InsP3 binding and InsP3-gated channel kinetics, and incorporated these data into a mathematical model capable of simulating channel kinetics. We found that cytosolic Ca increased the Kd of InsP3 binding approximately 3.5-fold, but did not influence the maximal number of binding sites. The ability of Ca to decrease InsP3 binding is consistent with the rightward shift in the bell-shaped Ca dependence of InsP3-gated Ca channel activity. High InsP3 concentrations are able to overcome the Ca-dependent inhibition of channel activity, apparently due to a low affinity InsP3 binding site (Kaftan, E.J., B.E. Ehrlich, and J. Watras. 1997. J. Gen. Physiol. 110:529-538). Constants from binding analyses and channel activity determinations were used to develop a mathematical model that fits the complex Ca-dependent regulation of the type 1 InsP3-gated Ca channel. This model accurately simulated both steady state data (channel open probability and InsP3 binding) and kinetic data (channel activity and open time distributions), and yielded testable predictions with regard to the regulation of this intracellular Ca channel. Information gained from these analyses, and our current molecular model of this Ca channel, will be important for understanding the basis and regulation of intracellular Ca waves and oscillations in intact cells.

Inositol 1,4,5-trisphosphate (InsP3) and calcium interact to increase the dynamic range of InsP3 receptor-dependent calcium signaling

The inositol 1,4,5-trisphosphate (InsP3)-gated Ca channel in cerebellum is tightly regulated by Ca (Bezprozvanny, I., J. Watras, and B.E. Ehrlich. 1991. Nature (Lond.). 351:751-754; Finch, E.A., T. J. Turner, and S.M. Goldin. 1991. Science (Wash. DC). 252:443-446; Hannaert-Merah, Z., J.F. Coquil, L. Combettes, M. Claret, J.P. Mauger, and P. Champeil. 1994. J. Biol. Chem. 269:29642-29649; Iino, M. 1990. J. Gen. Physiol. 95:1103-1122; Marshall, I., and C. Taylor. 1994. Biochem. J. 301:591-598). In previous single channel studies, the Ca dependence of channel activity, monitored at 2 microM InsP3, was described by a bell-shaped curve (Bezprozvanny, I., J. Watras, and B.E. Ehrlich. 1991. Nature (Lond.). 351:751-754). We report here that, when we used lower InsP3 concentrations, the peak of the Ca-dependence curve shifted to lower Ca concentrations. Unexpectedly, when we used high InsP3 concentrations, channel activity persisted at Ca concentrations as high as 30 microM. To explore this unexpected response of the channel, we measured InsP3 binding over a broad range of InsP3 concentrations. We found the well-characterized high affinity InsP3 binding sites (with Kd < 1 and 50 nM) (Maeda, N., M. Niinobe, and K. Mikoshiba. 1990. EMBO (Eur. Mol. Biol. Organ.) J. 9:61-67; Mignery, G., T.C. Sudhof, K. Takei, and P. De Camilli. 1989. Nature (Lond.). 342:192-195; Ross, C.A., J. Meldolesi, T.A. Milner, T. Satoh, S. Supattapone, and S.H. Snyder. 1989. Nature (Lond.). 339:468-470) and a low affinity InsP3 binding site (Kd = 10 microM). Using these InsP3 binding sites, we developed a new model that accounts for the shift in the Ca-dependence curve at low InsP3 levels and the maintained channel activity at high Ca and InsP3 levels. The observed Ca dependence of the InsP3-gated Ca channel allows the cell to abbreviate the rise of intracellular Ca in the presence of low levels of InsP3, but also provides a means of maintaining high intracellular Ca during periods of prolonged stimulation.

Early cellular events couple covalent binding of reactive metabolites to cell killing by nephrotoxic cysteine conjugates

Addition of the nephrotoxic cysteine conjugate, S-(1,2-dichlorovinyl)-L-cysteine (DCVC), to the LLC-PK1 line of renal epithelial cells leads to covalent binding of reactive intermediates followed by thiol depletion, lipid peroxidation, and cell death (Chen et al., 1990, J. Biol. Chem., 265:21603-21611). The present study was designed to determine if increased intracellular free calcium might play a role in this pathway of DCVC-induced toxicity by comparing the temporal relationships among increased intracellular free calcium, lipid peroxidation, and cytotoxicity. Intracellular free calcium increased 1 hr after DCVC treatment, long before LDH release occurred. The elevation of intracellular free calcium and cytotoxicity was prevented by inhibiting DCVC metabolism with AOA. The cell-permeable chelators, Quin-2AM and EGTA-AM, prevented the toxicity. Pretreatment of cells with a nontoxic concentration of ionomycin increased intracellular free calcium and potentiated DCVC-induced LDH release. However, the antioxidant, DPPD, which blocks lipid peroxidation and toxicity, did not affect the increase in intracellular free calcium, whereas buffering intracellular calcium with Quin-2AM or EGTA-AM blocked both lipid peroxidation and toxicity without preventing the depletion of nonprotein sulfhydryls by DCVC. Ruthenium red, an inhibitor of mitochondrial calcium uptake, also blocked cell death. We hypothesize that covalent binding of the reactive fragment from DCVC metabolism leads to deregulation of intracellular calcium homeostasis and elevation of intracellular free calcium. Increased intracellular free calcium may in turn be coupled to mitochondrial damage and the accumulation of endogenous oxidants which cause lipid peroxidation and cell death.

Inhibition of cyclic AMP accumulation in intact NCB-20 cells as a direct result of elevation of cytosolic Ca2+

Earlier studies established that adenylyl cyclase in NCB-20 cell plasma membranes is inhibited by concentrations of Ca2+ that are achieved in intact cells. The present studies were undertaken to prove that agents such as bradykinin and ATP, which elevate the cytosolic Ca2+ concentration ([Ca2+]i) from internal stores in NCB-20 cells, could inhibit cyclic AMP (cAMP) accumulation as a result of their mobilization of [Ca2+]i and not by other mechanisms. Both bradykinin and ATP transiently inhibited [3H]cAMP accumulation in parallel with their transient mobilization of [Ca2+]i. The [Ca2+]i rise stimulated by bradykinin could be blocked by treatment with thapsigargin; this thapsigargin treatment precluded the inhibition of cAMP accumulation mediated by bradykinin (and ATP). A rapid rise in [Ca2+]i, as elicited by bradykinin, rather than the slow rise evoked by thapsigargin was required for inhibition of [3H]cAMP accumulation. Desensitization of protein kinase C did not modify the inhibitory action of bradykinin on [3H]cAMP. Effects of Ca2+ on phosphodiesterase were also excluded in the present studies. The accumulated data are consistent with the hypothesis that hormonal mobilization of [Ca2+]i leads directly to the inhibition of cAMP accumulation in these cells and presumably in other cells that express the Ca(2+)-inhibitable form of adenylyl cyclase.

Interactions between phospholipase C-coupled and N-methyl-D-aspartate receptors in cultured cerebellar granule cells: protein kinase C mediated inhibition of N-methyl-D-aspartate responses

The N-methyl-D-aspartate (NMDA) receptor of rat cerebellar granule cells in primary culture is inhibited by phospholipase C-coupled receptor activation. In the absence of ionotropic agonist, cells modulate their cytoplasmic free Ca2+, [Ca2+]c, in response to stimulation of M3 muscarinic receptors, metabotropic glutamate receptors, and endothelin receptors by the respective agonists carbachol, trans-1-amino-1,3-cyclopentanedicarboxylic acid, and endothelin-1. The response is consistent with the ability of phospholipase C-coupled receptors to release a pool of intracellular Ca2+ and induce a subsequent Ca2+ entry into the cell; both of these responses can be abolished by discharge of internal Ca2+ stores with low concentrations of ionomycin or thapsigargin. In the case of cells stimulated with NMDA, the [Ca2+]c response to the phospholipase C-coupled agonists is complex and agonist dependent; however, in the presence of ionomycin each agonist produces a partial inhibition of the NMDA component of the [Ca2+]c signal. This inhibition can be mimicked by the protein kinase C activator 4 beta-phorbol 12,13-dibutyrate. It is concluded that NMDA receptors on cerebellar granule cells are inhibited by phospholipase C-coupled muscarinic M3, glutamatergic, and endothelin receptors via activation of protein kinase C.