Regulation of the cardiac calcium channel by protein phosphatases

Sarcolemmal α2-adrenoceptors in feedback control of myocardial response to sympathetic challenge

α2-adrenoceptor (α2-AR) isoforms, abundant in sympathetic synapses and noradrenergic neurons of the central nervous system, are integral in the presynaptic feed-back loop mechanism that moderates norepinephrine surges. We recently identified that postsynaptic α2-ARs, found in the myocellular sarcolemma, also contribute to a muscle-delimited feedback control capable of attenuating mobilization of intracellular Ca²⁺ and myocardial contractility. This previously unrecognized α2-AR-dependent rheostat is able to counteract competing adrenergic receptor actions in cardiac muscle. Specifically, in ventricular myocytes, nitric oxide (NO) and cGMP are the intracellular messengers of α2-AR signal transduction pathways that gauge the kinase-phosphatase balance and manage cellular Ca²⁺ handling preventing catecholamine-induced Ca²⁺ overload. Moreover, α2-AR signaling counterbalances phospholipase C - PKC-dependent mechanisms underscoring a broader cardioprotective potential under sympathoadrenergic and angiotensinergic challenge. Recruitment of such tissue-specific features of α2-AR under sustained sympathoadrenergic drive may, in principle, be harnessed to mitigate or prevent cardiac malfunction. However, cardiovascular disease may compromise peripheral α2-AR signaling limiting pharmacological targeting of these receptors. Prospective cardiac-specific gene or cell-based therapeutic approaches aimed at repairing or improving stress-protective α2-AR signaling may offer an alternative towards enhanced preservation of cardiac muscle structure and function.

Sarcolemmal α2-adrenoceptors control protective cardiomyocyte-delimited sympathoadrenal response

Sustained cardiac adrenergic stimulation has been implicated in the development of heart failure and ventricular dysrhythmia. Conventionally, α2 adrenoceptors (α2-AR) have been assigned to a sympathetic short-loop feedback aimed at attenuating catecholamine release. We have recently revealed the expression of α2-AR in the sarcolemma of cardiomyocytes and identified the ability of α2-AR signaling to suppress spontaneous Ca²⁺ transients through nitric oxide (NO) dependent pathways. Herein, patch-clamp measurements and serine/threonine phosphatase assay revealed that, in isolated rat cardiomyocytes, activation of α2-AR suppressed L-type Ca²⁺ current (I_CaL) via stimulation of NO synthesis and protein kinase G- (PKG) dependent activation of phosphatase reactions, counteracting isoproterenol-induced β-adrenergic activation. Under stimulation with norepinephrine (NE), an agonist of β- and α-adrenoceptors, the α2-AR antagonist yohimbine substantially elevated I_CaL at NE levels >10nM. Concomitantly, yohimbine potentiated triggered intracellular Ca²⁺ dynamics and contractility of cardiac papillary muscles. Therefore, in addition to the α2-AR-mediated feedback suppression of sympathetic and adrenal catecholamine release, α2-AR in cardiomyocytes can govern a previously unrecognized local cardiomyocyte-delimited stress-reactive signaling pathway. We suggest that such aberrant α2-AR signaling may contribute to the development of cardiomyopathy under sustained sympathetic drive. Indeed, in cardiomyocytes of spontaneously hypertensive rats (SHR), an established model of cardiac hypertrophy, α2-AR signaling was dramatically reduced despite increased α2-AR mRNA levels compared to normal cardiomyocytes. Thus, targeting α2-AR signaling mechanisms in cardiomyocytes may find implications in medical strategies against maladaptive cardiac remodeling associated with chronic sympathoadrenal stimulation.

Role of protein phosphatases in the run down of guinea pig cardiac Cav1.2 Ca2+ channels

This study aimed to investigate protein phosphatases involved in the run down of Cav1.2 Ca2+ channels. Single ventricular myocytes obtained from adult guinea pig hearts were used to record Ca2+ channel currents with the patch-clamp technique. Calmodulin (CaM) and ATP were used to restore channel activity in inside-out patches. Inhibitors of protein phosphatases were applied to investigate the role of phosphatases. The specific protein phosphatase type 1 (PP1) inhibitor (PP1 inhibitor-2) and protein phosphatase type 2A (PP2A) inhibitor (fostriecin) abolished the slow run down of Cav1.2 Ca2+ channels, which was evident as the time-dependent attenuation of the reversing effect of CaM/ATP on the run down. However, protein phosphatase type 2B (PP2B, calcineurin) inhibitor cyclosporine A together with cyclophilin A had no effect on the channel run down. Furthermore, PP1 inhibitor-2 mainly prolonged the open time constants of the channel, specifically, the slow open time. Fostriecin primarily shortened the slow close time constants. Our data suggest that PP1 and PP2A were involved in the slow phase of Cav1.2 Ca2+ channel run down. In addition, they exerted different effects on the open-close kinetics of the channel. All above support the view that PP1 and PP2A may dephosphorylate distinct phosphorylation sites on the Cav1.2 Ca2+ channel.

A compartmentalized mathematical model of the β1-adrenergic signaling system in mouse ventricular myocytes

The β₁-adrenergic signaling system plays an important role in the functioning of cardiac cells. Experimental data shows that the activation of this system produces inotropy, lusitropy, and chronotropy in the heart, such as increased magnitude and relaxation rates of [Ca²⁺]_i transients and contraction force, and increased heart rhythm. However, excessive stimulation of β₁-adrenergic receptors leads to heart dysfunction and heart failure. In this paper, a comprehensive, experimentally based mathematical model of the β₁-adrenergic signaling system for mouse ventricular myocytes is developed, which includes major subcellular functional compartments (caveolae, extracaveolae, and cytosol). The model describes biochemical reactions that occur during stimulation of β₁-adrenoceptors, changes in ionic currents, and modifications of Ca²⁺ handling system. Simulations describe the dynamics of major signaling molecules, such as cyclic AMP and protein kinase A, in different subcellular compartments; the effects of inhibition of phosphodiesterases on cAMP production; kinetics and magnitudes of phosphorylation of ion channels, transporters, and Ca²⁺ handling proteins; modifications of action potential shape and duration; magnitudes and relaxation rates of [Ca²⁺]_i transients; changes in intracellular and transmembrane Ca²⁺ fluxes; and [Na⁺]_i fluxes and dynamics. The model elucidates complex interactions of ionic currents upon activation of β₁-adrenoceptors at different stimulation frequencies, which ultimately lead to a relatively modest increase in action potential duration and significant increase in [Ca²⁺]_i transients. In particular, the model includes two subpopulations of the L-type Ca²⁺ channels, in caveolae and extracaveolae compartments, and their effects on the action potential and [Ca²⁺]_i transients are investigated. The presented model can be used by researchers for the interpretation of experimental data and for the developments of mathematical models for other species or for pathological conditions.

CaV1.2 signaling complexes in the heart

L-type Ca(2+) channels (LTCCs) are essential for generation of the electrical and mechanical properties of cardiac muscle. Furthermore, regulation of LTCC activity plays a central role in mediating the effects of sympathetic stimulation on the heart. The primary mechanism responsible for this regulation involves β-adrenergic receptor (βAR) stimulation of cAMP production and subsequent activation of protein kinase A (PKA). Although it is well established that PKA-dependent phosphorylation regulates LTCC function, there is still much we do not understand. However, it has recently become clear that the interaction of the various signaling proteins involved is not left to completely stochastic events due to random diffusion. The primary LTCC expressed in cardiac muscle, CaV1.2, forms a supramolecular signaling complex that includes the β2AR, G proteins, adenylyl cyclases, phosphodiesterases, PKA, and protein phosphatases. In some cases, the protein interactions with CaV1.2 appear to be direct, in other cases they involve scaffolding proteins such as A kinase anchoring proteins and caveolin-3. Functional evidence also suggests that the targeting of these signaling proteins to specific membrane domains plays a critical role in maintaining the fidelity of receptor mediated LTCC regulation. This information helps explain the phenomenon of compartmentation, whereby different receptors, all linked to the production of a common diffusible second messenger, can vary in their ability to regulate LTCC activity. The purpose of this review is to examine our current understanding of the signaling complexes involved in cardiac LTCC regulation.

Supramolecular assemblies and localized regulation of voltage-gated ion channels

This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.

Binding of protein phosphatase 2A to the L-type calcium channel Cav1.2 next to Ser1928, its main PKA site, is critical for Ser1928 dephosphorylation

The cAMP-dependent protein kinase (PKA) controls a large number of cellular functions. One critical PKA substrate in the brain and heart is the L-type Ca(2+) channel Ca(v)1.2, the activity of which is upregulated by PKA. The main PKA phosphorylation site is serine 1928 in the central pore forming alpha(1)1.2 subunit of Ca(v)1.2. PKA is bound to Ca(v)1.2 within a macromolecular signaling complex consisting of the beta(2) adrenergic receptor, trimeric G(s) protein, and adenylyl cyclase for fast, localized, and hence specific signaling [Davare, M. A., Avdonin, V., Hall, D. D., Peden, E. M., Buret, A., Weinberg, R. J., Horne, M. C., Hoshi, T., and Hell, J. W. (2001) Science 293, 98-101]. Protein phosphatase 2A (PP2A) serves to effectively balance serine 1928 phosphorylation by PKA through its association with the Ca(v)1.2 complex [Davare, M. A., Horne, M. C., and Hell, J. W. (2000) J. Biol. Chem. 275, 39710-39717]. We now show that native PP2A holoenzymes, as well as the catalytic subunit itself, bind to alpha(1)1.2 immediately downstream of serine 1928. Of those holoenzymes, only heterotrimeric PP2A containing B' and B' ' subunits copurify with alpha(1)1.2. Preventing the binding of PP2A by truncating alpha(1)1.2 28 residues downstream of serine 1928 hampers its dephosphorylation in intact cells. Our results demonstrate for the first time that a stable interaction of PP2A with Ca(v)1.2 is required for effective reversal of PKA-mediated channel phosphorylation. Accordingly, PKA as well as PP2A are constitutively associated with Ca(v)1.2 for its proper regulation by phosphorylation and dephosphorylation of serine 1928.

Regulation of L-type Ca2+ channels in the heart: overview of recent advances

Regulation of L-type Ca2+ channels is complex, because many factors, such as phosphorylation, divalent cations, and proteins, specified or unspecified, have been shown to affect the channel activities. An additional complication is that these factors interact with one another to achieve final outcomes. Recent molecular technologies have helped to shed light on the mechanisms governing the activity of L-type Ca2+ channels. In this review article, three major topics concerning regulation of L-type Ca2+ channels in the heart are discussed, i.e. c-AMP dependent channel phosphorylation, role of magnesium (Mg2+), and the phenomenon of channel run-down.

Protein phosphatase 1 and an opposing protein kinase regulate steady-state L-type Ca2+ current in mouse cardiac myocytes

Studies have suggested that integration of kinase and phosphatase activities maintains the steady-state L-type Ca(2+) current in ventricular myocytes, a balance disrupted in failing hearts. As we have recently reported that the PP1/PP2A inhibitor calyculin A evokes pronounced increases in L-type I(Ca), the goal of this study was to identify the counteracting kinase and phosphatase that determine 'basal'I(Ca) in isolated mouse ventricular myocytes. Whole-cell voltage-clamp studies, with filling solutions containing 10 mm EGTA, revealed that calyculin A (100 nm) increased I(Ca) at test potentials between -42 and +49 mV (44% at 0 mV) from a holding potential of -80 mV. It also shifted the V(0.5) (membrane potential at half-maximal) of both activation (from -17 to -25 mV) and steady-state inactivation (from -32 to -37 mV) in the hyperpolarizing direction. The broad-spectrum protein kinase inhibitor, staurosporine (300 nm), was without effect on I(Ca) when added after calyculin A. However, by itself, staurosporine decreased I(Ca) throughout the voltage range examined (50% at 0 mV) and blocked the response to calyculin A, indicating that the phosphatase inhibitor's effects depend upon an opposing kinase activity. The PKA inhibitors Rp-cAMPs (100 microm in the pipette) and H89 (1 microm) failed to reduce basal I(Ca) or to block the calyculin A-evoked increase in I(Ca). Likewise, calyculin A was still active with 10 mm intracellular BAPTA or when Ba(2+) was used as the charge carrier. These data eliminate roles for protein kinase A (PKA) and calmodulin-dependent protein kinase II (CaMKII) as counteracting kinases. However, the protein kinase C (PKC) inhibitors Ro 31-8220 (1 microm) and Gö 6976 (200 nm) decreased steady-state I(Ca) and blunted the effect of calyculin A. PP2A is not involved in this regulation as intracellular applications of 10-100 nm okadaic acid or 500 nm fostriecin failed to increase I(Ca). However, PP1 is important, as dialysis with 2 microm okadaic acid or 500 nm inhibitor-2 mimicked the increases in I(Ca) seen with calyculin A. These in situ studies identify constitutive activity of PP1 and the counteracting activity of certain isoforms of PKC, in pathways distinct from receptor-mediated signalling cascades, as regulatory components that determine the steady-state level of cardiac L-type I(Ca).

Expression of protein phosphatases during postnatal development of rabbit heart

Protein phosphatases play a major role in the regulation of L-type calcium current (I(Ca)) in heart cells. We previously showed developmental differences in the effects of inhibitors of protein phosphatases (PP's) on the modulation of I(Ca), with greater stimulatory effects on I(Ca) observed in newborn than in adult ventricular cells. We hypothesized that this developmental difference might be due to greater expression and levels of PP 1 and PP 2A in newborn than in adult ventricular cells. We thus determined the mRNA expression of alpha and beta subunits of PP 1 and the a subunit of PP 2A in adult and newborn rabbit ventricles and levels of PP 1 and PP 2A in total homogenates, particulate membranes, and in soluble fraction prepared from isolated ventricular myocytes from adult and newborn rabbits. RT-PCR analysis demonstrated the presence of mRNA of these subunits of PP's in both newborn and adult ventricles. Northern blot analysis using 32P labeled cDNA probes specific for PP 1alpha, PP 1beta and PP 2Aalpha showed that the expression of steady state mRNA levels for PP 1alpha, PP 1beta and PP 2Aalpha were much higher in newborn compared to adult rabbit ventricles. mRNA for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and for sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) in rabbit ventricles were measured as controls. GAPDH did not show significant developmental changes while mRNA for SERCA was higher in adult compared to newborns. Western blot analysis showed that PP 1 and PP 2A protein levels were also much higher in newborn compared to adult rabbit ventricular cells. Immunoblot analysis in particulate membranes and soluble fraction showed that PP1 was mainly membrane bound while PP 2 was present only in soluble fraction. These findings suggest that the two major protein phosphatases (PP 1 and PP 2A) in heart are expressed at much higher levels in newborn and decline to lower levels in adult ventricular myocytes. The presence of high levels of PP's and particularly PP 1 in newborn cells may be responsible for the greater dependence of newborn cells on the inhibition of PP as a mechanism of action of beta-agonist isoproterenol on I(Ca).

Calyculin A and outward K+ channel currents in rat tail artery smooth muscle cells

Protein dephosphorylation mediated by phosphatases represents an important mechanism for modulating the functions of the targeted proteins. Calyculin A has been extensively used as a specific inhibitor of protein phosphatases. However, the effect of calyculin A on K channel currents in vascular smooth muscle cells (SMCs) and the underlying mechanisms had been unknown. It was found in the current study that calyculin A inhibited the whole-cell outward K channel currents in rat tail artery SMCs in a concentration-dependent (median inhibitory concentration, 12.6 n ) and reversible fashion. The extracellular applied calyculin A induced a biphasic change in K current amplitude with an initial transient increase followed by a long-lasting inhibition (n = 6). The intracellularly applied calyculin A (100 nM ) caused a lesser inhibition (33 +/- 1%) of K channel currents than that caused by the extracellularly applied calyculin A (55.3 +/- 8% inhibition) and did not result in an initial increase in K channel currents. The inhibitory effect of the intracellularly applied calyculin A on K channel currents was reversed to a stimulatory effect after ATP was omitted from the intracellular solution. The K currents inhibited by calyculin A were conducted by the iberiotoxin-sensitive K channels in SMCs. Moreover, okadaic acid (0.03-3 microM ) did not cause any significant change in K(Ca) channel currents. In conclusion, calyculin A inhibited K(Ca) channel currents in vascular SMCs. This effect of calyculin A, however, was not mediated by the inhibition of protein phosphatases.

On the role of phosphatase in regulation of cardiac L-type calcium current by cyclic GMP

Does cGMP, via protein kinase G, inhibit cAMP-stimulated Ca(2+) current (I(Ca(L))) in mammalian ventricular myocytes by phosphorylating the calcium channel at a site different from that acted on by cAMP or by dephosphorylating the calcium channel through phosphatase(s)? We tested these possibilities in guinea pig ventricular myocytes superfused with Tyrode's solution (35 degrees C) and dialyzed with adenosine 5'-O-(3-thiotriphosphate) ([ATPgammaS](pip)). ATPgammaS is a kinase substrate but thiophosphorylated proteins are not phosphatase substrates. With 5 mM [ATPgammaS](pip), I(Ca(L)) increased gradually over 20 to 25 min and then rapidly in the presence of 3-isobutyl-1-methylxanthine. 8-Bromo-cGMP (8-Br-cGMP; 1 mM) did not inhibit I(Ca(L)) significantly (-3 +/- 11.8%, n = 21) in contrast to results with ATP dialysis (). Similar results were obtained with 0.1 mM carbachol (CCh). I(Ca(L)) increased after longer dialysis (>/=40 min) with ATPgammaS; again, 8-Br-cGMP had no effect. Also, isoproterenol (ISO) did not stimulate and CCh, alone or in the presence of ISO, did not inhibit I(Ca(L)). Block of CCh effect by ATPgammaS, although consistent with cGMP action in muscarinic inhibition, could be explained by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) formation from ATPgammaS via nucleoside diphosphate kinase. GTPgammaS uncouples muscarinic and beta-adrenoceptors from intracellular effectors. Failure of 8-Br-cGMP to reduce I(Ca(L)) irreversibly excludes calcium channel phosphorylation as an inhibitory mechanism. We propose that cGMP inhibits I(Ca(L)) by activating phosphatase(s) in guinea pig ventricular myocytes.

Effects of PP1/PP2A inhibitor calyculin A on the E-C coupling cascade in murine ventricular myocytes

Calyculin A was used to examine the importance of phosphatases in the modulation of cardiac contractile magnitude in the absence of any neural or humoral stimulation. Protein phosphatase (PP)1 and PP2A activity, twitch contractions, intracellular Ca(2+) concentration ([Ca(2+)](i)) transients, action potentials, membrane currents, and myofilament Ca(2+) sensitivity were measured in isolated mouse ventricular myocytes. Calyculin A (125 nM) inhibited PP1 and PP2A by 50% and 85%, respectively, whereas it doubled the twitch magnitude and increased twitch duration by 50% in field-stimulated cells. Calyculin A-evoked increases in L-type Ca(2+) current (70%) and the resulting [Ca(2+)](i) transient (83%) explain the positive inotropic response. However, increases in twitch and action potential durations did not result from increased myofilament Ca(2+) sensitivity or K(+) current inhibition, respectively. Comparison of the effects of calyculin A and isoproterenol on [Ca(2+)](i) transients and twitch contractions revealed that calyculin A had a much smaller lusitropic effect than the beta-agonist, indicating that calyculin A did not significantly increase sarcoplasmic reticulum Ca(2+) reuptake. Thus while cardiac contractile magnitude is controlled by a steady-state kinase/phosphatase balance, this regulation is not equally operative at all of the steps in the excitation-contraction coupling pathway and may in fact be most important to the regulation of the L-type Ca(2+) channel.

Regulation of cardiac and smooth muscle Ca(2+) channels (Ca(V)1.2a,b) by protein kinases

High voltage-activated Ca(2+) channels of the Ca(V)1.2 class (L-type) are crucial for excitation-contraction coupling in both cardiac and smooth muscle. These channels are regulated by a variety of second messenger pathways that ultimately serve to modulate the level of contractile force in the tissue. The specific focus of this review is on the most recent advances in our understanding of how cardiac Ca(V)1.2a and smooth muscle Ca(V)1.2b channels are regulated by different kinases, including cGMP-dependent protein kinase, cAMP-dependent protein kinase, and protein kinase C. This review also discusses recent evidence regarding the regulation of these channels by protein tyrosine kinase, calmodulin-dependent kinase, purified G protein subunits, and identification of possible amino acid residues of the channel responsible for kinase regulation.

Serine/threonine protein phosphatases and synaptic inhibition regulate the expression of cholinergic-dependent plateau potentials

We previously identified cholinergic-dependent plateau potentials (PPs) in CA1 pyramidal neurons that were intrinsically generated by interplay between voltage-gated calcium entry and a Ca(2+)-activated nonselective cation conductance. In the present study, we examined both the second-messenger pathway and the role of synaptic inhibition in the expression of PPs. The stimulation of m1/m3 cholinergic receptor subtypes and G-proteins were critical for activating PPs because selective receptor antagonists (pirenzepine, hexahydro-sila-difenidol hydrochloride, 4-diphenylacetoxy-N-methylpiperidine methiodide) and intracellular guanosine-5'-O-(2-thiodiphosphate) prevented PP generation in carbachol. Intense synaptic stimulation occasionally activated PPs in the presence of oxytremorine M, a cholinergic agonist with preference for m1/m3 receptors. PPs were consistently activated by synaptic stimulation only when oxytremorine M was combined with antagonists at both GABA(A) and GABA(B) receptors. These latter data indicate an important role for synaptic inhibition in preventing PP generation. Both intrinsically generated and synaptically activated PPs could not be elicited following inhibition of serine/threonine protein phosphatases by calyculin A, okadaic acid, or microcystin-L, suggesting that muscarinic-induced dephosphorylation is necessary for PP generation. PP genesis was also inhibited following irreversible thiophosphorylation by intracellular perfusion with ATP-gamma-S. These data indicate that the expression of cholinergic-dependent PPs requires protein phosphatase-induced dephosphorylation via G-protein-linked m1/m3 receptor(s). Moreover, synaptic inhibition via both GABA(A) and GABA(B) receptors normally prevents the synaptic activation of PPs. Understanding the regulation of PPs should provide clues to the role of this regenerative potential in both normal activity and pathophysiological processes such as epilepsy.

A modulatory role for protein phosphatase 2B (calcineurin) in the regulation of Ca2+ entry

The Ca2+/calmodulin-dependent protein phosphatase 2B (PP2B) also known as calcineurin (CN) has been implicated in the Ca2+-dependent inactivation of Ca2+ channels in several cell types. To study the role of calcineurin in the regulation of Ca2+-channel activity, phosphatase expression was altered in NG108-15 cells by transfection of sense and antisense plasmid constructs carrying the catalytic subunit of human PP2Bbeta3. Relative to mock-transfected (wild-type) controls, cells overexpressing calcineurin showed dramatically reduced high-voltage-activated Ca2+ currents which were recoverable by the inclusion of 1 microM FK506 in the patch pipette. Conversely, in cells with reduced calcineurin expression, high-voltage-activated Ca2+ currents were larger relative to controls. Additionally in these cells, low-voltage-activated currents were significantly reduced. Analysis of high-voltage-activated Ca2+ currents revealed that the kinetics of inactivation were significantly accelerated in cells overexpressing calcineurin. Following the delivery of a train of depolarizing pulses in experiments designed to produce large-scale Ca2+ influx across the cell membrane, Ca2+-dependent inactivation of high-voltage-activated Ca2+ currents was increased in sense cells, and this increase could be reduced by intracellular application of 1 mM BAPTA or 1 microM FK506. These data support a role of calcineurin in the negative feedback regulation of Ca2+ entry through voltage-operated Ca2+ channels.

[Ca(2+)](i)- and insulin-stimulating effect of the non-membranepermeable phosphatase-inhibitor microcystin-LR in intact insulin-secreting cells (RINm5F)

1. Microcystin-LR, a specific and effective inhibitor of serine/threonine phosphatases type 1/2A which does not permeate cells, was used to distinguish intracellular and extracellular effects of phosphatase inhibitors on insulin secretion by RINm5F cells. 2. Incubation of intact RINm5F cells with microcystin-LR (0.1 - 2 microM) almost doubled basal insulin release at 3 mM glucose but left maximal insulin release induced by KCl (30 mM) unaffected. 3. In parallel, there was an increase in cytosolic Ca(2+) by up to half maximum, which could be suppressed by the Ca(2+)-channel blocker D600. 4. In contrast, microcystin-LR incubation of intact cells did not affect phosphatase activity but significantly reduced phosphatase activity when used in cellular fractions. 5. From these data we conclude that microcystin-LR could affect Ca(2+)-channels and insulin release by inhibiting an extracellular phosphatase-like activity.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Effects of serine/threonine protein phosphatases on ion channels in excitable membranes

This review deals with the influence of serine/threonine-specific protein phosphatases on the function of ion channels in the plasma membrane of excitable tissues. Particular focus is given to developments of the past decade. Most of the electrophysiological experiments have been performed with protein phosphatase inhibitors. Therefore, a synopsis is required incorporating issues from biochemistry, pharmacology, and electrophysiology. First, we summarize the structural and biochemical properties of protein phosphatase (types 1, 2A, 2B, 2C, and 3-7) catalytic subunits and their regulatory subunits. Then the available pharmacological tools (protein inhibitors, nonprotein inhibitors, and activators) are introduced. The use of these inhibitors is discussed based on their biochemical selectivity and a number of methodological caveats. The next section reviews the effects of these tools on various classes of ion channels (i.e., voltage-gated Ca(2+) and Na(+) channels, various K(+) channels, ligand-gated channels, and anion channels). We delineate in which cases a direct interaction between a protein phosphatase and a given channel has been proven and where a more complex regulation is likely involved. Finally, we present ideas for future research and possible pathophysiological implications.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Effect of phosphorylation on L-type calcium current in ventricular myocytes dialysed with proteolytic enzymes

1. L-Type Ca2+ channels play important roles in cardiac excitation and conduction. The present study used the whole-cell patch-clamp technique to investigate properties of Ca2+ channels in guinea-pig isolated ventricular myocytes. The effects of internal application of the proteolytic enzymes trypsin and carboxypeptidase (CBP) on the whole-cell L-type Ca2+ current (ICa) were determined. When the effects of the enzymes on ICa had reached steady state, the effects of isoprenaline (ISP) or 2,3-butane-dione monoxime (BDM), which increase and decrease channel phosphorylation, respectively, were examined. The effects of these agents were compared with those observed in the absence of enzyme pretreatment. 2. The amplitude and inactivation characteristics of ICa during depolarizing voltage-clamp commands to +10 mV (0.1 Hz) were determined at 37 degrees C. 3. Trypsin and CBP (both at concentrations of 1 mg/mL in the pipette solution) increased the amplitude of ICa 4.2- and 2.8-fold, respectively, and each enzyme increased the time constant of the slowly inactivating current by 50%. 4. Trypsin decreased the potential at which ICa was half maximally activated from (mean +/- SD) -1.4 +/- 2.2 mV (n = 9) to -11.3 +/- 2.5 mV (n = 7). Although CBP increased ICa amplitude, it did not shift the half-maximal activation voltage. Maximum conductance was increased 5.3-fold by trypsin and 2.2-fold by CBP. 5. Isoprenaline (1 mumol/L) had no effects in myocytes dialysed with trypsin, but significantly increased the current in myocytes dialysed with CBP by 8%. 6. At 12 mmol/L, BDM had no effect on current amplitude in the presence of trypsin, but decreased the time constant of slow inactivation to control values. After dialysis with CBP, BDM significantly decreased the maximum current by 11% and also decreased the rate of slow inactivation towards control values. 7. These data suggest that trypsin and CBP may have digested a part of the calcium channel that normally restricts current flow, but to different extents. The enzymes interacted with BDM and ISP in a fashion suggesting that two sites may influence the amplitude of the current and at least two other sites may influence the time course of the slowly inactivating current.

Regulation of the L-type Ca2+ channel current in rat pinealocytes: role of basal phosphorylation

In the present study, the role of phosphoprotein phosphatase in the regulation of L-type Ca2+ channel currents in rat pinealocytes was investigated using the whole-cell version of the patch-clamp technique. The effects of three phosphatase inhibitors, calyculin A, tautomycin, and okadaic acid, were compared. Although all three inhibitors were effective in inhibiting the L-type Ca2+ channel current, calyculin A was more potent than either tautomycin or okadaic acid, suggesting the involvement of phosphoprotein phosphatase-1. To determine the kinase involved in the regulation of these channels, cells were pretreated with H7 (a nonspecific kinase inhibitor), H89 (a specific inhibitor of cyclic AMP-dependent kinase), KT5823 (a specific inhibitor of cyclic GMP-dependent kinase), or calphostin C (a specific inhibitor of protein kinase C). Pretreatment with either H7 or calphostin C decreased the inhibitory effect of calyculin A on the L-type Ca2+ channel current. In contrast, pretreatment with H89 or KT5823 had no effect on the inhibition caused by calyculin A. Based on these observations, we conclude that basal phosphatase activity, probably phosphoprotein phosphatase-1, plays an important role in the regulation of L-type Ca2+ channel currents in rat pinealocytes by counteracting protein kinase C-mediated phosphorylation.

cAMP-dependent phosphorylation sites and macroscopic activity of recombinant cardiac L-type calcium channels

The involvement of cAMP-dependent phosphorylation sites in establishing the basal activity of cardiac L-type Ca2+ channels was studied in HEK 293 cells transiently cotransfected with mutants of the human cardiac alpha1 and accessory subunits. Systematic individual or combined elimination of high consensus protein kinase A (PKA) sites, by serine to alanine substitutions at the amino and carboxyl termini of the alpha1 subunit, resulted in Ca2+ channel currents indistinguishable from those of wild type channels. Dihydropyridine (DHP)-binding characteristics were also unaltered. To explore the possible involvement of nonconsensus sites, deletion mutants were used. Carboxyl-terminal truncations of the alpha1 subunit distal to residue 1597 resulted in increased channel expression and current amplitudes. Modulation of PKA activity in cells transfected with the wild type channel or any of the mutants did not alter Ca2+ channel functions suggesting that cardiac Ca2+ channels expressed in these cells behave, in terms of lack of PKA control, like Ca2+ channels of smooth muscle cells.

Effect of misoprostol on myocardial contractility in rats treated with cyclosporin A

The nephrotoxic side effects of the immunosuppressant cyclosporin A in animals and humans are well known. Misoprostol, a prostaglandin E analog, is used clinically in organ-transplant recipients taking cyclosporin A to protect against these side effects. We reported previously that long-term treatment of rats with cyclosporin A causes a diminution in myocardial peak contractile stress. There is an associated spontaneous sarcomere activity and rest depression of force in the absence of a change in myofilaments sensitivity to intracellular Ca2+. Here we investigated the potential protective effects of misoprostol on the myocardium of cyclosporin A-treated rats. Rats were treated with either cyclosporin A, misoprostol, or their combination. Force-[Ca2+]o and -[Sr2+]o, and force-interval relations as well as the sarcomere length were studied in trabeculae isolated from the right ventricles. At suboptimal [Ca2+]o, cyclosporin A shifted the force-[Ca2+]o relation to the left but reduced peak contractile stress by approximately 35% at the highest (optimal) [Ca2+]o. Co-treatment with misoprostol prevented the leftward shift, and treatment with misoprostol alone did not cause a leftward shift. The diminution of peak stress, however, did not recover with misoprostol treatment, and stress was further reduced. Treatment with only misoprostol also reduced stress generated by the muscles more than that by cyclosporin A alone. Intriguingly, activation of the myofilaments by Sr2+ failed to recover peak stress to control levels in any group treated with misoprostol. Unlike cyclosporin A, however, rest potentiation of force was more pronounced, and spontaneous sarcomere activity was absent with misoprostol. No histopathologic changes were observed with cyclosporin A or misoprostol treatment. Misoprostol modifies the cyclosporin A-induced changes in the Ca2+ handling, but further decreases the stress generated by the muscles.

Cardiac protein phosphorylation: functional and pathophysiological correlates

Protein phosphorylation acts a pivotal mechanism in regulating the contractile state of the heart by modulating particular levels of autonomic control on cardiac force/length relationships. Early studies of changes in cardiac protein phosphorylation focused on key components of the excitation-coupling process, namely phospholamban of the sarcoplasmic reticulum and myofibrillar troponin I. In more recent years the emphasis has shifted towards the identification of other phosphoproteins, and more importantly, the delineation of the mechanistic and signaling pathways regulating the various known phosphoproteins. In addition to cAMP- and Ca(2+)-calmodulin-dependent kinase processes, these have included regulation by protein kinase C and the ever-emerging family of growth factor-related kinases such as the tyrosine-, mitogen- and stress-activated protein kinases. Similarly, the role of protein dephosphorylation by protein phosphatases has been recognized as integral in modulating normal cardiac cellular function. Recent studies involving a variety of cardiovascular pathologies have demonstrated that changes in the phosphorylation states of key cardiac regulatory proteins may underlie cardiac dysfunction in disease states. The emphasis of this comprehensive review will be on discussing the role of cardiac phosphoproteins in regulating myocardial function and pathophysiology based not only on in vitro data, but more importantly, from ex vivo experiments with corroborative physiological and biochemical evidence.

Modulation of L-type calcium current by internal potassium in guinea pig ventricular myocytes

OBJECTIVES

The early phase of myocardial ischemia is characterized by a considerable K+ efflux from cardiac myocytes, causing decreasing internal ([K+]i) and increasing external ([K+]o) K+ concentrations. The change in [K+]i and [K+]o is one of the factors thought to initiate the ischemia-induced changes in electrical activity. Nevertheless, little is known about the influence of [K+]i and [K+]o on the L-type calcium current.

METHODS

The whole-cell patch-clamp technique combined with an internal perfusion system was used to test possible actions of altered [K+]i and [K+]o on L-type current carried by Ca2+ and Ba2+ in isolated guinea pig ventricular myocytes.

RESULTS

Changing the [K+]i in the range of 110-170 mM revealed a sigmoidal concentration-response relationship between the L-type current and [K+]i. The maximum change in current amplitude was more than 40% with a half-saturation concentration of 136 mM which is near the physiological [K+]i. Ca2+ influx during action potential clamp increased by approximately 42% after raising [K+]i from 130 to 170 mM. Internal perfusion with Cs+ demonstrated that Cs+ is less effective than K+ in regulating the L-type current. By using ATP-analogues, [K+]i was shown to affect the L-type channel in a phosphorylation-independent way. Changes in [K+]o only modulated the L-type current via alterations in [K+]i.

CONCLUSIONS

The decrease in [K+]i during early ischemia is, per se, sufficient to reduce the L-type current by up to 15%, thereby decreasing the action potential duration, and Ca2+ influx into the cells. This may act in addition to well-known mechanisms such as changes in internal pH and falling ATP levels, which influence the L-type current. Moreover, the phenomenon may complicate the interpretation of electrophysiological measurements of L-type current under conditions where [K+]i is not precisely controlled.

Mechanisms of the contractile effects of 2,3-butanedione-monoxime in the mammalian heart

We studied the mechanisms of action of a negative inotropic compound, 2,3-butanedione-monoxime (BDM), which has been suggested to be a cardioprotective agent. In guinea-pig papillary muscles the negative inotropic effect of BDM start at 100 mumol/l amounting to 18.32 +/- 2.09% of predrug value at 10 mmol/l without any effects on time parameters (n = 12, each). 30 mmol/l BDM totally abolished force of contraction; this effect was reversible after washout. In the presence of the phosphatase-inhibitor cantharidin (30 mumol/l) the concentration response curve on force of contraction was shifted to higher concentrations of BDM. 100 mmol/l BDM decreased the phosphorylation state of the inhibitory subunit of troponin (TnI) and phospholamban (PLB) in [32P]-labeled guinea-pig ventricular myocytes to 76.5 +/- 4.7% and 49.7 +/- 4.2%, respectively (n = 7). Furthermore, BDM enhanced the activity of phosphorylase phosphatases in guinea-pig ventricular homogenates amounting to a stimulation to 203.5 +/- 10.4% at 100 mmol/l whereas type 1 phosphorylase phosphatase activity increased only by 24.5% (n = 5). PLB phosphatase activity was enhanced to 155.9 +/- 11.7% by 100 mmol/l BDM (n = 5). It is concluded that the effects of BDM on contractile parameters are accompanied by decreased phosphorylation of the cardiac regulatory proteins TnI and PLB which could in part be due to activation of type 1 or 2A phosphatase activity. Hence, it is suggested that BDM affects the phosphorylation state of TnI and PLB not directly, but via activation of their phosphatases.

Modulation of the skeletal muscle ryanodine receptor by endogenous phosphorylation of 160/150-kDa proteins of the sarcoplasmic reticulum

This paper demonstrates and characterizes the inhibition of ryanodine binding caused by the phosphorylation of the 160/150-kDa proteins in skeletal muscle sarcoplasmic reticulum (SR). Inhibition of ryanodine binding was obtained by preincubation of SR membranes with ATP + NaF . The inhibition was characterized by the following findings: (a) If ATP was replaced by AdoPP[NH]P, inhibition of ryanodine binding activity was not observed. (b) The inhibitory effect of preincubation with ATP + NaF, like the phosphorylation of 150/160-kDa proteins, was Ca2+ dependent. (c) Inhibition of ryanodine binding, as the protein phosphorylation, was not observed if NaF (> 30 mM) was replaced with okadaic acid. (d) The optimal pH for the inhibition and the phosphorylation was about 7.0. (e) Both the phosphorylation of the 160/150-kDa proteins and inhibition of ryanodine binding were prevented by dichlorobenzimidazole riboside and hemin, inhibitors of casein kinase II. (f) Dephosphorylation of the 160/150-kDa proteins prevented the inhibition of ryanodine binding. (g) The presence of NP-40 during the phosphorylation prevented both the 160/150-kDa phosphorylation and the inhibition of ryanodine binding. Furthermore, a linear relationship was obtained between the degree of ryanodine binding inhibition and the level of phosphorylation of the 160/150-kDa proteins, as controlled by ATP or NaF concentrations. The binding affinity for Ca2+ of the ryanodine receptor (RyR) was modified by phosphorylation of the 160/150-kDa proteins, decreasing by up to 100-fold. The phosphorylation of the SR membranes resulted in an elimination of ryanodine binding sites with slight effect on the ryanodine binding affinity. These results suggest the modulation of the properties of the RyR by phosphorylation/dephosphorylation of the 160/150-kDa proteins. The identification of the phosphorylated 160/150-kDa proteins, their kinase, and the structural interactions between them and the RyR are presented in the accompanying paper.

Effect of arachidonic acid on the L-type calcium current in frog cardiac myocytes

1. External application of the unsaturated fatty acid arachidonic acid (AA) to frog ventricular cells caused a large inhibition (approximately 85%) of the L-type calcium current (ICa,L) previously stimulated by the beta-adrenergic agonist isoprenaline (Iso). The concentration producing half-maximal inhibition (K1/2) was 1.52 microM. The inhibitory effect did not affect the peak current-voltage relationship but produced a negative shift in the inactivation curve. 2. The inhibitory effect of AA also occurred in cells internally perfused with cAMP and non-hydrolysable analogues of cAMP. These data suggest that AA is acting by a mechanism located beyond adenylyl cyclase and does not involve changes in intracellular cAMP levels. 3. AA also inhibited the calcium current stimulated by internal perfusion with the catalytic subunit of protein kinase A (PKA), suggesting that AA acts downstream of channel phosphorylation. 4. The inhibitory effect of AA on the isoprenaline- or cAMP-stimulated ICa,L is largely reduced in cells internally perfused with the thiophosphate donor analogue of ATP, ATP gamma S, or protein phosphatase 1 and 2A inhibitors like microcystin (MC) or okadaic acid (OA). External application of the phosphatase inhibitor calyculin (Caly) also reduced the AA effect. These data suggested that the AA effect on ICa,L involves activation of protein phosphatase activity. 5. The effect of AA on ICa,L was not affected by staurosporine, an inhibitor of protein kinases. It was also unaffected in cells internally perfused with GTP gamma S. These results suggest that neither a PKC- nor a G-protein-mediated mechanism are likely to be involved in the effect of AA on ICa,L. 6. A saturated fatty acid, myristic acid (MA), had no inhibitory effect on the isoprenaline-stimulated Ca2+ current, whereas, in the same cells arachidonic acid produced approximately 85% inhibition of ICa,L. 7. The inhibitory effect of AA was not affected by exposing the cells to indomethacin (Indo), an inhibitor of the metabolism of AA by cyclo-oxygenase, nor nordihydroguaiaretic acid (NDGA), an inhibitor of the lipoxygenase pathway. However, the non-metabolizable analogue of AA, 5,8,11,14-eicosatetraynoic acid (ETYA), was without effect on the isoprenaline-stimulated ICa,L. 8. These results suggest that AA inhibits ICa,L via a mechanism which involves, in part, stimulation of protein phosphatase activity. This process could provide a new mechanism in the modulation of calcium channel activity.

Chronic nifedipine treatment diminishes cardiac inotropic response to nifedifine: functional upregulation of dihydropyridine receptors

Chronic treatment with nifedipine induces up-regulation of functional active Ca2+ channels in cardiac muscle membranes. Adult male New Zealand White rabbits (NZW) were treated with nifedipine (20 mg/day) for 25 days. In isovolumic perfused hearts at constant coronary flow and heart rate (HR) the left ventricular developed pressure (LVDP) and its first derivative (dP/dt) were monitored. Basal contractility and contractility at different end-diastolic volumes (EDV) were higher in nifedipine-treated animals, with no changes in diastolic chamber stiffness. Dose response to nifedipine in pretreated animals showed less decrease in contractility than in controls [ED50 = 1.09 +/- 0.09 x 10-7 (control) and 1.55 +/- 0.17 x 10-7 M nifedipine (treated) (p < 0.05)]. Ca2+ channel density was assessed by specific binding at the dihydropyridine receptor with [methyl-3H]PN 200-110. In cardiac membranes, maximal binding capacity (Bmax) was 269 +/- 38 (n = 7, control) and 429 +/- 46 fmol/mg protein (n = 7, treated) (p < 0.05), without significant changes in dissociation constant. In addition, we noted no changes in dihydropyridine (DHP) binding sites in aortic membranes. Our results offer a possible explanation for the lack of decrease in contractility despite the persistent hypotensive effect in hypertensive patients during chronic treatment with nifedipine.

Basal dephosphorylation controls slow gating of L-type Ca2+ channels in human vascular smooth muscle

The role of cellular phosphatase activity in regulation of smooth muscle L-type Ca2+ channels was investigated using tautomycin, a potent and specific inhibitor of serin/threonin phosphatases type 1 and 2A. Tautomycin (1-100 nM) inhibited Ca2+ channel activity in smooth muscle cells isolated from human umbilical vein. Tautomycin-induced inhibition of Ca2+ channel activity was due to a reduction of channel availability which originated mainly from prolongation of the lifetime of unavailable states of the channel. Pretreatment of smooth muscle cells with the protein kinase inhibitor H-7 (10 microM) prevented the inhibitory effect of tautomycin. Our results suggest modulation of slow gating between available and unavailable states as a mechanism of phosphorylation-dependent down-regulation of Ca2+ channels in vascular smooth muscle.

The effect of a chemical phosphatase on single calcium channels and the inactivation of whole-cell calcium current from isolated guinea-pig ventricular myocytes

A chemical phosphatase, butanedione monoxime (BDM, at 12-20 mM), reduced open probability (P0) of single cardiac L-type Ca2+ channels in cell-attached patches from guinea-pig ventricular myocytes, without effect on the amplitude of single-channel current, the mean open time or the mean shorter closed time, but it increased mean longer closed time and caused a fall in channel availability. A decrease in the mean time between first channel opening and last closing within a trace was principally due to an inhibition of the longer periods of activity. As a result, the time course of the mean currents, which resolved into an exponentially declining and a sustained component, was changed by an increase in the rate of the exponential phase and a profound reduction of the sustained current. Essentially similar results were obtained when studying whole-cell Ba2+ currents. The inactivation of the whole-cell Ca2+ currents was composed of two exponentially declining components with the slower showing a significantly greater sensitivity to BDM, an effect that was much more pronounced in myocytes exposed to isoprenaline with adenosine 5'-O-(3-thiotriphosphate) (ATP[gamma S]) in the pipette solution. The actions of BDM, which are the opposite of those produced by isoprenaline, suggest that the level of phosphorylation affects processes involved in the slow regulation of channel activity under basal conditions and that several sites (and probably several kinases) are involved. Channels with an inherently slow inactivation would seem to be converted into channels with a rapid inactivation by a dephosphorylation process.

Protein kinase inhibitor H-89 reverses forskolin stimulation of cardiac L-type calcium current

Calcium currents (ICa) and barium currents (IBa) were measured in freshly isolated single ferret ventricular myocytes, using the whole cell patch-clamp and perforated patch-clamp techniques with Na and K currents blocked by tetraethylammonium and Cs. The membrane potential (Em) dependence of activation and steady-state inactivation curves were determined using a Boltzmann relation, where E0.5 is the Em at half-maximal conductance. Forskolin (1 microM) increased the rate of ICa inactivation, especially in perforated patch, but slowed IBa inactivation. The acceleration is likely to be due to greater Ca-dependent inactivation of ICa, where the slowing of IBa inactivation may be due to protein kinase A-dependent slowing of Em-dependent inactivation. Forskolin (1-10 microM) also increased ICa amplitude by two- to threefold and shifted the E0.5 for both activation and inactivation to more negative potentials by 7-8 mV. The effect of forskolin on the amplitude of ICa could be reversed by an inhibitor of adenosine 3',5'-cyclic monophosphate-dependent protein kinase, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89; 1-10 microM). However, H-89 did not reverse the shift of E0.5 induced by forskolin. H-89 application by itself does not decrease basal ICa but does shift the E0.5 of both activation and inactivation to more negative values of Em. It is possible that H-89 reverses the shift induced by regulatory phosphorylation (due to forskolin) but induces a coincidental negative shift itself.

Norepinephrine modulates excitability of neonatal rat optic nerves through calcium-mediated mechanisms

We report that norepinephrine markedly increases excitability of neonatal rat optic nerves. To investigate the mechanisms of the norepinephrine-induced excitability increase, we studied isolated optic nerves from 42 neonatal (< three days old) and five adult (> three months old) Long-Evan's hooded rats. Norepinephrine (10(-6), 10(-5) and 10(-4) M) rapidly and reversibly increased the amplitude (mean +/- S.D.: 3.5 +/- 1.7%, 12.1 +/- 2.8% and 35.6 +/- 8.4%) of compound action potentials elicited by submaximal stimulation of neonatal optic nerves. The beta-1 adrenoceptor antagonist atenolol (10(-5) M) blocked the norepinephrine-induced increase in excitability but the alpha antagonist phentolamine (10(-5) M) did not. The beta agonist isoproterenol (10(-5) and 10(-4) M) increased response amplitudes (8.7 +/- 4.1% and 25.8 +/- 4.6%) but the alpha-1 agonist methoxamine and alpha-2 agonist clonidine did not. The beta antagonist propranolol blocked the isoproterenol effect. Replacing Ca2+ with Mg2+ or adding 0.8 mM of Cd2+ reversibly blocked the norepinephrine effects. Extracellular K+ concentrations did not change in optic nerves during norepinephrine application. Blockade of K+ channels with apamin (10(-6) M) or tetraethylammonium (10(-3) M) did not prevent the excitatory effects of norepinephrine. Adult rat optic nerves were insensitive to both norepinephrine (10(-4) M) and isoproterenol (10(-4) M). Our results indicate that norepinephrine increases neonatal optic axonal excitability through Ca(2+)-dependent mechanisms. The data suggest that the adrenoceptors are situated on the axons, that the excitability changes are not due to changes in extracellular K+ concentration or K+ channels sensitive to apamin or tetraethylammonium. The sensitivity of rat optic nerves to norepinephrine declined with age. Axonal adrenoceptors may play a role in optic axonal development and injury.

Stimulation of protein phosphatases as a mechanism of the muscarinic-receptor-mediated inhibition of cardiac L-type Ca2+ channels

Acetylcholine decreases currents through cardiac L-type Ca2+ channels after stimulation with agents which elevate levels of cyclic adenosine monophosphate, such as isoproterenol, but there is still a controversy over the mechanisms of this muscarinic effect. We tested the hypothesis of whether, after isoproterenol stimulation, protein phosphatases are activated by acetylcholine. Whole-cell currents were recorded from guinea-pig ventricular myocytes. The effect of 10(-5) M acetylcholine on currents induced by 10(-8) M isoproterenol was studied in the absence or presence of protein phosphatase inhibitors. Three agents reduced the acetylcholine response: okadaic acid (3 or 9 x 10(-6) M) and cantharidin (3 x 10(-6) M) added to the pipette solution, and bath-applied fluoride (3 mM). In contrast, pipette application of other phosphatase inhibitors, namely the inhibitor PPI2 (1000 U/ml), ciclosporin (10(-5) M), or calyculin A (10(-6) M) did not significantly diminish the acetylcholine effect. Interestingly, there was no correlation between the effects of the compounds on basal Ca2+ current and their interference with the muscarinic response. An activation of type 2A phosphatases by acetylcholine would explain these findings. Indeed, okadaic acid is 3 orders of magnitude more potent in vitro in its inhibition of this isoform (purified from cardiac myocytes) than is calyculin A, while type-1 phosphatases are inhibited equally. The data support the attractive possibility that stimulation of protein phosphatases is part of the signal transduction cascade of Ca2+ channel inhibition by acetylcholine.

Action of calpastatin in prevention of cardiac L-type Ca2+ channel run-down cannot be mimicked by synthetic calpain inhibitors

Activity of L-type Ca2+ channels in a membrane patch disappears rapidly when the patch is excised from the cell into an artificial solution. This channel run-down observed in isolated membrane patches can however, be prevented by application of calpastatin, an endogenous protease inhibitor, and ATP. The high specificity of calpastatin for the protease calpain would clearly point to a participation of calpain activity in the run-down of Ca2+ channels. In an attempt to examine a possible involvement of calpain, three synthetic and rather specific calpain inhibitors were substituted for calpastatin. One of these inhibitors chosen for its membrane permeability in addition allowed calpain activity to be inhibited even before patch excision. The potency of these compounds in inhibiting calpain, specifically mu- and m-calpain, was first determined in a biochemical assay and then compared with their efficacy in preventing Ca2+ channel run-down. Surprisingly, calpastatin was least effective in calpain inhibition but by far the most potent in prevention of Ca2+ channel run-down. In addition run-down of Ca2+ channel activity was examined for its reversibility, which would not be expected upon involvement of a proteolytic process. However, Ca2+ channel activity clearly recovered after run-down by application of calpastatin. In contrast, synthetic calpain inhibitors were unable to reverse Ca2+ channel run-down. These results indicate that proteolysis might only be partially responsible for channel run-down and suggest an as yet unidentified function for calpastatin beyond its inhibitory action on calpain in the regulation of Ca2+ channel activity.

Modulation of calcium currents by a D1 dopaminergic protein kinase/phosphatase cascade in rat neostriatal neurons

In rat neostriatal neurons, D1 dopamine receptors regulate the activity of cyclic AMP-dependent protein kinase (PKA) and protein phosphatase 1 (PP1). The influence of these signaling elements on high voltage-activated (HVA) calcium currents was studied using whole-cell voltage-clamp techniques. The application of D1 agonists or cyclic AMP analogs reversibly reduced N- and P-type Ca2+ currents. Inhibition of PKA antagonized this modulation, as did inhibition of PP1, suggesting that the D1 effect was mediated by a PKA enhancement of PP1 activity directed toward Ca2+ channels. In a subset of neurons, D1 receptor-mediated activation of PKA enhanced L-type currents. The differential regulation of HVA currents by the D1 pathway helps to explain the diversity of effects this pathway has on synaptic integration and plasticity in medium spiny neurons.

The effects of various drugs on the myocardial inotropic response

1. The signal transduction process mediated by cyclic AMP that leads to the characteristic positive inotropic effect (PIE) in association with a positive lusitropic effect (acceleration of rate of twitch relaxation) has been well established. Relationships between accumulation of cyclic AMP, changes in intracellular Ca2+ transients and the PIE differ, however, depending on the mechanism of particular drugs that affect different steps in the metabolism of cyclic AMP. Selective partial agonists of beta 1-adrenoceptors and inhibitors of phosphodiesterase (PDE) III cause the accumulation of less cyclic AMP for a given PIE than does isoproterenol. In addition, in aequorin-microinjected canine ventricular muscle, selective inhibitors of PDE III, OPC 18790 and Org 9731, produced smaller decreases in the responsiveness of myofilaments to Ca2+ ions than isoproterenol, while a partial agonist of beta 1-adrenoceptors, denopamine, elicits a decrease in Ca2+ responsiveness of the same extent as does isoproterenol. 2. Activation of myocardial alpha 1-adrenoceptors, as well as stimulation of receptors for endothelin and angiotensin II, which accelerates hydrolysis of phosphoinositide (PI) to result in production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are associated with very similar inotropic regulation: (1) the dependence on the species of animals of induction of the PIE; (2) an excellent correlation between the extent of acceleration of hydrolysis of PI and the PIE; (3) isometric contraction curves associated with a negative lusitropic effect; (4) the PIE associated with increases in myofibrillar responsiveness to Ca2+ ions; and (5) the selective inhibition of the PIE by an activator of protein kinase C (PKC), phorbol 12,13-dibutyrate (PDBu), with little effect on the PIE of isoproterenol and Bay k 8644. 3. A novel class of cardiotonic agents, namely, Ca2+ sensitizers such as EMD 53998 and Org 30029, act on the Ca(2+)-binding site of troponin C, increasing the affinity of these sites for Ca2+ ions, or at the actin-myosin interface to facilitate the cycling of cross-bridges. These agents produce a PIE with little change or decrease in Ca2+ transients and may bring about a significant breakthrough in the development of drugs for reversal of myocardial failure in the treatment of congestive heart failure.

Inhibition and rapid recovery of Ca2+ current during Ca2+ release from sarcoplasmic reticulum in guinea pig ventricular myocytes

We have investigated the modulation of the L-type Ca2+ channel by Ca2+ released from the sarcoplasmic reticulum (SR) in single guinea pig ventricular myocytes under whole-cell voltage clamp. [Ca2+]i was monitored by fura 2. By use of impermeant monovalent cations in intracellular and extracellular solutions, the current through Na+ channels, K+ channels, nonspecific cation channels, and the Na+-Ca2+ exchanger was effectively blocked. By altering the amount of Ca2+ loading of the SR, the time course of the Ca2+ current (ICa) could be studied during various amplitudes of Ca2+ release. In the presence of a large Ca2+ release, fast inhibition of ICa occurred, whereas on relaxation of [Ca2+]i, fast recovery was observed. The time course of this transient inhibition of ICa reflected the time course of [Ca2+]i. However, the inhibition seen in the first 50 ms, ie, the time of net Ca2+ release from the SR, exceeded the inhibition observed later during the pulse, suggesting the existence of a higher [Ca2+] near the channel during this time. Transient inhibition of ICa during Ca2+ release was observed to a similar degree at all potentials. It could still be observed in the presence of intracellular ATP-gamma-S and of cAMP. Therefore, we conclude that the modulation of ICa by Ca2+ release from the SR is not related to dephosphorylation. It could be related to a reduction in the driving force and to a direct inhibition of the channel by [Ca2+]i. The observation that the degree of inhibition does not depend on membrane potential suggests that the Ca2+ binding site for this modulation is located outside the pore.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of slow calcium channels of myocardial cells and vascular smooth muscle cells by cyclic nucleotides and phosphorylation

The slow Ca2+ channels (L-type) of the heart are stimulated by cAMP. Elevation of cAMP produces a very rapid increase in number of slow channels available for voltage activation during excitation. The probability of a Ca2+ channel opening and the mean open time of the channel are increased. Therefore, any agent that increases the cAMP level of the myocardial cell will tend to potentiate ICa, Ca2+ influx, and contraction. The action of cAMP is mediated by PK-A and phosphorylation of the slow Ca2+ channel protein or an associated regulatory protein (stimulatory type). The myocardial slow Ca2+ channels are also regulated by cGMP, in a manner that is opposite or antagonistic to that of cAMP. We have demonstrated this at both the macroscopic level (whole-cell voltage clamp) and the single-channel level. The effect of cGMP is mediated by PK-G and phosphorylation of a protein, as for example, a regulatory protein (inhibitory-type) associated with the Ca2+ channel. Introduction of PK-G intracellularly causes a relatively rapid inhibition of ICa(L) in both chick and rat heart cells. Such inhibition occurs for both the basal and stimulated ICa(L). In addition, the cGMP/PK-G system was reported to stimulate a phosphatase that dephosphorylates the Ca2+ channel. In addition to the slower indirect pathway--exerted via cAMP/PK-A--there is a faster more-direct pathway for ICa(L) stimulation by the beta-adrenergic receptor. This latter pathway involves direct modulation of the channel activity by the alpha subunit (alpha s*) of the Gs-protein. In vascular smooth muscle cells the two pathways (direct and indirect) also appear to be present, although the indirect pathway produces inhibition of ICa(L). PK-C and calmodulin-PK also may play roles in regulation of the myocardial slow Ca2+ channels. Both of these protein kinases stimulate the activity of these channels. Thus, it appears that the slow Ca2+ channel is a complex structure, including perhaps several associated regulatory proteins, which can be regulated by a number of factors intrinsic and extrinsic to the cell, and thereby control can be exercised over the force of contraction of the heart.

Developmental changes in modulation of calcium currents of rabbit ventricular cells by phosphodiesterase inhibitors

BACKGROUND

We have previously shown major differences in beta-adrenergic and muscarinic modulation of L-type calcium currents (ICa) in newborn and adult rabbit heart. However, little is known about developmental changes in modulation of ICa by phosphodiesterases (PDEs), which also regulate intracellular cAMP concentration by its hydrolysis.

METHODS AND RESULTS

Enzymatically isolated adult and newborn (1- to 3-day-old) rabbit ventricular myocytes were used to study the effects of PDE inhibitors on ICa measured by the whole-cell patch-clamp method. 3-Isobutyl-1-methyl-xanthine (IBMX), a nonselective PDE inhibitor, increased ICa in a dose-dependent manner for both groups. The maximal effect of IBMX, expressed as percentage increase in ICa over control levels, was greater for newborn myocytes than for adult myocytes, but the effects of IBMX applied alone were observed only at concentrations > 10 mumol/L. The concomitant use of 0.1 mumol/L isoproterenol produced a significant potentiation of the IBMX effect on ICa, with a significant additive effect of IBMX in newborn myocytes even at 0.05 mumol/L IBMX. The concomitant use of a subthreshold concentration of IBMX (0.1 mumol/L) did not potentiate the dose dependence of adult ICa on isoproterenol but did markedly potentiate the dose dependence of newborn ICa on isoproterenol. The Emax and EC50 of isoproterenol in the presence of 0.1 mumol/L IBMX on newborn ICa were 235% and 8 nmol/L, respectively, whereas the Emax and EC50 of isoproterenol in the absence of IBMX on newborn ICa were 111% and 81 nmol/L, respectively. The addition of 50 mumol/L IBMX to 10 mumol/L isoproterenol markedly increased the newborn ICa density up to a level equivalent to that reached with 200 mumol/L cAMP in the pipette (14.9 +/- 1.2 versus 13.4 +/- 0.7 pA/pF). Our data suggest that the inhibition constant (Ki) of IBMX for inhibiting PDEs that participate in the regulation of ICa is much lower in newborn than in adult myocytes. Milrinone 1 mumol/L, a selective PDE III inhibitor, increased the 0.1 mumol/L isoproterenol-stimulated ICa of adult myocytes but had no significant additive effect for the 0.1 mumol/L isoproterenol-stimulated ICa of newborn myocytes. Rolipram 1 mumol/L, a selective PDE IV inhibitor, increased the 0.1 mumol/L isoproterenol-stimulated ICa for newborn myocytes but had no significant additive effect for the 0.1 mumol/L isoproterenol-stimulated ICa for adult myocytes.

CONCLUSIONS

These results suggest that the most important PDE isozyme for regulation of ICa of rabbit myocytes changes from PDE IV to PDE III during the postnatal period.

Developmental changes in the actions of phosphatase inhibitors on calcium current of rabbit heart cells

We used whole-cell voltage clamp to compare the modulation of calcium current density (ICa, picoampere per picofarad) of freshly isolated, adult and newborn rabbit heart in response to intracellular application of microcystin and okadaic acid, both of which block phosphatase activity of phosphatase type 1 and 2A. Newborn cells showed a much larger response to the intracellular application of either microcystin or okadaic acid than did adult cells. In newborn cells, the application of microcystin produced an increase in ICa which appeared to maximize ICa, as shown by the rise in ICa to levels which could be reached by application of 10 microM forskolin or by the intracellular application of 200 microM 3',5'-cyclic adenosine monophosphate (cAMP). In adult cells, the maximal response to microcystin was considerably less than that obtainable with forskolin or cAMP. After achieving a maximal response with microcystin, the addition of forskolin increased ICa further in adult cells but elicited no additional response in newborn cells. The treatment of cells with 0.1 microM isoproterenol, a concentration approximately equal to that required for a half-maximal response, strongly potentiated the effect of microcystin in newborn cells, but not in adult cells. We propose that newborn rabbit heart cells compared with adult rabbit heart cells have a greater level of protein phosphatase activity (perhaps combined with a somewhat greater kinase activity), a greater proportion of the protein phosphatase activity in the form of protein phosphatase type 1 (which is inhibited by isoproterenol) and a greater dependence on the inhibition of protein phosphatase as a mechanism of action of isoproterenol, compared with the increase in kinase activity on calcium channels.