On the mechanism of muscarinic inhibition of the cardiac Ca current

Medium-chain fatty acids modulate myocardial function via a cardiac odorant receptor

Several studies have demonstrated the expression of odorant receptors (OR) in various human tissues and their involvement in different physiological and pathophysiological processes. However, the functional role of ORs in the human heart is still unclear. Here, we firstly report the functional characterization of an OR in the human heart. Initial next-generation sequencing analysis revealed the OR expression pattern in the adult and fetal human heart and identified the fatty acid-sensing OR51E1 as the most highly expressed OR in both cardiac development stages. An extensive characterization of the OR51E1 ligand profile by luciferase reporter gene activation assay identified 2-ethylhexanoic acid as a receptor antagonist and various structurally related fatty acids as novel OR51E1 ligands, some of which were detected at receptor-activating concentrations in plasma and epicardial adipose tissue. Functional investigation of the endogenous receptor was carried out by Ca²⁺ imaging of human stem cell-derived cardiomyocytes. Application of OR51E1 ligands induced negative chronotropic effects that depended on activation of the OR. OR51E1 activation also provoked a negative inotropic action in cardiac trabeculae and slice preparations of human explanted ventricles. These findings indicate that OR51E1 may play a role as metabolic regulator of cardiac function.

Differential effects of lysophosphatidylcholine and ACh on muscarinic K(+),non-selective cation and Ca(2+) currents in guinea-pig atrial cells

We compared the effects of lysophosphatidylcholine (LPC) and acetylcholine (ACh) on IK(ACh), ICa and a non-selective cation current (INSC) in guinea-pig atrial myocytes to clarify whether LPC and ACh activate similar Gi/o-coupled effector systems.　IK(ACh), ICa and INSC were analyzed in single atrial myocytes by the whole cell patch-clamp.　LPC induced INSC in a concentration-dependent manner in atrial cells.　ACh activated IK(ACh), but failed to evoke INSC.　LPC also activated IK(ACh) but with significantly less potency than ACh.　The effects of both ligands on IK(ACh) were inhibited by intracellular loading of pre-activated PTX.　This treatment also inhibited LPC-induced INSC, indicating that IK(ACh) and INSC induced by LPC are both mediated by Gi/o.　LPC and ACh had similar potencies in inhibiting ICa, which was pre-augmented by forskolin, indicating that LPC and ACh activate similar amounts of α-subunits of Gi/o.　The different effects of LPC and ACh on IK(ACh) and INSC may suggest that LPC and ACh activate Gi/o having different types of βγ subunits, and that LPC-induced INSC may be mediated by βγ subunits of Gi/o, which are less effective in inducing IK(ACh).

The importance of Ca(2+)-dependent mechanisms for the initiation of the heartbeat

Mechanisms underlying pacemaker activity in the sinus node remain controversial, with some ascribing a dominant role to timing events in the surface membrane (“membrane clock”) and others to uptake and release of calcium from the sarcoplasmic reticulum (SR) (“calcium clock”). Here we discuss recent evidence on mechanisms underlying pacemaker activity with a particular emphasis on the many roles of calcium. There are particular areas of controversy concerning the contribution of calcium spark-like events and the importance of I(f) to spontaneous diastolic depolarisation, though it will be suggested that neither of these is essential for pacemaking. Sodium-calcium exchange (NCX) is most often considered in the context of mediating membrane depolarisation after spark-like events. We present evidence for a broader role of this electrogenic exchanger which need not always depend upon these spark-like events. Short (milliseconds or seconds) and long (minutes) term influences of calcium are discussed including direct regulation of ion channels and NCX, and control of the activity of calcium-dependent enzymes (including CaMKII, AC1, and AC8). The balance between the many contributory factors to pacemaker activity may well alter with experimental and clinical conditions, and potentially redundant mechanisms are desirable to ensure the regular spontaneous heart rate that is essential for life. This review presents evidence that calcium is central to the normal control of pacemaking across a range of temporal scales and seeks to broaden the accepted description of the “calcium clock” to cover these important influences.

Arrhythmogenesis in Brugada syndrome: impact and constrains of current concepts

Brugada syndrome (BrS), an inherited arrhythmogenic disease first described in 1992, is characterized by ST segment elevations on the electrocardiogram in the right precordium and by a high occurrence of arrhythmias including the life-threatening ventricular tachycardia/fibrillation. Knowledge of the underlying mechanisms of formation of arrhythmogenic substrate in BrS is essential, namely for the risk stratification of BrS patients and their therapy which is still restrained almost exclusively to the implantation of cardioverter/defibrillator. In spite of many crucial findings in this field published within recent years, the final consistent view has not been established so far. Hence, BrS described 20 years ago remains an actual topic of both clinical and experimental studies. This review presents an overview of the current knowledge related to the pathogenesis of BrS arrhythmogenic substrate, namely of the genetic basis of BrS, functional consequences of mutations related to BrS, and arrhythmogenic mechanisms in BrS.

Ca(2+)-stimulated adenylyl cyclases regulate the L-type Ca(2+) current in guinea-pig atrial myocytes

Ca(2+)-stimulated adenylyl cyclases (ACs) have recently been shown to play important roles in pacemaking in the sino-atrial node. Here we present evidence that Ca(2+)-stimulated ACs are functionally active in guinea-pig atrial myocytes. Basal activity of an AC in isolated atrial myocytes was demonstrated by the observations that MDL 12,330A (10 μm), an AC inhibitor, reduced L-type Ca(2+) current (I(CaL)) amplitude, while inhibition of phosphodiesterases with IBMX (100 μm) increased I(CaL) amplitude. Buffering of cytosolic Ca(2+) by exposure of myocytes to BAPTA-AM (5 μm) reduced I(CaL) amplitude, as did inhibition of Ca(2+) release from the sarcoplasmic reticulum with ryanodine (2 μm) and thapsigargin (1 μm). [Ca(2+)]i-activated calmodulin kinase II (CaMKII) inhibition with KN-93 (1 μm) reduced I(CaL), but subsequent application of BAPTA-AM further reduced I(CaL). This effect of BAPTA-AM, in the presence of CaMKII inhibition, demonstrates that there is an additional Ca(2+)-modulated pathway (not dependent on CaMKII) that regulates I(CaL) in atrial myocytes. The effects of BAPTA could be reversed by forskolin (10 μm), a direct stimulator of all AC isoforms, which would restore cAMP levels. In the presence of BAPTA-AM, the actions of IBMX were reduced. In addition, inclusion of cAMP in the patch electrode in the whole-cell configuration prevented the effects of BAPTA. These effects are all consistent with a role for Ca(2+)-stimulated AC in the regulation of atrial myocyte I(CaL).

sGC{alpha}1 mediates the negative inotropic effects of NO in cardiac myocytes independent of changes in calcium handling

In the heart, nitric oxide (NO) modulates contractile function; however, the mechanisms responsible for this effect are incompletely understood. NO can elicit effects via a variety of mechanisms including S-nitrosylation and stimulation of cGMP synthesis by soluble guanylate cyclase (sGC). sGC is a heterodimer comprised of a β(1)- and an α(1)- or α(2)-subunit. sGCα(1)β(1) is the predominant isoform in the heart. To characterize the role of sGC in the regulation of cardiac contractile function by NO, we compared left ventricular cardiac myocytes (CM) isolated from adult mice deficient in the sGC α(1)-subunit (sGCα(1)(-/-)) and from wild-type (WT) mice. Sarcomere shortening under basal conditions was less in sGCα(1)(-/-) CM than in WT CM. To activate endogenous NO synthesis from NO synthase 3, CM were incubated with the β(3)-adrenergic receptor (β(3)-AR) agonist BRL 37344. BRL 37344 decreased cardiac contractility in WT CM but not in sGCα(1)(-/-) myocytes. Administration of spermine NONOate, an NO donor compound, did not affect sarcomeric shortening in CM of either genotype; however, in the presence of isoproterenol, addition of spermine NONOate reduced sarcomere shortening in WT but not in sGCα(1)(-/-) CM. Neither BRL 37344 nor spermine NONOate altered calcium handling in CM of either genotype. These findings suggest that sGCα(1) exerts a positive inotropic effect under basal conditions, as well as mediates the negative inotropic effect of β(3)-AR signaling. Additionally, our work demonstrates that sGCα(1)β(1) is required for NO to depress β(1)/β(2)-AR-stimulated cardiac contractility and that this modulation is independent of changes in calcium handling.

Control of cardiac contractility in the rat working heart-brainstem preparation

A great deal of knowledge exists regarding neural control of myocardial function in the rat. Most of the studies addressing this issue were conducted either under general anaesthesia or in isolated hearts in vitro. Our principal aim was to provide a detailed quantitative description of mechanisms controlling cardiac contractility in the rat, in an anaesthetic-free preparation with a preserved functional brainstem. Furthermore, while vagally mediated negative inotropy is a well-known phenomenon, at present there is no direct evidence for its presence in the rat; we searched for such evidence. To this end, in the arterially perfused working heart-brainstem preparation of the rat, we measured left ventricular pressure (LVP) and computed its first derivative (LVdP/dt). We made the following new observations. (i) Zatebradine (cardiac sodium pacemaker current blocker) caused a bradycardia associated with increases in LVP and LVdP/dt; the latter effect was via a frequency-dependent mechanism. (ii) We confirmed that in the rat, the force-frequency relationship (dependence of contractility on heart rate) is positive over a low range of heart rates, and negative and linear at physiological levels of heart rate, and provided its quantitative description. (iii) The increase in systemic pressure caused a rise in contractility, and vagal blockade or destruction of the central nervous system did not alter this inotropic effect, suggesting that it was mediated by intrinsic cardiac mechanisms. (iv) Vagal stimulation caused complex polyphasic changes in LVdP/dt and LVP in unpaced preparations; during pacing, it caused slowly developing falls in LVdP/dt that could be prevented by atropine. We conclude that control of ventricular contractility in the rat heart differs from that in other mammals not only by its negative frequency dependence, but also in the potent influence of aortic pressure on LVdP/dt. At the level of autonomic neural control, our newly found, vagally mediated negative inotropic effect adds to the accumulating body of data regarding both the presence and the functional importance of parasympathetic innervation of the ventricular myocardium.

Effects of muscarinic receptor stimulation on Ca2+ transient, cAMP production and pacemaker frequency of rabbit sinoatrial node cells

We investigated the contribution of the intracellular calcium (Ca_i²⁺) transient to acetylcholine (ACh)-mediated reduction of pacemaker frequency and cAMP content in rabbit sinoatrial nodal (SAN) cells. Action potentials (whole cell perforated patch clamp) and Ca_i²⁺ transients (Indo-1 fluorescence) were recorded from single isolated rabbit SAN cells, whereas intracellular cAMP content was measured in SAN cell suspensions using a cAMP assay (LANCE^®). Our data show that the Ca_i²⁺ transient, like the hyperpolarization-activated “funny current” (I_f) and the ACh-sensitive potassium current (I_K,ACh), is an important determinant of ACh-mediated pacemaker slowing. When I_f and I_K,ACh were both inhibited, by cesium (2 mM) and tertiapin (100 nM), respectively, 1 μM ACh was still able to reduce pacemaker frequency by 72%. In these I_f and I_K,ACh-inhibited SAN cells, good correlations were found between the ACh-mediated change in interbeat interval and the ACh-mediated change in Ca_i²⁺ transient decay (r² = 0.98) and slow diastolic Ca_i²⁺ rise (r² = 0.73). Inhibition of the Ca_i²⁺ transient by ryanodine (3 μM) or BAPTA-AM (5 μM) facilitated ACh-mediated pacemaker slowing. Furthermore, ACh depressed the Ca_i²⁺ transient and reduced the sarcoplasmic reticulum (SR) Ca²⁺ content, all in a concentration-dependent fashion. At 1 μM ACh, the spontaneous activity and Ca_i²⁺ transient were abolished, but completely recovered when cAMP production was stimulated by forskolin (10 μM) and I_K,ACh was inhibited by tertiapin (100 nM). Also, inhibition of the Ca_i²⁺ transient by ryanodine (3 μM) or BAPTA-AM (25 μM) exaggerated the ACh-mediated inhibition of cAMP content, indicating that Ca_i²⁺ affects cAMP production in SAN cells. In conclusion, muscarinic receptor stimulation inhibits the Ca_i²⁺ transient via a cAMP-dependent signaling pathway. Inhibition of the Ca_i²⁺ transient contributes to pacemaker slowing and inhibits Ca_i²⁺-stimulated cAMP production. Thus, we provide functional evidence for the contribution of the Ca_i²⁺ transient to ACh-induced inhibition of pacemaker activity and cAMP content in rabbit SAN cells.

Ca2+-stimulated adenylyl cyclase isoform AC1 is preferentially expressed in guinea-pig sino-atrial node cells and modulates the I(f) pacemaker current

Ca(2+)-stimulated adenylyl cyclases (AC) are known to play important roles in neurons but have not previously been reported in the heart. Here we present the first evidence for selective expression of Ca(2+)-stimulated AC in the sino-atrial node (SAN) but not in ventricular muscle of the guinea-pig heart. The AC1 isoform of Ca(2+)-stimulated AC was shown to be present in SAN, both as mRNA using RT-PCR and as protein using immuno-blotting with a specific antibody. Confocal immuno-fluorescence studies detected membrane localization of AC1 in SAN cells, but no AC1 in ventricular muscle. Ca(2+)-stimulated AC8 may also be present in SAN. The functional importance of AC activity was investigated by monitoring activation of I(f) (gated by hyperpolarization and regulated by cAMP, which shifts activation to more depolarized voltages). Basal activity of AC in isolated SAN myocytes was demonstrated by the observations that an inhibitor of AC activity (MDL 12330A, 10 microm) shifted activation in the hyperpolarizing direction, while inhibition of phosphodiesterases (IBMX, 100 microm) shifted I(f) activation in the depolarizing direction. Buffering cytosolic Ca(2+) with the Ca(2+) chelator BAPTA (by exposure to BAPTA-AM) shifted activation of I(f) in the hyperpolarizing direction, and under these conditions the AC inhibitor MDL had little or no further effect. The actions of BAPTA were overcome by exposure to forskolin (10 microm), a direct stimulator of all AC isoforms, to restore cAMP levels. These effects are consistent with the functional importance of Ca(2+)-stimulated AC, which is expected to be fundamental to initiation and regulation of the heartbeat.

Ahnak is critical for cardiac Ca(v)1.2 calcium channel function and its β‐adrenergic regulation

Defective L-type Ca2+ channel (I(CaL)) regulation is one major cause for contractile dysfunction in the heart. The I(CaL) is enhanced by sympathetic nervous stimulation: via the activation of beta-adrenergic receptors, PKA phosphorylates the alpha1C(Ca(V)1.2)- and beta2-channel subunits and ahnak, an associated 5643-amino acid (aa) protein. In this study, we examined the role of a naturally occurring, genetic variant Ile5236Thr-ahnak on I(CaL). Binding experiments with ahnak fragments (wild-type, Ile5236Thr mutated) and patch clamp recordings revealed that Ile5236Thr-ahnak critically affected both beta2 subunit interaction and I(CaL) regulation. Binding affinity between ahnak-C1 (aa 4646-5288) and beta2 subunit decreased by approximately 50% after PKA phosphorylation or in the presence of Ile5236Thr-ahnak peptide. On native cardiomyocytes, intracellular application of this mutated ahnak peptide mimicked the PKA-effects on I(CaL) increasing the amplitude by approximately 60% and slowing its inactivation together with a leftward shift of its voltage dependency. Both mutated Ile5236Thr-peptide and Ile5236Thr-fragment (aa 5215-5288) prevented specifically the further up-regulation of I(CaL) by isoprenaline. Hence, we suggest the ahnak-C1 domain serves as physiological brake on I(CaL). Relief from this inhibition is proposed as common pathway used by sympathetic signaling and Ile5236Thr-ahnak fragments to increase I(CaL). This genetic ahnak variant might cause individual differences in I(CaL) regulation upon physiological challenges or therapeutic interventions.

Responses of the sustained inward current to autonomic agonists in guinea-pig sino-atrial node pacemaker cells

1. The present study was undertaken to examine the responses of the sustained inward current (I(st)) to beta-adrenergic and muscarinic agonists in guinea-pig sino-atrial (SA) node cells using the whole-cell patch-clamp technique. I(st) was detected as the nicardipine (1 microM)-sensitive inward current at potentials between approximately -80 and +20 mV in the presence of low concentration (0.1 mM) of extracellular Ca2+, where the L-type Ca2+ current (I(Ca,L)) was practically abolished. 2. Beta-adrenergic agonist isoprenaline (ISO) in nanomolar concentrations not only increased the amplitude of I(st) but also shifted the membrane potential producing the peak amplitude (Vpeak) to a negative direction by approximately 15 mV without appreciably affecting potential range for the current activation. The stimulatory effect of ISO was concentration-dependent with an EC50 of 2.26 nM and the maximal effect (96.4+/-22.9% increase, n=6) was obtained at 100 nM ISO, when evaluated by the responses at -50 mV. 3. Bath application of acetylcholine (ACh) significantly inhibited I(st), which had been maximally augmented by 100 nM ISO; this inhibitory effect of ACh was concentration-dependent with an IC50 of 133.9 nM. High concentration (1000 nM) of ACh depressed basal I(st) by 10.5+/-2.0% (n=3). 4. In action potential clamp experiments, I(st) was also detected under control conditions and was markedly potentiated by exposure to ISO. 5. These results strongly suggest that I(st) not only contributes to the spontaneous action potentials of mammalian SA node cells but also plays a substantial role in mediating autonomic regulation of SA node pacemaker activity.

Species- and tissue-dependent effects of NO and cyclic GMP on cardiac ion channels

Biochemical studies have established the presence of a NO pathway in the heart, including sources of NO and various effectors. Several cardiac ion channels have been shown to be modified by NO, such as L-type Ca(2+), ATP-sensitive K(+), and pacemaker f-channels. Some of these effects are mediated by cGMP, through the activity of three main proteins: the cGMP-dependent protein kinase (PKG), the cGMP-stimulated phosphodiesterase (PDE2) and the cGMP-inhibited PDE (PDE3). Other effects appear independent of cGMP, as for instance the NO modulation of the ryanodine receptor-Ca(2+) channel. In the case of the cardiac L-type Ca(2+) channel current (I(Ca,L)), both cGMP-dependent and cGMP-independent effects have been reported, with important tissue and species specificity. For instance, in rabbit sinoatrial myocytes, NO inhibits the beta-adrenergic stimulation of I(Ca,L) through activation of PDE2. In cat and human atrial myocytes, NO potentiates the cAMP-dependent stimulation of I(Ca,L) through inhibition of PDE3. In rabbit atrial myocytes, NO enhances I(Ca,L) in a cAMP-independent manner through the activation of PKG. In ventricular myocytes, NO exerts opposite effects on I(Ca,L): an inhibition mediated by PKG in mammalian myocytes but by PDE2 in frog myocytes; a stimulation attributed to PDE3 inhibition in frog ventricular myocytes but to a direct effect of NO in ferret ventricular myocytes. Finally, NO can also regulate cardiac ion channels by a direct action on G-proteins and adenylyl cyclase.

Receptor subtypes mediating the inotropic effects and Ca(2+) signaling induced by endothelin-1 through crosstalk with norepinephrine in canine ventricular myocardium

In canine ventricular myocardium, endothelin-1 (ET-1) alone induced only a weak transient negative inotropic effect (NIE). However, ET-1 induced a marked sustained positive inotropic effect (PIE) subsequent to a transient NIE in the presence of norepinephrine (NE) at low concentrations (0.1 - 1 nM) and elicited a pronounced sustained NIE in the presence of NE at high concentrations (around 100 nM). Thus, the extent of beta-adrenoceptor stimulation induced by NE played a crucial role in determining the characteristics of the inotropic effects of ET-1. The characteristics of ET receptor subtypes involved in contractile regulation and Ca(2+) signaling induced by ET-1 were determined. The ET-1-induced transient NIE and decrease in Ca(2+) transients were abolished by the selective ET(A)-receptor antagonist FR319317, but not by the selective ET(B)-receptor antagonist BQ-788. The sustained PIE and the increase in Ca(2+) transients induced by ET-1 were abolished by FR319317, but not inhibited by BQ-788. In contrast, the sustained NIE of ET-1 was abolished by the non-selective ET antagonist TAK-044, markedly attenuated by FR319317, and partially inhibited by BQ-788. ET-1 alone elicited a PIE in the presence of BQ-788, which indicates that the activation of ET(B)-receptors counteracts the development of the PIE of ET-1. The current findings indicate that both ET(A) and ET(B) receptors are involved in the regulation of Ca(2+) signaling and contractility in canine ventricular myocardium.

Genomic and non-genomic regulation of L-type calcium channels in rat ventricle by thyroid hormone

Hyperthyroidism is associated with low exercise tolerance despite high cardiac output and sometimes with the development of heart failure. L-type calcium channels may play a role in the mechanism, but this has not been fully understood. We examined the effects of thyroid hormone on gene expression and function of L-type calcium channels in rat ventricles by the ribonuclease protection assay and whole-cell patch-clamp technique, respectively. The effects of bisoprolol, beta-blocking agent, on the regulation of calcium channel by thyroid hormone was also studied. In hyperthyroid animals, the mRNA of the calcium channel alpha1c subunit was reduced on day 4, compared with that in euthyroid animals, and remained low on day 8. Bisoprolol did not affect the thyroid hormone mediated decrease in alpha1c subunit mRNA. While L-type calcium current was greater in hyperthyroid than euthyroid myocytes on day 4, it was smaller on day 8. In addition, the isoproterenol-induced increase in calcium current in euthyroid rats was attenuated in hyperthyroid rats. Acetylcholine decreased calcium current in hyperthyroid myocytes, but not in euthyroid myocytes. In conclusion, L-type calcium current was increased by thyroid hormone in rat ventricular myocytes by the activation of the adenylate cyclase cascade, despite a decreased calcium channel gene expression. These genomic and non-genomic modifications may play an important role in the association of high cardiac output with low exercise tolerance, and in the development of heart failure in hyperthyroidism.

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 kinase C regulates functional coupling of beta1-adrenergic receptors to Gi/o-mediated responses in cardiac myocytes

The effect of protein kinase C (PKC) activation on beta1-adrenergic receptor (beta1-AR) regulation of the cardiac L-type Ca2+ current (ICa,L) was studied using the whole-cell patch clamp technique. Treatment of guinea pig ventricular myocytes with phorbol-12,13-dibutyrate (PDBu) caused a significant decrease in ICa,L sensitivity to stimulation by submaximal beta1-AR activation using isoproterenol (Iso). This decrease in sensitivity was also associated with the ability of higher concentrations of Iso to directly inhibit the stimulatory response. PDBu treatment produced similar effects on H2 histamine receptor-mediated ICa,L responses. In the presence of PDBu, higher concentrations of Iso inhibited the histamine stimulated ICa,L, and this effect was blocked by a selective beta1-AR antagonist. Higher concentrations of histamine also inhibited the Iso stimulated ICa,L, and this effect was blocked by a selective H2 receptor antagonist. The effects of PDBu were blocked by the PKC inhibitor bisindolylmaleimide I, and they were not mimicked by the inactive phorbol ester 4alpha-phorbol-12,13-didecanoate. The inhibitory effects of Iso and histamine were significantly reduced when Gi/o mediated responses were blocked with pertussis toxin. These results suggest that PKC promotes coupling of cardiac beta1-adrenergic and H2 histamine receptors to Gi/o mediated inhibitory responses.

Muscarinic regulation of cardiac ion channels

The parasympathetic component of the autonomic nervous system plays an important role in the physiological regulation of cardiac function by exerting significant influence over the initiation as well as propagation of electrical impulses, in addition to being able to regulate contractile force. These effects are mediated in whole or in part through changes in ion channel activity that occur in response to activation of M(2) muscarinic cholinergic receptors following release of the neurotransmitter acetylcholine. The coupling of M(2) receptor activation to most changes in cardiac ion channel function can be explained by one of two general paradigms. The first involves direct G protein-dependent regulation of ion channel activity. The second involves indirect regulation of ion channel activity through modulation of cAMP-dependent responses. This review focuses on recent advances in our understanding of the mechanisms by which M(2) muscarinic receptor activation both inhibits and facilitates cAMP-dependent ion channel responses in the heart.

Circadian variation of cardiac K+ channel gene expression

BACKGROUND

Many cardiac arrhythmias have their own characteristic circadian variations. Because the expression of many genes, including clock genes, is regulated variably during a day, circadian variations of ion channel gene expression, if any, could contribute to the fluctuating alterations of cardiac electrophysiological characteristics and subsequent arrhythmogenesis.

METHODS AND RESULTS

To examine whether cardiac K+ channel gene expression shows a circadian rhythm, we analyzed the mRNA levels of 8 Kv and 6 Kir channels in rat hearts every 3 hours throughout 1 day. Among these channels, Kv1.5 and Kv4.2 genes showed significant circadian variations in their transcripts: approximately 2-fold increase of Kv1.5 mRNA from trough at Zeitgeber time (ZT) 6 to peak at ZT18 and a completely reverse pattern in Kv4.2 mRNA ( approximately 2-fold increase from trough at ZT18 to peak at ZT6). Actually, along with the variations in the immunoreactive proteins, the density of the transient outward and steady-state currents in isolated myocytes and the responses of atrial and ventricular refractoriness to 4-aminopyridine in isolated-perfused hearts showed differences between ZT6 and ZT18, a circadian pattern comparable to that of Kv1.5 and Kv4.2 gene expression. Reversal of light stimulation almost inverted these circadian rhythms, although pharmacological autonomic blockade only partially attenuated the rhythm of Kv1.5 but not of Kv4.2 transcripts.

CONCLUSIONS

Among all the cardiac K+ channels, Kv1.5 and 4.2 channels are unique in showing characteristic circadian patterns in their gene expression, with Kv1.5 increase during the dark period partially dependent on beta-adrenergic activities and Kv4.2 increase during the light period independent of the autonomic nervous function.

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.

Temperature-sensitive intracellular Mg2+ block of L-type Ca2+ channels in cardiac myocytes

We examined the concentration-dependent blocking effects of intracellular Mg2+ on L-type Ca2+ channels in cardiac myocytes using the whole cell patch-clamp technique. The increase of L-type Ca2+ channel current (I(Ca)) (due to relief of Mg2+ block) occurred in two temporal phases. The rapid phase (runup) transiently appeared early (<5 min) in dialysis of the low-Mg2+ solution; the slow phase began later in dialysis (>10 min). Runup was not blocked by intracellular GTP (GTP(i)). The late phase of the I(Ca) increase (late I(Ca)) was suppressed by GTP(i) (0.4 mM) and was observed in myocytes of the guinea pig or frog at higher (32 or 24 degrees C, respectively) rather than lower temperatures (24 or 17.5 degrees C, respectively). At pMg = 6.0, raising the temperature from 24 to 32 degrees C evoked late I(Ca) with a Q10 of 14.5. Restoring the temperature to 24 degrees C decreased I(Ca) with a Q10 of only 2.4. The marked difference in the Q10 values indicated that late I(Ca) (pMg = 5-6) is an irreversible phenomenon. Phosphorylation suppressed the intracellular [Mg2+] dependency of late I(Ca). This effect of phosphorylation together with the inhibitory action of GTP(i) on Mg2+-dependent blocking of I(Ca) are common properties of mammalian and amphibian cardiomyocytes.

Electrophysiological analysis of the negative chronotropic effect of endothelin-1 in rabbit sinoatrial node cells

1. Electrophysiological effects of endothelin-1 (ET-1) were studied in rabbit sinoatrial node (SAN) using conventional microelectrode and whole-cell voltage and current recordings. 2. In rabbit SAN, RT-PCR detected ET(A) endothelin receptor mRNA. ET-1 (100 nM) increased the cycle length of action potentials (APs) from 305 +/- 15 to 388 +/- 25 ms; this effect was antagonised by the ET(A) receptor-selective antagonist BQ-123 (1 microM). ET-1 increased AP duration (APD50) by 22%, depolarised the maximum diastolic potential (MDP) from -59 +/- 1 to -53 +/- 2 mV, shifted the take-off potential by +5 mV and decreased the pacemaker potential (PMP) slope by 15%. Under exactly the same experimental conditions, ET-1 caused a positive chronotropic effect in guinea-pig SAN with a decrease of 13% in APD50, a shift of -4 mV in the take-off potential and an increase of 8% in the PMP slope. 3. Rabbit SAN exhibited two major cell types, distinguished both by their appearances and by their electrophysiological responses to ET-1. Whereas the spontaneous pacing rate and the PMP slope were similarly decreased by ET-1 (10 nM) in both cell types, ET-1 depolarised MDP from -67 +/- 1 to -62 +/- 4 mV in spindle-shaped cells but hyperpolarised it from -73 +/- 1 to -81 +/- 3 mV in rod-shaped cells. ET-1 decreased APD50 by 8 and 52% and shifted the take-off potential by +5 and -9 mV in spindle- and rod-shaped cells, respectively. 4. ET-1 decreased the high-threshold calcium current (I(CaL)) by about 50% in both cell types, without affecting its voltage dependence, and decreased the delayed rectifier K+ current (I(K)) with significant shifts (of +4.7 and +14.0 mV in spindle- and rod-shaped cells, respectively) in its voltage dependence. It was exclusively in rod-shaped cells that ET-1 activated a sizeable amount of time-independent inward-rectifying current. 5. The hyperpolarisation-activated current (I(f)), observed exclusively in spindle-shaped cells, was significantly increased by ET-1 at membrane potentials between -74.7 and -84.7 mV whereas it was significantly decreased at more negative potentials. ET-1 significantly decreased the slope of the current-voltage (I-V) relation of the I(f) tail without changing its half-maximum voltage. 6. The overall negative chronotropic influence of ET-1 on the whole rabbit SAN is interpreted as resulting from the integration of its different actions on spindle- and rod-shaped SAN cells through electrotonic interaction.

Nitric oxide synthase expression and function in embryonic and adult cardiomyocytes

Nitric oxide (NO) is an important signalling molecule that plays a relevant role in different cell systems, among them the adult heart. The effects of NO are primarily mediated through modulation of Ca(2+) homeostasis, myofibrillar contractility, and metabolic regulation in cardiomyocytes. Recent evidence also suggests an important role of NO for cardiomyogenesis by modulating proliferation and differentiation and regulating cardiac function. In the embryonic, but also the healthy and diseased, adult mammalian heart, the inducible (iNOS) and the endothelial (eNOS) nitric oxide synthases (NOS) are detected. However, the expression pattern of NO and its function differ during development. Furthermore, under pathophysiological conditions NOS expression can also change and cause impairment of cardiac performance and cytotoxic effects. The present review focuses on the role and function of NO during cardiomyogenesis, the mechanisms responsible for eNOS availability, and the paracrine effects of NO generated by cardiomyocytes.

ACh-induced rebound stimulation of L-type Ca(2+) current in guinea-pig ventricular myocytes, mediated by Gbetagamma-dependent activation of adenylyl cyclase

1. The effects that muscarinic receptor stimulation have on the cAMP-dependent regulation of L-type Ca(2+) currents were studied in isolated guinea-pig ventricular myocytes using the whole-cell configuration of the patch-clamp technique. 2. The muscarinic agonist ACh inhibited the Ca(2+) current stimulated by the beta-adrenergic agonist isoprenaline (Iso), and washout of ACh revealed a stimulatory response that appeared as a transient rebound increase in the amplitude of the Ca(2+) current. The ACh-induced stimulatory effect was not observed in the absence of Iso. 3. ACh-induced rebound stimulation was also observed in the presence of H(2) histamine receptor activation and cholera toxin treatment, which like beta-adrenergic receptor activation enhance adenylyl cyclase (AC) activity in a stimulatory G protein (G(s))-dependent manner. ACh-induced rebound stimulation was not observed in the presence of forskolin, which enhances AC activity in a G(s)-independent manner. 4. Pertussis toxin (PTX) treatment blocked both the stimulatory and inhibitory effects of ACh. Intracellular dialysis with QEHA, a peptide that binds free G protein betagamma subunits, selectively antagonized the stimulatory effect, leaving an enhanced inhibitory effect. 5. Evidence for the expression of AC4, an isoform of AC that can be stimulated by Gbetagamma but only in the presence of Galpha(s), was obtained by Western blot analysis of guinea-pig ventricular myocyte membrane preparations. 6. These results suggest that muscarinic receptor stimulation facilitates as well as inhibits cAMP-dependent regulation of the Ca(2+) current and that the net response is a balance between these two actions. We suggest that the stimulatory effect is due to a direct activation of AC4 by the betagamma subunits of a PTX-sensitive G protein.

cGMP-mediated inhibition of cardiac L-type Ca(2+) current by a monoclonal antibody against the M(2) ACh receptor

The effects of a monoclonal antibody (B8E5) directed against the second extracellular loop of the muscarinic M(2) receptor were studied on the L-type Ca(2+) currents (I(Ca,L)) of guinea pig ventricular myocytes using the whole cell patch-clamp technique. Similar to carbachol, B8E5 reduced the isoproterenol (ISO)-stimulated I(Ca,L) but did not significantly affect basal I(Ca,L). Atropine blocked the inhibitory effect of B8E5. The electrophysiological parameters of ISO-stimulated I(Ca,L) were not modified in presence of B8E5. Inhibition of I(Ca,L) by B8E5 was still observed when intracellular cAMP was either enhanced by forskolin or maintained constant by using a hydrolysis-resistant cAMP analog (8-bromoadenosine 3',5'-cyclic monophosphate) or by applying the phosphodiesterase inhibitor IBMX. The effect of B8E5 was mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate, a potent stimulator of cGMP-dependent protein kinase, and prevented by a selective inhibitor of nitric oxide-sensitive guanylyl cyclase [1H-(1,2,4)oxadiazolo[4,3-a]quinoxaline-1-one]. These results indicate that the antibody B8E5 inhibits the beta-adrenergic-stimulated I(Ca,L) through activation of the M(2) muscarinic receptor and further suggest that the antibody acts not via the classical pathway of decreasing intracellular cAMP, but rather by increasing cGMP.

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

The role of nitric oxide in bradycardia of rats with obstructive cholestasis

Nitric oxide (NO) has an important role in controlling heart rate and contributes to the cholinergic antagonism of the positive chronotropic response to adrenergic stimulation. Based on evidence of NO overproduction in cholestasis and also on the existence of bradycardia in cholestatic subjects, this study aimed to evaluate the chronotropic effect of epinephrine in isolated atria of cholestatic rats and determine whether alterations in epinephrine-induced chronotropic responses of cholestatic rats are corrected after systemic inhibition of NO synthase (NOS) with N(G)-nitro-L-arginine (L-NNA). Male Sprague-Dawley rats were used. Cholestasis was induced by surgical ligation of the bile duct under general anesthesia and sham-operated animals were considered as control. The animals were divided into three groups, which received either L-arginine (200 mg/kg/day), L-NNA (10 mg/kg/day) or saline. One week after the operation, a lead II ECG was recorded from the animals, then spontaneously beating atria were isolated and chronotropic responses to epinephrine were evaluated in a standard oxygenated organ bath. The results showed that plasma gamma-glutamyl transpeptidase and alanine aminotransferase activity was increased by bile-duct ligation, and that L-aginine treatment partially, but significantly, prevented the elevation of these markers of liver damage. The results showed that heart rate of cholestatic animals was significantly less than that of sham-operated control rats in vivo and this bradycardia was corrected with daily administration of L-NNA. The basal spontaneous beating rate of atria in cholestatic animals was not significantly different from that of sham-operated rats in vitro. Meanwhile, cholestasis induced a significant decrease in chronotropic effect of epinephrine. These effects were corrected by daily administration of L-NNA. Surprisingly L-arginine was as effective as L-NNA and increased the chronotropic effect of epinephrine in cholestatic rats but not in sham-operated animals. Systemic NOS inhibition corrected the decreased chronotropic response to adrenergic stimulation in cholestatic rats, and suggests an important role for NO in the pathophysiology of heart rate complications in cholestatic subjects. The opposite effect of chronic L-arginine administration in cholestasis and in control rats could be explained theoretically by an amelioration of cholestasis-induced liver damage by chronic L-arginine administration in bile duct-ligated rats.

Ventricular activation during sympathetic imbalance and its computational reconstruction

We characterized the epicardial activation sequence during a norepinephrine (NE)-induced ventricular arrhythmia in anesthetized pigs and studied factors that modulated it. Subepicardial NE infusion caused the QRS complex to invert within a single beat (n = 35 animals, 101 observations), and the earliest epicardial activation consistently shifted to the randomly located infusion site (n = 14). This preceded right atrial activation, whereas the total ventricular epicardial activation time increased from 20 +/- 4 to 50 +/- 9 ms (P < 0.01). These events were accompanied by a ventricular tachycardia and a drop in left ventricular pressure, which were fully reversed after the infusion was stopped. Epicardial pacing at the infusion site mimicked all electrical and hemodynamic changes induced by NE. The arrhythmia was prevented by propranolol and abolished by cardiac sympathetic or vagal nerve stimulation. Focal automaticity was computationally reconstructed using a two-dimensional sheet of 256 x 256 resistively coupled ventricular cells, where calcium handling was abnormally high in the central region. We conclude that adrenergic stimulation to a small region of the ventricle elicits triggered automaticity and that computational reconstruction implicates calcium overload. Interventions that reduce spatial inhomogeneities of intracellular calcium may prevent this type of arrhythmia.

NO is involved in MCh-induced accentuated antagonism via type II PDE in the canine blood-perfused SA node

The possible role of type II (cGMP-stimulated cAMP hydrolysis) phosphodiesterase (PDE) in the accentuated antagonism of muscarinic effects on heart rate during beta-stimulation via endogenous nitric oxide (NO) was evaluated. The canine isolated sinoatrial node preparation was cross circulated with arterial blood of a support dog. The sinoatrial rate of the preparation was 96 +/- 5 beats/min (n = 16) at control. Methacholine (MCh; 0.01-1 microg) injected into the right coronary artery in a bolus fashion caused dose-dependent decreases in sinoatrial rate. Under an intra-arterial infusion of isoproterenol (1 microM), resulting in approximately 50% increase in sinoatrial rate, MCh-induced decreases were markedly augmented from -18 +/- 3% to -44 +/- 4% at 0.3 mg of MCh. When N(G)-nitro-L-arginine methyl ester (100 microM) or N(G)-monomethyl-L-arginine (100 microM) were continuously infused, the augmented MCh-induced decreases in sinoatrial rate were significantly suppressed (-29 +/- 3% or -25 +/- 3%, respectively, P < 0.01). Pretreatment with either 3-isobutyl-1-methylxanthine (IBMX; 20 microM), a non-selective PDE inhibitor, or amrinone (20 microM), a selective type III (cGMP inhibited cAMP hydrolysis) PDE inhibitor, doubled the isoproterenol-induced increase in the sinoatrial rate. However, the augmented MCh-induced decreases in sinoatrial rate were significantly depressed by IBMX (from -23 +/- 5% to -14 +/- 1%, P < 0.01) but not by amrinone (to -20 +/- 3%). These results suggest that MCh-induced accentuated antagonism in the sinoatrial node pacemaker activity can be modulated by endogenous NO via an activation of the type II cyclic GMP-stimulated cAMP PDE.

Muscarinic inhibitory and stimulatory regulation of the L-type Ca2+ current is not altered in cardiac ventricular myocytes from mice lacking endothelial nitric oxide synthase

Using conventional and perforated patch-clamp techniques, the inhibitory and stimulatory effects of acetylcholine (ACh) on beta-adrenergic regulation of the L-type Ca2+ current (ICa) were studied in ventricular myocytes from wild-type mice (WT) and from mice lacking endothelial nitric oxide synthase (eNOS or NOS3; NOS3-KO mice). To validate the direct comparison of ACh effects on beta-adrenergic responses, the sensitivity of ICa to the beta-adrenergic agonist isoprenaline (Iso) was studied in both WT and NOS3-KO mouse myocytes. ICa sensitivity to Iso was not found to be significantly different in WT and NOS3-KO myocytes: Iso increased ICa with an EC50 of 4.9 and 3.7 nM in WT and NOS3-KO myocytes, respectively. ACh-induced inhibition of ICa did not significantly differ in ventricular myocytes from WT and NOS3-KO mice. ACh (10 microM) inhibited the stimulatory effect of 3 nM Iso by 39 and 35% in WT and NOS3-KO myocytes, respectively. Exposure to and subsequent washout of ACh in the continuous presence of submaximally stimulating concentrations of Iso (1-3 nM) resulted in a transient rebound stimulation of ICa in both WT and NOS3-KO mouse myocytes. The magnitude of the stimulatory effect of ACh did not significantly differ in WT and NOS3-KO mice. These results indicate that nitric oxide (NO) generated by NOS3 does not significantly affect the beta-adrenergic responsiveness of ICa. The results also confirm previous work indicating that NO generated by NOS3 is not obligatory for muscarinic inhibition of the beta-adrenergically regulated ICa in ventricular myocytes. Finally these results demonstrate for the first time that NO generated by NOS3 is not involved in muscarinic rebound stimulation of ICa in ventricular myocytes.

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.

EP receptor-mediated inhibition by prostaglandin E(1) of cardiac L-type Ca(2+) current of rabbits

Prostaglandin E(1) (PGE(1)) has cardioprotective effects on the ischemic-reperfused heart. To clarify the mechanisms underlying the protective action of PGE(1) on myocardium, we examined the effect of PGE(1) on the L-type Ca(2+) current (I(Ca)) using single atrial cells from rabbits. PGE(1) did not show a significant effect on basal I(Ca) but inhibited the I(Ca) prestimulated by isoproterenol (Iso, 30 nM). This inhibition was concentration dependent (EC(50) = 0.027 microM). Both sulprostone, a specific PGE receptor subtype (EP(1) and EP(3)) agonist, and 11-deoxy-PGE(1), an EP(3) agonist, inhibited the Iso-stimulated I(Ca), similar to PGE(1). Pretreatment with pertussis toxin (PTX) abolished the PGE(1) inhibition of I(Ca). Both the application of forskolin plus IBMX and intracellular dialysis with 8-bromoadenosine 3',5'-cyclic monophosphate eliminated the effect of PGE(1). PGE(1) did not show any further inhibition of I(Ca) when the effect of Iso was almost fully antagonized by acetylcholine. Methylene blue (guanylate cyclase inhibitor), KT-5823 (cGMP-dependent protein kinase inhibitor), and erythro-9-(2-hydroxy-3-nonyl)adenine (type II phosphodiesterase inhibitor) did not significantly change the inhibitory effect of PGE(1). These findings suggest that 1) PGE(1) inhibits Iso-stimulated I(Ca) by binding to the EP(3) receptor and 2) the PTX-sensitive and cAMP-dependent pathway is involved in the PGE(1) inhibition of I(Ca), but the nitric oxide-cGMP-dependent pathway is not. The PGE(1)-induced antiadrenergic effect shown in this study may contribute to the PGE(1) protection of myocardium against ischemia.

Dynamic nonlinear vago-sympathetic interaction in regulating heart rate

Although the characteristics of the static interactions between the sympathetic and parasympathetic nervous systems in regulating heart rate have been well established, how the dynamic interaction modulates the heart rate response remains unknown. Thus, we investigated the dynamic interaction by estimating the transfer function from nerve stimulation to heart rate, using band-limited Gaussian white noise, in anesthetized rabbits. Concomitant tonic vagal stimulation at 5 and 10 Hz increased the gain of the transfer function relating dynamic sympathetic stimulation to heart rate by 55.0%+/-40.1% and 80.7%+/-50.5%, respectively (P < 0.05). Concomitant tonic sympathetic stimulation at 5 and 10 Hz increased the gain of the transfer function relating dynamic vagal stimulation to heart rate by 18.2%+/-17.9% and 24.1%+/-18.0%, respectively (P < 0.05). Such bidirectional augmentation was also observed during simultaneous dynamic stimulation of the sympathetic and vagal nerves independent of their stimulation patterns. Because of these characteristics, changes in sympathetic or vagal tone alone can alter the dynamic heart rate response to stimulation of the other nerve. We explained this phenomenon by assuming a sigmoidal static relationship between autonomic nerve activity and heart rate. To confirm this assumption, we identified the static and dynamic characteristics of heart rate regulation by a neural network analysis, using large-amplitude Gaussian white noise input. To examine the mechanism involved in the bidirectional augmentation, we increased cytosolic adenosine 3',5'-cyclic monophosphate (cAMP) at the postjunctional effector site by applying pharmacological interventions. The cAMP accumulation increased the gain of the transfer function relating dynamic vagal stimulation to heart rate. Thus, accumulation of cAMP contributes, at least in part, to the sympathetic augmentation of the dynamic vagal control of heart rate.

Elevated cAMP suppresses muscarinic inhibition of L-type calcium current in guinea pig ventricular myocytes

We investigated the effect of carbachol (CCh) on L-type Ca2+ current (ICa(L)) enhanced by dialyzed adenosine 3',5'-cyclic monophosphate (cAMP) and/or bath-applied 3-isobutyl-1-methylxanthine (IBMX) in guinea pig isolated ventricular myocytes. At pipette concentrations ([cAMP]pip) from 30 microM to 1 mM, cAMP increased ICa(L) to 25.8 +/- 0.9 microA/cm2 (682 +/- 24.8% increase above control). CCh (100 microM) did not inhibit ICa(L) at any [cAMP]pip. IBMX, a nonselective phosphodiesterase (PDE) inhibitor, increased ICa(L) maximally at 300 microM IBMX (17.9 +/- 0.7 microA/cm2; 449 +/- 20% increase). CCh (100 microM) inhibited ICa(L) by 92 +/- 9.5% at 30 microM IBMX and 78 +/- 4.6% at 100 microM IBMX; this effect was reduced or absent at higher IBMX concentrations (300 and 1,000 microM). Coadministration of cAMP and IBMX also progressively suppressed inhibition by CCh. CCh had a negligible effect on ICa(L) at 750 microM IBMX in the absence of pipette cAMP and at 50 microM IBMX in the presence of 100 microM [cAMP]pip. ACh-activated K+ current (IK(ACh)) was unchanged in atrial myocytes dialyzed with 100 microM cAMP; this excludes a phosphorylation-dependent desensitization of the muscarinic receptor (mAChR) or Gi by cAMP. LY83583 (100 microM), an inhibitor of cyclic guanosine monophosphate (cGMP) production, attenuated inhibition of ICa(L) by CCh in the presence of IBMX. 8-Bromo-cGMP (8-Br-cGMP), an activator of cGMP-dependent protein kinase (PKG), mimicked CCh in its actions on ICa(L) raised by both cAMP (no significant change) and IBMX (49 +/- 5.1% inhibition). Okadaic acid, an inhibitor of type 1 and 2A phosphatases, blocked inhibition of IBMX-stimulated ICa(L) by either CCh or 8-Br-cGMP. Thus the ability of CCh to inhibit ICa(L) appears caused by cGMP/PKG activation of an okadaic acid-sensitive protein phosphatase, and elevated levels of cAMP protect against this action.

Parasympathetic inhibition of sympathetic effects on pacemaker location and rate in hearts of anesthetized dogs

INTRODUCTION

The site of impulse origin in the right atrium generally is considered to be a single static locus within the sinoatrial (SA) node. Previous investigators showed that the pacemaker site may shift due to changes in sympathetic or parasympathetic neural activity. We investigated the interactions between sympathetic and parasympathetic influences on the site of impulse initiation in the right atrium in anesthetized dogs.

METHODS AND RESULTS

We determined the site of impulse initiation and the spread of excitation over the anterior and posterior regions of the right atrium by a matrix of 48 unipolar recording electrodes. We assessed the spread of excitation at 3-msec intervals by constructing isochronal activation sequence maps. Sympathetic stimulation increased the frequency of atrial excitation (i.e., the heart rate), but also shifted the earliest activation region (EAR) from a locus in the SA node to a locus in the superior vena cava (the superior pacemaker site). Vagus stimulation decreased the heart rate and shifted the EAR to a lower site in the SA node or a site in the inferior right atrium along the sulcus terminalis (the inferior pacemaker site). A short period of vagus stimulation during a more prolonged sympathetic stimulation elicited a larger decrease in rate than did vagus stimulation alone and shifted the EAR from the superior site to the SA node or to the inferior site. After atropine, combined stimulation shifted the EAR to the superior site, but propranolol did not change EAR location.

CONCLUSION

Our results suggest that parasympathetic activity predominates over sympathetic activity not only on heart rate, but also on the location of the EAR in the anesthetized dog.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Parasympathetic modulation of sinoatrial node pacemaker activity in rabbit heart: a unifying model

We have extended our compartmental model [Am. J. Physiol. 266 (Cell Physiol. 35): C832-C852, 1994] of the single rabbit sinoatrial node (SAN) cell so that it can simulate cellular responses to bath applications of ACh and isoprenaline as well as the effects of neuronally released ACh. The model employs three different types of muscarinic receptors to explain the variety of responses observed in mammalian cardiac pacemaking cells subjected to vagal stimulation. The response of greatest interest is the ACh-sensitive change in cycle length that is not accompanied by a change in action potential duration or repolarization or hyperpolarization of the maximum diastolic potential. In this case, an ACh-sensitive K+ current is not involved. Membrane hyperpolarization occurs in response to much higher levels of vagal stimulation, and this response is also mimicked by the model. Here, an ACh-sensitive K+ current is involved. The well-known phase-resetting response of the SAN cell to single and periodically applied vagal bursts of impulses is also simulated in the presence and absence of the beta-agonist isoprenaline. Finally, the responses of the SAN cell to longer continuous trains of periodic vagal stimulation are simulated, and this can result in the complete cessation of pacemaking. Therefore, this model is 1) applicable over the full range of intensity and pattern of vagal input and 2) can offer biophysically based explanations for many of the phenomena associated with the autonomic control of cardiac pacemaking.

Effects of phosphorylation-related drugs on slow Ca2+ tail current in guinea-pig detrusor cells

In isolated guinea-pig detrusor cells, large conditioning depolarizations evoke slowly deactivating Ca2+ tail currents, considered to represent the second open state. The possible involvement of channel phosphorylation in this open state was examined. Application of isoprenaline caused a marginal increase in Ca2+ channel current evoked by simple depolarization, while forskolin did not. During application of either drug, slow-tail currents were never observed after simple depolarizations. The conditions necessary to induce slow-tail currents were not changed, even when cyclic AMP, ATP-gamma-S (adenosine 5'-O-(3-thiotriphosphate)), GDP-beta-S (guanosine 5'-O-(2-thiodiphosphate)) (in the pipette) or H-7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride) (to the bathing solution) was applied. The frequent depolarization protocol, known to facilitate Ca2+ current via Ca2+ and cyclic AMP-dependent phosphorylation mechanism(s) in cardiac myocytes, did not induce slow-tail currents. These results suggest that the transition of Ca2+ channels to the second open state during large depolarization is not a result of (voltage-operated) channel phosphorylation itself. Possible underlying mechanisms are discussed.

Cold acclimation of guinea pig depressed contraction of cardiac papillary muscle

Guinea pigs were exposed to 5 degrees C for 3 wk, and the contractions of myocardial papillary muscle were compared with preparations dissected from control animals kept at approximately 25 degrees C. Developed tension of the papillary muscle per cross-sectional area was significantly (t-test, P < 0.05) decreased after cold exposure (19,200 +/- 8,160 vs. 3,020 +/- 2,890 dyne/cm2; 1 Hz). Time to peak tension was significantly faster in cold-exposed guinea pigs (126.4 +/- 11.1 ms; 1 Hz) than in controls (162.7 +/- 8. 7 ms). The magnitude of the developed tension after application of ryanodine (2 mM) to muscles from cold-exposed animals was decreased to 37.5 +/- 8.3% of control at 1 Hz, whereas in muscles from control animals, tension was decreased to 82.4 +/- 7.7%. The ryanodine-sensitive component of contraction was not significantly changed in control guinea pigs at frequencies >0.5 Hz, whereas in muscles from cold-acclimated guinea pigs, there was a "positive staircase." These results suggested that reversal of the Na+/Ca2+ exchanger is predominantly involved in the positive staircase in control guinea pigs, whereas rate-dependent increases in the Ca2+ store in the sarcoplasmic reticulum may be involved in the staircase after cold acclimation.

Regulation of the L-type Ca2+ channel during cardiomyogenesis: switch from NO to adenylyl cyclase-mediated inhibition

In adult mammalian cardiomyocytes, stimulation of muscarinic receptors counterbalances the beta-adrenoceptor-mediated increase in myocardial contractility and heart rate by decreasing the L-type Ca2+ current (ICa) (1, 2). This effect is mediated via inhibition of adenylyl cyclase and subsequent reduction of cAMP-dependent phosphorylation of voltage-dependent L-type Ca2+ channels (3). Little is known, however, about the nature and origin of this pivotal inhibitory pathway. Using embryonic stem cells as an in vitro model of cardiomyogenesis, we found that muscarinic agonists depress ICa by 58 +/-3% (n=34) in early stage cardiomyocytes lacking functional beta-adrenoceptors. The cholinergic inhibition is mediated by the nitric oxide (NO)/cGMP system since it was abolished by application of NOS inhibitors (L-NMA, L-NAME), an inhibitor of the soluble guanylyl cyclase (ODQ), and a selective phosphodiesterase type II antagonist (EHNA). The NO/cGMP-mediated ICa depression was dependent on a reduction of cAMP/protein kinase A (PKA) levels since application of the catalytic subunit of PKA or of the PKA inhibitor PK) prevented the carbachol effect. In late development stage cells, as reported for ventricular cardiomyocytes (2, 4), muscarinic agonists had no effect on basal ICa but antagonized beta-adrenoceptor-stimulated ICa by 43 +/-4% (n=16). This switch in signaling pathways during development is associated with distinct changes in expression of the two NO-producing isoenzymes, eNOS and iNOS, respectively. These findings indicate a fundamental role for NO as a signaling molecule during early embryonic development and demonstrate a switch in the signaling cascades governing ICa regulation.

Accumulation of cAMP augments dynamic vagal control of heart rate

Recent investigations in our laboratory using a Gaussian white noise perturbation technique have shown that simultaneous sympathetic stimulation augmented the gain of the transfer function from vagal stimulation frequency to heart rate response. However, the mechanism of that augmentation remains to be elucidated. In this study, we examined in anesthetized rabbits how three pharmacological interventions known to cause intracellular accumulation of cAMP affected the transfer function. Isoproterenol (0.3 microg . kg-1 . min-1 iv) increased the dynamic gain of transfer function from 7.12 +/- 0.67 to 12.4 +/- 1.21 beats . min-1 . Hz-1 (P < 0.05) without changing the corner frequency or the lag time. Similar augmentations were observed when forskolin (5 microg . kg-1 . min-1 iv) or theophylline (20 mg/kg iv) was administered under conditions of beta-adrenergic blockade. These results suggest that the accumulation of cAMP at postjunctional effector sites contributes, at least in part, to the sympathetic augmentation of the dynamic vagal control of heart rate.

Potentiating effect of acetylcholine on stimulation by isoproterenol of L-type Ca2+ current and arrhythmogenic triggered activity in guinea pig ventricular myocytes

INTRODUCTION

The objective of this study was to determine whether the effect of isoproterenol (Iso) to increase L-type Ca2+ current [I(Ca(L))] and action potential duration (APD) was potentiated in ventricular myocytes following termination of an exposure of these cells to acetylcholine (ACh), and whether this potentiating effect of ACh could be arrhythmogenic.

METHODS AND RESULTS

Transmembrane currents and potentials of guinea pig isolated ventricular myocytes were measured using the whole cell, patch clamp technique. Stimulation of I(Ca(L)) and prolongation of APD caused by Iso (10 nmol/L) were attenuated in the presence of ACh (10 micromol/L), but were transiently enhanced by 111% +/- 20% and 214% +/- 44%, respectively, following termination of a 2- to 4-minute exposure of myocytes to ACh. No changes were observed in the absence of Iso. Both the amplitude and incidence of Iso-induced transient inward current, afterdepolarizations, and sustained triggered activity were greater immediately after termination of exposure to ACh than before application of ACh.

CONCLUSION

Stimulation by Iso of I(Ca(L)) is transiently enhanced in guinea pig ventricular myocytes following termination of exposure of these cells to ACh. The rebound increase of Iso-stimulated I(Ca(L)) is associated with an increase of APD and induction of arrhythmogenic triggered activity.

Inhibition of nitric oxide synthase slows heart rate recovery from cholinergic activation

The role of nitric oxide (NO) in the cholinergic regulation of heart rate (HR) recovery from an aspect of simulated exercise was investigated in atria isolated from guinea pig to test the hypothesis that NO may be involved in the cholinergic antagonism of the positive chronotropic response to adrenergic stimulation. Inhibition of NO synthesis with NG-monomethyl-L-arginine (L-NMMA, 100 micro M) significantly slowed the time course of the reduction in HR without affecting the magnitude of the response elicited by bath-applied ACh (100 nM) or vagal nerve stimulation (2 Hz). The half-times (t1/2) of responses were 3.99 +/- 0.41 s in control vs. 7. 49 +/- 0.68 s in L-NMMA (P < 0.05). This was dependent on prior adrenergic stimulation (norepinephrine, 1 micro M). The effect of L-NMMA was reversed by L-arginine (1 mM; t1/2 4.62 +/- 0.39 s). The calcium-channel antagonist nifedipine (0.2 micro M) also slowed the kinetics of the reduction in HR caused by vagal nerve stimulation. However, the t1/2 for the reduction in HR with antagonists (2 mM Cs+ and 1 micro M ZD-7288) of the hyperpolarization-activated current were significantly faster compared with control. There was no additional effect of L-NMMA or L-NMMA+L-arginine on vagal stimulation in groups treated with nifedipine, Cs+, or ZD-7288. We conclude that NO contributes to the cholinergic antagonism of the positive cardiac chronotropic effects of adrenergic stimulation by accelerating the HR response to vagal stimulation. This may involve an interplay between two pacemaking currents (L-type calcium channel current and hyperpolarization-activated current). Whether NO modulates the vagal control of HR recovery from actual exercise remains to be determined.

Do beta 2-adrenergic receptors modulate Ca2+ in adult rat ventricular myocytes?

We examined the role of beta 2-adrenergic receptors (ARs) in modulating calcium homeostasis in rat ventricular myocytes. Zinterol (10 microM), an agonist with a 25-fold greater affinity for beta 2-ARs over beta 1-ARs, modestly enhanced L-type calcium current (ICa) magnitude by approximately 30% and modestly accelerated the rate of Ca2+ concentration ([Ca2+]) decline (approximately 35%) but had little effect on the magnitude of the [Ca2+] transient (a nonsignificant 6% increase). However, 1 microM of the highly selective beta 1-AR antagonist CGP-20712A completely blocked the ICa increase induced by 10 microM zinterol. Pretreatment of cells with pertussis toxin (PTX) did not alter ICa enhancement by 10 microM zinterol, although it did abolish the ability of acetylcholine to block the forskolin-induced enhancement of ICa. Zinterol (10 microM) approximately doubled adenosine 3',5'-cyclic monophosphate (cAMP) accumulation, although one-half of this increase was blocked by CGP-20712A. In contrast, 1 microM of the nonselective beta-agonist isoproterenol increased cAMP production 15-fold. Thus we found no evidence that activation of beta 2-ARs modulates calcium homeostasis in rat ventricular myocytes, even after treatment with PTX.