It is becoming largely accepted that the hormones arginine-vasopressin (AVP) and oxytocin (OT) from the hypothalamic-neurohypophysial system (HNS) are not only important to the central nervous system (CNS) but they also have physiological actions in the peripheral nervous system (PNS) and in organs outside the nervous system, including the heart. The aim of this article is to present an overview of the effects of AVP and OT on neuronal cells and to compare them with what happens in cardiac cells, with an emphasis on Ca2+ signal control.


THE NEUROPEPTIDES VASOPRESSIN AND OXYTOCIN IN THE HYPOTHALAMO-NEUROHYPOPHYSEAL AXIS
Stimulus-secretion coupling and excitation-secretion coupling

The concept of ‘Stimulus-secretion coupling’ proposed for secretory events of neuro/endocrine cells by W.W. Douglas (1) was an analogy of the concept of ‘excitation-contraction coupling’ proposed for skeletal and cardiac muscle contraction. The reason why Bill Douglas used the word ‘stimulus’ instead of ‘excitation’ was because some endocrine cells do generate action potentials during secretion when they are vigorously stimulated and therefore ‘excitation’ is not mandatory for secretion. Moreover, because the stimulus-secretion coupling concept can be extensively used for many non-excitable secretory cells such as exocrine gland and epithelial cells. The soma of the neurones in the neurohypophysis are located in the supraoptic and paraventricular nucleus (SON and PVN, respectively) of the hypothalamus and do generate action potentials. Therefore, neurosecretion from the nerve ending of these neurones in the posterior pituitary gland is thought to share similar mechanisms with the release of neurotransmitters in other CNS and PNS neurones. Thus, stimulus-secretion coupling in the neurohypophysis can be termed as excitation-secretion coupling, however, we use ‘stimulus-secretion coupling’ to pay respect to the contribution of W.W. Douglas in this field.

The structure of neurohypophysis and neurohypophyseal hormones

In the posterior pituitary gland, neurones the cell bodies of which are located in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) secrete two peptide hormones, arginine-vasopressin (AVP) and oxytocin (OT) into the systemic circulation. The amino acid sequences of AVP and OT are shown in Fig. 1. The receptors for AVP and OT are seven transmembrane, G-protein-coupled receptors. The major functions of AVP and OT are antiduretic actions exerted at the collecting duct of nephrons, vasoconstriction exerted at vascular smooth muscle cells, and constriction of the uterus and myoepithelium cells in the mammary gland, respectively. The release of AVP is stimulated when the plasma osmolarity or plasma Na+ concentration is increased or the blood pressure is decreased. On the other hand, the release of OT is stimulated during parturition or lactation. The excitatory information for AVP and OT neurones is mediated by neurones in the brain stem and circumventricular organs. It reaches the SON and PVN, and then arrives at the neurohypophysis.

Fig. 1. The hypothalamus-pituitary gland axis. A. The magnocellular neurosecretory neurones whose cell bodies are located at the supraoptic nucleus (SON) and paraventricular nucleus (PVN) synthesize arginine-vasopressin (AVP) and oxytocin (OT). These two types of neurones send their axons to the neurohypophysis and secrete AVP and OT into the systemic circulation. OC: Optic chiasm. B. Amino acid sequences of AVP and OT.

The neurohypophysis consists of nerve endings of SON and PVN neurones, and pituicytes, glial like cells, surrounding the nerve endings (Fig. 2). By mechanical trituration of isolated neurohypophysis, neurosecretosome preparations can be obtained (2). This is very useful because various functions of the nerve ending can be studied just as with of isolated cells. In Fig. 3, an electron microscope picture is shown. Neurosecretosomes possess many exocytotic granules as well as mitochondria. With this preparation, hormone secretion (2), the intracellular Ca2+ concentration ([Ca2+]i) (3), and electrophysiological parameters can be measured.

Fig. 2. The structure of the neurohypophysis. Nerve endings of SON and PVN neurones at the neurohyophysis are schematically shown. The nerve endings are surrounded by glial cells in this tissue, pituicytes.

Fig. 3. An electron micrograph of ‘neurosecretosomes’. Neuroseceretosomes can be made by mechanical trituration of the neurohypophysial tissue followed by centrifugation on a with sucrose gradient (2). The neurosecretosomes have a diameter of 2-10 µm and possess secretory vesicles and mitochondria.

The regulatory mechanism of neurohypophyseal hormone secretion

The release of AVP and OT strongly depends on the frequency and pattern of action potentials generated at the SON and PVN, propagated to the nerve endings at the neurohypophysis. There have been numerous reports on the physiological ligands that affect action potential discharge. In this chapter we focus on SON neurones, because all SON neurones send nerve terminals exclusively to the neurohypophysis, whereas only a portion of PVN neurones do so. Most physiological ligands act mainly on the somatodendritic region of the SON neurone rather than the nerve terminals. Thus, the major step of the control of AVP and OT release depends on the action potential generation at the SON.

The control of action potential discharge

Morphologically, it has been shown that SON neurones receive glutamatergic and GABAergic synaptic inputs as excitatory and inhibitory signals, respectively. By the use of slice-patch-clamp technique, they are recorded as fast Excitatory Postsynaptic Currents (EPSCs) and Inhibitory Postsynaptic Currents (IPSCs) (5) Beside the two synaptic inputs, there have been many reports on the excitatory and inhibitory synaptic inputs into the SON, which are listed below.

Postsynaptic modulation. Neurotransmitters/neuromodulators that have been reported to possess postsynaptic actions on SON neurones are as excitatory agents are, glutamate, histamine, dopamine, acetylcholine, prostaglandin E2, F2a, angiotensin II, neurotensin, PACAP (pituitary adenylate cyclase activating polypeptide) and CCK (cholecystokin), and as inhibitory agents, GABA (-aminobutyric acid), opioids, galanin, adenosine, NO (nitric oxide) and ANP (atrial natriuretic peptide). CCK is known to act only on OT neurones, but many other agents are reported to act both on AVP and OT neurones.

It has been reported that SON neurones themselves can sense the osmolarity by the stretch-inactivated non-selective cation channels (6). Moreover, many of the excitatory agents acting on SON neurones activate non-selective cation channels. Charles Bourque and his colleagues using single channel measurements (7) have found that neurotensin, CCK, and angiotensin II, all three activate the same cation channels as does the hyperosmolarity does. Although it remains to be examined if the other excitatory neuotransmitters/modulators affect the same cation channels as the three peptides and osmolarity, it is certain that the major mechanism by which SON neurones are excited is via non-selective cation channels (Fig. 4). AVP and OT that are released in the SON are also known to act on the SON neurones themselves via autoreceptors (8). AVP is reported to act on AVP neurones exclusively, and modulates the action potential discharge in three different manners (9). Specifically, it increases firing rate for neurones firing at a slow rate, decreases firing rate for neurones firing at a high rate, and has no effect on neurones firing at a moderate rate. On the other hand, OT is reported to simply increase the firing rate of OT neurones.

Fig. 4. Non-selective cation channels in SON neurones. The change in the osmolarity gates non-selective cation channels (stretch-inactivated channels) of SON neurones. Besides the osmolarity, some neuropeptides and prostaglandins activate non-selective cation channels and regulate the excitability of SON neurones. NT: Neurotensin.

Presynaptic modulation. Many of the neurotransmitters/modulators known to activate postsynaptic receptors of SON neurones are also reported to act on presynaptic receptors at terminals of GABA and glutamate neurones that innervate the SON neurone (10). Fig. 5 illustrates various patterns of such neurotransmitters affecting SON neurones by acting at the pre- as well as post-synaptic receptors. Glutamate and GABA act on their own terminals via autoreceptors and suppress their own release. Beside these controls, it is evident that glutamate suppresses GABA release and GABA suppresses glutamate release in a reciprocal manner. Noradrenaline suppresses GABA release, and facilitates glutamate release. It also acts on the postsynaptic receptors to cause depolarization of SON neurones, and these three different mechanisms all lead to excitation of SON neurones. On the other hand, NO and PGE2 are reported to affect GABA release only without affecting glutamate release. Finally, PACAP has been reported to act exclusively on the postsynaptic sites without causing any presynaptic actions (11).

Fig. 5. The paterns of regulation of the two major synaptic inputs into the SON, glutamatergic EPSP and GABAergic IPSPs. Various neurotransmitters/neuromodulators act not only on the postsynaptic receptors at the SON neurone but also at presynaptic receptors located at glutamatergic and GABAergic neurones innervating SON neurones. Red solid and black dotted lines in the figure represent stimulatory or excitatory and inhibitory actions, respectively. Open and closed boxes represent presynaptic and postsynaptic receptors, respectively. The receptors for NO mediating this inhibitory action are not known but it seems that the action of NO is not mediated by cGMP (20).

Stimulus-secretion coupling

Studies using the perforated-patch-clamp technique in the neurosecretosomes prepared from the neurohypophysis (namely, the nerve terminals of SON neurones) have revealed that the terminals possess voltage-gated Na+, K+, and Ca2+ channels (4). Furthermore, it has been reported that they have L, N, P/Q, and R type Ca2+ channels. [Ca2+]i measurements with fluorescent dyes such as fura-2 in the terminals were also performed and large increases in [Ca2+]i have been recorded in the preparations during electrical stimulation or high K+ simulation (3). The evidence showing that [Ca2+]i as well as peptide release is strongly suppressed by the Ca2+ channel blockers known to be effective at the terminals suggests that the concept of stimulus-secretion coupling proposed by W.W. Douglas can be applied to this terminals. Namely, action potentials arriving at the terminals in the neurohypophysis activate voltage-gated Ca2+ channels in this region to cause Ca2+-dependent exocytosis of AVP and OT.

Many of the above-mentioned neurotransmitters/modulators not only affect excitability of SON neurones, but also modulate the firing patterns of these neurones. As shown in Fig. 6, AVP neurones exhibit typical phasic bursts, whereas the non stimulated OT neurones show regular continuous firing but show very frequent bursts during lactation. Basically, the release of AVP and OT increase in proportion to the firing rate, probably because higher [Ca2+]i is reached in the terminal during higher action potential discharge. But when the firing frequency is increased further, the release of the peptides tends to decrease and the amount of release plotted against the firing frequency become a bell-shaped. It was reported that the most optimal firing frequency to evoke the release of neurohypolyseal hormones is equal to 13 Hz (12).

Fig. 6. The firing patterns of AVP and OT neurones. AVP neurones exhibit typical phasic bursts, whereas OT neurones do not. The action potentials of the OT neurones in this figure were obtained from anesthetized female rats during lactation. The high frequency bursts are due to suckling.

It has also been also indicated that there is an optimal interval between phasic bursts to evoke AVP release. As shown in Fig. 7, bursts with the appropriate resting period caused much more effective AVP release than bursts without the interval. Interestingly, the interval between the bursts that caused the most effective AVP release was 10-21 s, which is very close to the physiological intervals between bursts reported for rat AVP neurones (13). The reduction of AVP release observed during very high firing frequency and the importance of the interval between the bursts are thought to be due to inactivation of voltage-gated Ca2+ channels and the fatigue of exocytotic machinery.

Fig. 7. The effect of burst patterns on AVP release and [Ca2+]i. A. The amount of AVP released from isolated and incubated neurohypophyses during stimulation with 50 mM K+, or electrical stimulation mimicking physiological bursting (4 times, with or without 21 s intervals). The electrical stimulation with the 21 s interval caused more effective release of AVP. B. Four burst stimulations with 5-180 s intervals were applied and AVP release was measured. The bar graphs represent the total AVP release during the 4 bursts; the closed circles show the rate of AVP release (pg/min). The bursts with 10-21 s intervals were most effective. C. [Ca2+]i measurement with fura-2 in the neurohypophysis. The [Ca2+]i during basal, 50mM K+ stimulation, and the 5 bursts with or without 21 s intervals are shown. Just as AVP release, the peak [Ca2+]i increased more effectively when stimulated with the bursts with the intervals.

Somato-dendritic release (SDR)

Accumulated evidence indicates that the release of AVP and OT occurs not only at the terminals but also at the somatodendritic region at the SON. The latter phase of peptide secretion is called somato-dendritic release (SDR). Recently, reports from several laboratories have indicated that AVP and OT that are released in this fashion into the SON may play an important role in the regulation of functions of SON neurones. The initial evidence suggesting SDR was obtained by electron microscopy, which revealed exocytosis in the dendrites (14). Later, it was been reported that AVP and OT may increase their own release in the SON (15, 16), and that various stimuli such as hyperosmolarity, dehydration, parturition and lactation all increased SDR of AVP and OT. Moreover, SDR is reported to be stimulated by several neurotransmitters (17). It is important that agents causing release of these peptides not always stimulate SDR, and vice versa (18). Nevertheless, SDR is triggered by an increase in [Ca2+]i (19), similarly as the release from the axonal terminals. Accordingly, similar stimulus-secretion coupling with Ca2+ as the key molecule can be postulated for SDR. Moreover, AVP and OT released into the SON are thought to act on the SON neurones themselves in an autocrine fashion (Fig. 8). It has been reported that AVP acts exclusively on AVP neurones, and OT acts only on OT neurones to evoke an increase in [Ca2+]i (17). Taken together, it seems likely that AVP and OT released inside the nucleus may regulate SON neurone functions under physiological conditions. How they are regulated and when such regulations become important may be understood by future investigation of SDR.

Fig.8. Somatodendritic peptide release in the SON. AVP and OT are released not only from the nerve terminals at the neurohypophysis but also in the somatodendritic region in the SON in the exocytotic manner. AVP released from the AVP neurone acts at the AVP neurone itself via autoreceptor or neighbouring AVP neurones to control the firing frequency, whereas OT acts only via autorecepor on the OT neurone to increase the firing frequency (9).

INSIGHTS INTO THE ROLE OF THE NEUROPEPTIDES AVP AND OT IN THE HEART
AVP regulates cardiac function

General considerations. As discussed above, AVP plays an important role in the regulation of plasma osmolarity and renal hemodynamic alterations, water retention and blood pressure (21), and also in cardiac pathophysiology. For instance, in congestive heart failure, AVP worsens heart failure by causing vasoconstriction of arteries and veins, potentially contributing to remodelling of the left ventricle and causing fluid retention and worsening of hyponatremia (22). Beside its action on the vascular system, it has recently been suggested that AVP directly stimulates directly cardiac myocytes and takes part in Ca2+ handling, protein synthesis and differentiation process.

Synthesis and release of AVP in the heart. Though the brain is the main source of AVP where it plays a major role as a neurotransmitter, local AVP production and secretion by the heart has been recently suggested, especially in buffer-perfused rat hearts (23, 24). Interestingly we demonstrated in 8-day-old rat atrial tissue that 1 µM mibefradil (T-type Ca2+ channel blocker) inhibited the secretion of atrial natriuretic factor (ANF) (25). Therefore Ca2+ influx could play an important role in peptide secretion in the heart already at the postnatal stage.

AVP modulates contractility, hypertrophy and differentiation of cardiac myocytes. So far no direct effect of AVP on cardiac myocytes contractility has been reported. Nevertheless there seems to be an indirect dual action of AVP on cardiac inotropy depending on its concentration in Wistar rats and guinea pigs. At concentrations above the physiological range (400-500 pg/ml), AVP decreases contractility due to a coronary ischemia induced by a V1a-mediated coronary vasoconstriction. At physiological concentrations (50-100 pg/mol), coronary perfusion of AVP triggers a positive inotropy reaction (26).

Cardiac hypertrophy is an adaptative response of the heart that is characterised by the increase of the size of the myocytes due to the addition of new sarcomeres, the contractile units. There are two types of hypertrophies: compensatory hypertrophy represents a reversible physiological adaptation, whereas non-compensatory hypertrophy corresponds to a pathophysiological adaptation which is “not” reversible and leads to heart failure (cardiac cells become much less effective in terms of contraction) if the cardiac muscle is not able to recover its normal function. This in turn may cause severe cardiac insufficiency. Furthermore, these pathological states are often accompanied by several forms of arrhythmias (i.e. disturbance of the rhythmic response of the myocytes to the imposed initial electrical stimulation).

An increasing number of important studies report the involvement of particular hormones at different stages of these pathologies, especially those acting through G-protein-coupled receptors. The action of G-protein type S (Gs)-coupled receptors signalling is well known, especially the role of ß-adrenergic stimulation (through ß1 and ß2 receptors) on the induction of cardiac hypertrophy and failure via protein kinase A-induced phosphorylation of L-type Ca2+ channels (LCCs), ryanodine receptors (RyRs) and contractile proteins (27,28). Besides Gs proteins, it is now assumed that agonists of the 7-transmembrane (7TM) spanning proteins coupled to Gq also play a major role in the regulation of cardiac function, particularly in terms of Ca2+ handling and rhythmic activity.

In neonatal cardiomyocytes, AVP promotes hypertrophy by increasing protein synthesis and stimulating Ca2+ release from InsP3-sensitive intracellular stores (29). The augmentation of the rate of protein synthesis by AVP also occurs in adult hearts and relies on V1 receptors using a mechanism that differs from the cAMP-dependent pathway (30). In addition to its role in cardiac hypertrophy, AVP is involved in myocyte differentiation, particularly in the genesis of cardiac contracting cells from stem cells (31). Interestingly the same group showed an increase of the action potential duration and of the LCC current densities in ventricular-like cells isolated from AVP-stimulated embryoid bodies (32).

AVP intracellular signalling. We have seen that AVP promotes cardiac hypertrophy in neonatal myocytes. This effect seems to be dependent on the vasopressin V1a receptor (coupled to the Gq/G11 signalling pathway) leading to extracellular signal-regulated kinase (ERK)1/2 activation (33). It is noteworthy that another study revealed the positive modulation of arachidonic acid release by AVP by activation of extracellular Ca2+ influx, PKC and p42 mitogen-activated protein kinase (MAPK) in H9c2 cells, a cardiac cell line (34). Interestingly the same induction of ERKs (p42/p44 MAPK) was also found in rat cardiomyocytes stimulated by AVP and this phenomenon was shown to be dependent of V1 receptor, tyrosine kinase, PKC and MAPK kinase (MEK) (35).

Gq-proteins are heterotrimeric GTPases composed of , ß, and subunits that are coupled to various signalling cascades. The hormonal activation of Gq-coupled receptors triggers the detachment of the subunit Gq that stimulates phospholipase PLC-ß to produce the intracellular messengers inositol trisphosphate (InsP3) and di-acyl-glycerol (DAG) from phosphoinositol-4,5-biphosphate (PIP2). InsP3 diffuses into the cytosol and elicits the release of Ca2+ from intracellular stores through InsP3Rs leading to elementary Ca2+ signals and to global Ca2+ oscillations which mainly rely on positive and negative feedback of the cytosolic Ca2+ concentration [Ca2+]c on InsP3Rs (36). In addition DAG recruits and activates diverse isoforms of protein kinase C (PKC) to the membrane (37). PKCs, in turn, mediate a wide range of phosphorylation events that regulate fast phenomena such as ionic channel activity, but also long-term events such as gene expression for instance by the activation of the MAPK cascades.

AVP evokes the elevation of intracellular Ca2+ concentration in neonatal cardiomyocytes as well. This phenomenon is PLC-dependent and relies on InsP3-sensitive stores, the presence of Mg2+ and extracellular Ca2+ but is independent of PKC, cyclooxygenase activation, mitochondria and ryanodine receptors (38). Moreover AVP enhances the LCC current in guinea pig ventricular myocytes (39) by increasing the channel opening probability and opening time (40). In another model, human coronary myocytes (HCM), [Ca2+]i is modulated by an atypical Na+ current (41). We provided evidence that endothelin (100 nM) and AVP (100 nM) induced large increases of [Ca2+]i in HCM when these hormones were applied for less than 30 s (42). Surprisingly, cyclosporin A, an immunosuppressive agent, enhanced the [Ca2+]i responses to endothelin and AVP (Fig. 9), suggesting a mechanism for its side effect on hypertension (42).

Fig. 9. AVP-induced [Ca2+]i increase in human coronary myocytes is modified by an immunosuppressive agent (CsA). The bar diagrams show the mean peak amplitude of the [Ca2+]i responses to an acute stimulation with AVP without (-CsA) or with chronic treatment (+CsA) during 24 hrs of CsA. Data represent the mean of 6 experiments ± S.E.M. (P <0.001 between non-treated and treated with CsA).

Furthermore, the AVP signalling pathway seems to be linked to another signalling cascade, the nitric oxide (NO) synthesis. In neonatal rat cardiac myocytes stimulated by interleukin-1 beta, AVP augments NO production and activates the expression of the inducible NO synthase through a V1a receptor-dependent mechanism (43). As described above, AVP mediates the differentiation of embryonic stem cells into cardiac cells. This effect is blocked by an inhibitor of NO synthase. However, in that case the stimulation of the endothelial NO synthase expression by AVP to the V2 receptor (coupled to the cAMP pathway) (32). Finally, beside the MAPK cascade, the Ca2+-calmodulin-calcineurin-NFAT pathway appears to be involved in the transcription regulation of the human atrial myosin light chain 1 induced by AVP in H9c2 cells (44). The binding of AVP to its receptors in the heart triggers a plethora of transductional and transcriptional processes. Therefore it is nowadays very challenging to design drugs acting on V1 and V2 receptors in order to address specific heart pathology issues

AVP receptors as target for therapies. Importantly, in patients with chronic heart failure, AVP synthesis and release are substantially up-regulated (26). This suggests that AVP effects should then be controlled especially with regard to cardiomyocytes. An elegant method is based on using AVP receptor antagonists. For instance, in neonatal rat cardiac muscle cells, YM087, a nonpeptide AVP receptor antagonist with a high affinity for V1a (Ki=0.63 nM), inhibits the AVP-induced increase of protein synthesis in a concentration-dependent manner (45). The problem with the treatment of congestive heart failure is to find drugs affecting both cardiac dysfunction and renal water balance function. Recently, two new compounds SR 49059 and SR 121463B, V1a and V2 antagonists respectively, succeeded in regulating both cardiac hypertrophy and water retention in rats with aortocaval fistula (21). Interestingly, clinical trials have been undertaken in patients with heart failure. Conivaptan (combined V1a and V2 receptor antagonist), tolvaptan and lixivaptan (V2 receptor antagonists), managed to reduce water retention while the concentration of Na2+ in the serum was preserved. Nevertheless, none of these compounds so far showed a significant improvement in patients’ health (22).

Action of OT in the heart

Though the role of OT in the heart is not yet as comprehensively described as for AVP, an increasing amount of data suggest the presence of an OT system in cardiac cells together with the involvement of OT in diverse signalling pathways and cardiac differentiation. More than ten years ago, the first evidence for the existence of OT receptors in atria and ventricles was provided by Gutkowska and collaborators (46). Actually the authors showed in this study the amplification of OT receptor transcripts by PCR and undertook a competitive binding assay, without isolation of the receptors or structural components. In the same study, using OT antagonists, they demonstrated that OT receptors were responsible for the release of atrial natriuretic peptide (ANP) to slow the heart rate and reduce blood pressure. Later the same group could differentiate mouse P19 embryonic stem cells into cardiomyocyte-like beating cells with OT and DMSO, this effect being OT receptor-dependent (47). Recently, these authors demonstrated that they could block the OT-induced cardiac myogenesis by applying an inhibitor of NO synthases, indicating the central role of NO signalling in cardiac differentiation mediated by OT (48).

In addition, another investigation confirmed the cross-talk of NO synthases and OT in neonatal rat heart where OT chronic application increased the amount of NO synthase (eNOS) transcripts (49). Moreover, in a similar study, Pournajafi-Nazarloo and co-workers (50) shed some light on the increase of estrogen receptor alpha mRNAs by OT, estrogen receptors playing an important role in cardioprotection. Whether Ca2+ is critical in the effects of OT still needs further experimental examination, especially with regards to exocytosis leading to ANP release.

AVP and OT in the heart: conclusion

Hence, AVP and OT appear to be members of the “neurohormone” family such as endothelin-1 (ET-1), phenylephrine (PE) and angiotensin II. Many studies have reported that the chronic application of ET-1 and PE to neonatal cardiac myocytes induces hypertrophic responses (51,52). In adult heart cells, ET-1 and PE are believed to activate a remodelling of the Ca2+ handling when chronically applied (53) as described in Fig. 10. The term “neurohormones” is based on the fact that these hormones acting on cardiac cells are also produced in neuronal structures, such as dorsal root ganglia (54), and can be released for instance from post-ganglionic sympathetic neurones for instance (55). The aim of the present work was to discuss the possible roles of the neuropeptides AVP and OT on cardiac tissue. These hormones mainly act through the Gq-coupled signalling pathway and regulate [Ca2+]i. Importantly, AVP and OT are involved in contractility, hypertrophy and differentiation. The use of agents affecting their receptors might be a promising alternative to help diminishing the consequences of severe cardiovascular diseases. The study of chronic effects of AVP and OT on isolated cardiomyocytes is rendered difficult because of the dedifferentiation process through which the adult myocytes undergo during culture and would certainly benefit from the use of a reliable long-term culture model (56).

Fig. 10. Influence of Gq signalling on Ca2+ handling in cardiac myocytes. Gq pathway is illustrated in this scheme with a particular emphasis of its involvement on Ca2+ release during excitation-contraction coupling. Ca2+ release from the sarcoplasmic reticulum (SR) is expressed as highly localised signals (sparks) or global events (waves). The termination of the Ca2+ rise is mainly due to activation of the Na+/Ca2+ exchanger at the plasma membrane and to the sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA). The examples of signalling by endothelin-1 (ET-1) and phenylephrine (PE) are shown, but the same cascade of activation could be attributed to AVP and OT. This cartoon was adapted with permission from Viero’s thesis (2008) (59).

GENERAL OUTLOOK
We now know more about the mechanisms of action of AVP and OT in the central nervous system (57) and in the peripheral nervous system (58), especially with regards to Ca2+ handling. Thus far, not much has been understood on the interaction between Ca2+ signalling and second messenger systems and on involvement of various subtypes of AVP and OT receptors in the heart. In the light of what was shown in the of the hypothalamo-neurohypophyseal system, the cross-talk between AVP and OT action and other neurotransmitters/neuromodulators still needs to be deciphered in cardiac tissues.

List of abbreviations

AVP: ANP: CCK: CNS: DAG: ERK: ET-1: GABA: Gq: HCM: HNS: InsP3: LCC: MAPK: MEK: NO: OT: PACAP: PE: PIP2: PKC: PLC: PNS: PVN: RyR: SON:

arginine vasopressin atrial natriuretic peptide cholecystokin central nervous system di-acyl-glycerol extracellular signal-regulated kinase endothelin-1 -aminobutyric Acid G protein type q human coronary myocyte hypothalamic-neurohypophysial system inositol trisphosphate L-type Ca2+ channel mitogen-activated protein kinase MAPK kinase nitric oxide oxytocin pituitary adenylate cyclase activating polypeptide phenylephrine phosphoinositol-4,5-biphosphate protein kinase C phospholipase C peripheral nervous system paraventricular nucleus ryanodine receptor supraoptic nucleus

Acknowledgments: This work was supported by the grants in aid by the ministry of Education, Sports and Science 18380175-Japan and by the Japanese Society for Promotion of Science senior visiting fellowship (2004 and 2008) for G. Dayanithi. We thank the publisher Kagakuhyoronsha, Tokyo, Japan for permitting us to use some of the illustrations presented in this manuscript and Dr. Keith Langley, Montpellier-France for critical reading and language editing of the manuscript.

Conflicts of interest statement: None declared.


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