Noradrenaline release and the effect of endogenous activation of the phospholipase C/protein kinase C signalling pathway in rat atria

Functional development of the ovarian noradrenergic innervation

A substantial fraction of the noradrenergic innervation targeting the mammalian ovary is provided by neurons of the celiac ganglion. Although studies in the rat have shown that noradrenergic nerves reach the ovary near the time of birth, it is unknown how the functional capacity of this innervation unfolds during postnatal ovarian development. To address this issue, we assessed the ability of the developing ovary to incorporate and release (3)H-norepinephrine. Incorporation of (3)H-norepinephrine was low during the first 3 wk of postnatal life, but pharmacological inhibition of norepinephrine (NE) neuronal uptake with cocaine showed that an intact transport mechanism for NE into nerve terminals is already in place by the first week after birth. Consistent with this functional assessment, the mRNA encoding the NE transporter was also expressed in the celiac ganglion at this time. During neonatal-infantile development [postnatal (PN) d 5-20], the spontaneous, vesicle-independent outflow of recently taken up NE was high, but the NE output in response to K(+)-induced depolarization was low. After PN d 20, spontaneous outflow decreased and the response to K(+) increased markedly, reaching maximal values by the time of puberty. Tyramine-mediated displacement of NE stored in vesicles, which displace vesicular NE, showed that vesicle-dependent NE storage becomes functional by PN d 12 and that vesicular release increases during the juvenile-peripubertal phases of sexual development. These results indicate that vesicular release of NE from ovarian noradrenergic nerves begins to operate by the third week of postnatal life, becoming fully functional near the time of puberty.

Molecular mechanisms underlying the modulation of exocytotic noradrenaline release via presynaptic receptors

The release of noradrenaline from nerve terminals is modulated by a variety of presynaptic receptors. These receptors belong to one of the following three receptor superfamilies: transmitter-gated ion channels, G protein-coupled receptors (GPCR), and membrane receptors with intracellular enzymatic activities. For representatives of each of these three superfamilies, receptor activation has been reported to cause either an enhancement or a reduction of noradrenaline release. As these receptor classes display greatly diverging structures and functions, a multitude of different molecular mechanisms are involved in the regulation of noradrenaline release via presynaptic receptors. This review gives a short overview of the presynaptic receptors on noradrenergic nerve terminals and summarizes the events involved in vesicle exocytosis in order to finally delineate the most important signaling cascades that mediate the modulation via presynaptic receptors. In addition, the interactions between the various presynaptic receptors are described and the underlying molecular mechanisms are elucidated. Together, these presynaptic signaling mechanisms form a sophisticated network that precisely adapts the amount of noradrenaline being released to a given situation.

Chronic Exercise Improves Myocardial Inotropic Reserve Capacity Through α1-Adrenergic and Protein Kinase C-Dependent Effects in Senescent Rats

We have previously demonstrated that α1-adrenergic (AR)-mediated contraction is diminished in the senescent rat heart, in part due to alterations in protein kinase C (PKC) signaling. Since chronic exercise training (EX) can exert independent effects on increasing α1-AR contraction in the adult rat heart, we sought to determine whether age-related defects in α1-AR contraction could be reversed by chronic EX. We further hypothesized that improved α1-AR contraction by EX may be PKC dependent. Adult (4 months; Y) and aged (24 months; O) male F344 rats were treadmill-trained (n = 12–13/group; TR) at ∼70% of VO2max for 12 weeks or remained sedentary (YSED, YTR, OSED, OTR). Training status was verified by plantaris citrate synthase activity and left ventricular (LV) contractile responses (dP/dt) to α1-AR stimulation were assessed in Langendorff-perfused hearts using the α1-AR agonist phenylephrine (PE; 10−5 M) with and without the PKC inhibitor chelerythrine (CE; 10−6 M). α1-AR stimulation elicited greater increases in LV dP/dt in hearts isolated from OTR (4525.4 ± 224.1 mmHg/s) versus OSED (3658.9 ± 291.0 mmHg/s), while CE abolished PE-induced effects (OTR, 4069.2 ± 341.2) versus (OSED, 3608.9 ± 321.2) (p <.01). Upon western blotting, phosphospecific antibodies directed at PKCε (pSer729) revealed greater levels in LV isolated from YTR versus YSED, and EX ameliorated aged-related reductions in OSED (p <.001). Basal PKCε mRNA levels were also greater in YTR and OTR versus YSED (p <.01). PE-induced increases in phosphor-PKCδ (pThr507) levels observed in OSED were attenuated in OTR (p <.03). Chronic EX was also associated with significant reductions in PKCα (pSer657) levels following PE in OTR (p <.002). The results indicate that age-related reductions in α1-AR contraction can be partially reversed by EX in the rat heart. These results further suggest that alterations in PKC levels underlie, at least in part, EX-induced improvements in α1-AR contraction.

Involvement of B-50 (GAP-43) phosphorylation in the modulation of transmitter release by protein kinase C

1. Protein kinase C (PKC) is a family of enzymes that is activated by diacylglycerol (DAG) following phospholipase (PL) C activation. Protein kinase C may also be activated by metabolites and arachidonic acid generated by breakdown of membrane phospholipids by PLD and PLA2, respectively. Subsequent to PKC activation, key protein substrates are phosphorylated, resulting in the facilitation of transmitter release. 2. Phorbol esters are compounds that mimic the actions of DAG on PKC and have been shown to facilitate stimulation-induced (S-I) transmitter release in rat brain. However, some phorbol esters that have a high affinity for PKC have no effect on transmitter release, whereas others with a lower affinity for PKC markedly elevate S-I transmitter release. 3. The structure and, more importantly, the lipophilicity of the phorbol esters determines their ability to access and activate the intraneuronal pools of PKC that are involved with transmitter release. In studies in which cell membranes were intact, phorbol esters did not display the characteristics expected based on their affinities for PKC in contrast with studies in disrupted synaptosomes. This supports the hypothesis that the membrane plays a critical role in determining the effects of phorbol esters on PKC. 4. B-50, a PKC substrate thought to be involved in transmitter release, also appears to be differentially phosphorylated by various phorbol esters. The effects on B-50 phosphorylation in intact synaptosomes, but not disrupted synaptosomes, are well correlated with the effects of phorbol esters on S-I transmitter release. 5. B-50 is colocalized with actin, which has also been suggested to play an important role in facilitating the movement of reserve pools of transmitter vesicles to the readily releasable state. Therefore, it is possible that the phosphorylation status of B-50 directly influences the organization of actin filaments, thereby allowing transmitter output to be sustained under high levels of stimulation.

Diminished alpha1-adrenergic-mediated contraction and translocation of PKC in senescent rat heart

Myocardial reserve function declines with aging due in part to reduced alpha- and beta-adrenergic receptor (AR)-mediated contractile augmentation. Whereas specific age-associated deficits in beta-AR signaling have been identified, it is not known which components of the alpha1-AR signaling cascade, e.g., protein kinase C (PKC) and associated anchoring proteins (receptors for activated C kinase; RACKs), underlie deficits in alpha1-AR contractile function with aging. We therefore assessed cardiac contraction (dP/dt) in Langendorff perfused hearts isolated from adult (5 mo) and senescent (24 mo) Wistar rats following maximal alpha1-AR stimulation with phenylephrine (PE), and we measured the subcellular distribution of PKCalpha and PKCepsilon, and their respective anchoring proteins RACK1 and RACK2 by Western blotting. The maximum dP/dt response to PE (10(-5) M) was significantly reduced by 41% in 24-mo-old vs. 5-mo-old (P < 0.01). Inhibitory effects of PKC blockade (chelerythrine; 10 microM) on dP/dt following alpha1-AR stimulation with PE observed in adult hearts were absent in 24-mo-old hearts (P < 0.01). In 5-mo-old hearts, PE elicited reductions in soluble PKCalpha and PKCepsilon levels, while increasing particulate PKCalpha and PKCepsilon levels to a similar extent. In contrast, soluble PKCalpha and PKCepsilon levels in 24-mo-old hearts were increased in response to PE; particulate PKCepsilon and PKCalpha were unchanged or reduced and associated with significant reductions in particulate RACK1 and RACK2. The results indicate, for the first time, that selective translocation of PKCalpha and PKCepsilon in response to alpha1-AR stimulation is disrupted in the senescent myocardium. That age-related reductions in particulate RACK1 and RACK2 levels were also observed provide evidence that alterations in PKC-anchoring proteins may contribute to impaired PKC translocation and defective alpha1-AR contraction in the aged rat heart.

Activation of signal-transduction mechanisms may underlie the therapeutic effects of an applied electric field

Successful treatment of various medical complaints with an applied electric field has been reported over the years. The identities of the cellular mechanisms that are influenced by this type of treatment and facilitate the positive effects, remain elusive. A study of many in vitro and in vivo reports revealed that the beneficial effects can be attributed to the activation of membrane proteins, and specifically proteins involved in signal-transduction mechanisms. Not only may the proteins be affected but it is now well established that enhanced Ca(2+)influx, observed to follow electric stimulation of cells, also contributes to many calcium-dependent cellular processes which can be linked to the therapeutic effects discussed in this paper. An hypothesis of the physical changes caused by an applied, relatively small (10(3)to 10(4)V m(-1)range), electric field with low to moderate frequency (below 150 Hz), is postulated.

Protein kinase C: a physiological mediator of enhanced transmitter output

Protein kinase C (PKC), activated by either diacylglycerol and/or arachidonic acid, through the activation of presynaptic receptors or nerve or nerve depolarization is involved is involved in the enhancement of transmitter release from many neural types. This facilities is most likely mediated by the phosphorylation of proteins involved in vesicle dynamics although a role for ion channels cannot be ruled out. PKC is not fundamental to the release process but rather has a modulatory role of PKC is to help maintain transmitter output during prolonged or elevated levels of activation and this seems to parallel suggestions that PKC is involved in the movement of reserve pools of vesicles into release-study sites. presynaptic facilitatory actions mediated by PKC are also involved in integrated modulatory functions such as long term potentiation, again where it elevates or maintains transmitter output. Although studies have tried to identify specific roles for various PKC isoforms, the actions of phorbol esters in elevators transmitter release do not fit with known potencies on individual isoforms and lit suggests that PKC may be located at an intraneuronal location which is difficult to access for lipophilic phorbol esters and further work is required in this area.