GTP gamma S causes contraction of skinned frog skeletal muscle via the DHP-sensitive Ca2+ channels of sealed T-tubules

In vivo expression of G-protein beta1gamma2 dimer in adult mouse skeletal muscle alters L-type calcium current and excitation-contraction coupling

A number of G-protein-coupled receptors are expressed in skeletal muscle but their roles in muscle physiology and downstream effector systems remain poorly investigated. Here we explored the functional importance of the G-protein betagamma (Gbetagamma) signalling pathway on voltage-controlled Ca(2+) homeostasis in single isolated adult skeletal muscle fibres. A GFP-tagged Gbeta(1)gamma(2) dimer was expressed in vivo in mice muscle fibres. The GFP fluorescence pattern was consistent with a Gbeta(1)gamma(2) dimer localization in the transverse-tubule membrane. Membrane current and indo-1 fluorescence measurements performed under voltage-clamp conditions reveal a drastic reduction of both L-type Ca(2+) current density and of peak amplitude of the voltage-activated Ca(2+) transient in Gbeta(1)gamma(2)-expressing fibres. These effects were not observed upon expression of Gbeta(2)gamma(2), Gbeta(3)gamma(2) or Gbeta(4)gamma(2). Our data suggest that the G-protein beta(1)gamma(2) dimer may play an important regulatory role in skeletal muscle excitation-contraction coupling.

GTP gammaS removal of D-600 block of skeletal muscle excitation-contraction coupling

G proteins interacting with dihydropyridine receptors (DHPR) in transverse tubules (TT) of skeletal muscle may have a role in skeletal excitation-contraction (EC) coupling. The aim of this study was to determine the effects of G protein-specific nucleotides [guanosine 5'-O-(3-thiotriphosphate) (GTP gammaS) and guanosine 5'-O-(2-thiodiphosphate) (GDP betaS)] on the EC coupling mechanism in the presence of D-600, an agent that blocks EC coupling by immobilizing the voltage-sensing subunit of the DHPR in its inactivated state. By use of the mechanically peeled single-fiber preparation from rabbit adductor magnus skeletal muscle, 50 microM GTP gammaS and 500 microM GDP betaS were applied with the fiber in a D-600-induced state of blocked EC coupling. Neither nucleotide served as an independent stimulus for sarcoplasmic reticulum (SR) Ca2+ release when added to the TT polarizing bath under conditions of D-600 block. The presence of GTP gammaS or GDP betaS during a complete EC coupling cycle removed the D-600 block of EC coupling, despite continuous bath D-600. After the nucleotides were washed out, in the continued presence of D-600, the D-600 block of EC coupling was reestablished. In contrast, GTP gammaS added only during the period of TT depolarization under D-600 block did not remove the D-600 block of EC coupling, even though GTP gammaS did stimulate SR Ca2+ release. GTP gammaS had no effect on submaximum (0.5-1.0 mM) caffeine contractures and thus is unlikely to be acting through the Ca2+-induced Ca2+ release mechanism of the SR. These data suggest that the molecular binding site for GTP gammaS and GDP betaS is likely to be in the TT near the DHPR, perhaps on a G protein.

Electrophysiological and iontophoretic aspects of the habenular influence on hippocampal neurones

In previous experimental studies, carried out on cats, we demonstrated that electrical stimulation of lateral habenula (LH) at 0.5-3.0 Hz or 5-20 Hz had a double effect (low frequency-excitation; high frequency-inhibition) on the spontaneous firing rate of single hippocampal neurones. Our results, in agreement with similar case studies, allowed us to hypothesise that in the habenular modulation of the hippocampus the raphe nucleus is probably involved. In fact, all the effects of LH stimulation were antagonised by the iontophoretic intrahippocampal application of methysergide. In the present series of experiments, performed on rats, it was possible to demonstrate that LH stimulation at 1-10 Hz causes an excitation of a progressively major number of hippocampal neurones depending upon the increase of frequency stimulation. The absence of habenulo-induced effects after a iontophoretic application of methysergide on single hippocampal units suggests the involvement of the raphe nucleus. Furthermore, in consideration of recent anatomical evidences demonstrating an excitatory projection between LH and raphe nucleus, intraraphal N-methyl-D-aspartate (NMDA) application, performed through a Hamilton microsyringe, induces an inhibitory effect. All the results suggest that in the raphe context it is possible to hypothesise the presence of an intrinsic interneurone, directly activated by the excitatory projection arising from the LH; this interneurone is likely inhibitory on the serotonergic raphe-hippocampus efferent neurone. This functional organization is responsible for the effect of LH stimulation at different frequencies as well as for the effects of intraraphal NMDA application.

Capacitative Ca2+ influx in adrenal glomerulosa cells: possible role in angiotensin II response

We examined the effect of the depletion of intracellular Ca2+ stores on Ca2+ influx in rat glomerulosa cells. Depletion of intracellular Ca2+ stores was achieved by inhibiting sarco/endoplasmic reticulumtype Ca(2+)-ATPase with thapsigargin or 2,5,di-(t-butyl)-1,4-benzohydroquinone (t-BHQ). Both inhibitors induced a sustained rise in cytoplasmic Ca2+ concentration. The initial rise was observed also in Ca(2+)-free medium, while the sustained phase disappeared, indicating that the latter requires Ca2+ influx. In Ca(2+)-free medium, the readdition of Ca2+ induced a steeper and higher rise in intracellular Ca2+ concentration in thapsigargin-treated cells than in controls, supporting the role of Ca2+ influx. In normal medium, the addition of Cd2+ (80 microM) evoked an immediate inhibition of the sustained phase of thapsigargin response. The response to thapsigargin was insensitive to nifedipine. Thapsigargin failed to enhance Mn2+ quenching of fura 2. Our results provide evidence for the existence of capacitative Ca2+ influx in rat glomerulosa cells and indicate that dihydropyridine-sensitive Ca2+ channels do not participate in capacitative Ca2+ entry. High concentrations of thapsigargin and t-BHQ, similar to the reported effects of angiotensin II and vasopressin, inhibited K(+)-induced Ca2+ signals. These effects appear, however, to be independent of the depletion of internal Ca2+ stores.

G protein effects on Ca2+ release and excitation-contraction coupling in skeletal muscle fibers

Skeletal muscle excitation-contraction (EC) coupling may involve secondary mechanisms, such as those involving G proteins. The aim of this study was to identify possible G protein effects on sarcoplasmic reticulum (SR) Ca2+ release, in general, and on voltage-dependent EC coupling, in particular. Effects of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) were studied using a single peeled rabbit skeletal muscle fiber preparation that is capable of releasing SR Ca2+ in response to transverse tubule (TT) depolarization. Because of possible nonspecific and residual effects of 200 microM GTP gamma S, a lower concentration of 50 microM GTP gamma S was used to stimulate G proteins in the peeled fiber system. Under conditions for steady-state (resting) polarization of TT, GTP gamma S rarely elicited SR Ca2+ release. When the TTs are in steady-state (resting) depolarization, 50 microM GTP gamma S or GTP elicited SR Ca2+ release and associated tension transients in only 69% of fibers tested. In contrast GTP gamma S always augmented Ca2+ release during TT depolarization-induced EC coupling. These results indicate the presence of at least two excitatory G proteins for SR Ca2+ release, only one of which is a modulator, albeit nonessential, of peeled fiber EC coupling.

Plasmalemmal dihydropyridine receptors modify the function of subplasmalemmal inositol 1,4,5-trisphosphate receptors: a hypothesis

Experimental observations on rat glomerulosa cells inspired a model which postulates that plasmalemmal dihydropyridine receptors are in juxtaposition and interaction with inositol 1,4,5-trisphosphate receptors in subplasmalemmal calciosomes. Activation of dihydropyridine receptors promotes the Ca2+ releasing effect of inositol 1,4,5-trisphosphate. The most important observations compatible with the model are the following: (1) angiotensin II does not influence Ca2+ influx during the peak phase of Ca2+ signal; (2) dihydropyridine drugs modify the initial peak of the Ca2+ signal induced by angiotensin II; (3) inhibitors of the dihydropyridine receptor reduce the initial Ca2+ signal also in the presence of 5 mM Ni2+, an inhibitor of voltage dependent Ca2+ influx; and (4) changes in extracellular K+ concentration within the physiological range also modify the cytoplasmic Ca2+ response to angiotensin II.

The role of phosphoinositide metabolism in Ca2+ signalling of skeletal muscle cells

1. The mobilization of Ca2+ from intracellular stores by D-myo-inositol 1,4,5-triphosphate[Ins(1,4,5)P3] is now widely accepted as the primary link between plasma membrane receptors that stimulate phospholipase C and the subsequent increase in intracellular free Ca2+ that occurs when such receptors are activated (Berridge, 1993). Since the observations of Volpe et al. (1985) which showed that Ins(1,4,5)P3 could induce Ca2+ release from isolated terminal cisternae membranes and elicit contracture of chemically skinned muscle fibres, research has focused on the role of Ins(1,4,5)P3 in the generation of SR Ca2+ transients and in the mechanism of excitation-contraction coupling (EC-coupling). 2. The mechanism of signal transduction at the triadic junction during EC-coupling is unknown. Asymmetric charge movement and mechanical coupling between highly specialized triadic proteins has been proposed as the primary mechanism for voltage-activated generation of SR Ca2+ signals and subsequent contraction. Ins(1,4,5)P3 has also been proposed as the major signal transduction molecule for the generation of the primary Ca2+ transient produced during EC-coupling. 3. Investigations on the generation of Ca2+ transients by Ins(1,4,5)P3 have been conducted on ion channels incorporated into lipid bilayers, skinned and intact fibres and isolated membrane vesicles. Ins(1,4,5)P3 induces SR Ca2+ release and the enzymes responsible for its synthesis and degradation are present in muscle tissue. However, the sensitivity of the Ca2+ release mechanism to Ins(1,4,5)P3 is highly dependent on experimental conditions and on membrane potential. 4. While Ins(1,4,5)P3 may not be the major signal transduction molecule for the generation of the primary Ca2+ signal produced during voltage-activated contraction, this inositol polyphosphate may play a functional role as a modulator of EC-coupling and/or of the processes of myoplasmic Ca2+ regulation occurring on a time scale of seconds, during the events of contraction.

Isoproterenol and GTP gamma S inhibit L-type calcium channels of differentiating rat skeletal muscle cells

In adult skeletal muscle, G-proteins have been shown to modulate the calcium channels both directly and through a cAMP-dependent phosphorylating mechanism. We have investigated the action of G-proteins on the L-type calcium current in cultured rat muscle cells (myoballs) under voltage clamp in whole cell or perforated patch modes. Intracellular photolytic release of 200 microM GTP gamma S inhibited the L-type calcium current. Inclusion of 500 microM uncaged GTP gamma S in the patch pipette in the whole cell configuration reduced the calcium current by a similar amount. Under perforated patch conditions external application of 10 microM of the beta-adrenergic agonist isoproterenol also reduced the calcium current. Pretreatment of the cells with pertussis toxin reversed the effect of GTP gamma S and removed that of isoproterenol. We conclude that rat myoballs contain beta-adrenergic receptors that inhibit the L-type calcium current, and that this inhibition is mediated by a pertussis toxin-sensitive G-protein.

Identification of heterotrimeric and low molecular weight GTP-binding proteins in rabbit skeletal muscle longitudinal sarcoplasmic reticulum

Direct photoaffinity labeling of proteins of longitudinal sarcoplasmic reticulum (LSR) of rabbit skeletal muscle with [32P]GTP revealed GTP-binding proteins of about 52, 45 and 30 kDa. ADP-ribosylation with [32P]NAD in the presence of cholera toxin (CTX) or pertussis toxin (PTX) indicates the existence of a CTX substrate (about 45 kDa); no PTX substrates were observed. Western blots of LSR probed with RM/1, an antiserum against a decapeptide from the C-terminus of Gs alpha, showed an immunoreactive band at about 45 kDa. [32P]GTP overlays of Western blots of LSR showed a heavily-labeled protein of about 29 kDa and one or more additional slightly smaller proteins that were more weakly labeled. When LSR was subjected to mild trypsin hydrolysis, the Western blot overlay revealed three bands at about 23, 25 and 29 kDa. Western blots of LSR proteins showed no significant immunoreactivity with the anti-(pan)-ras monoclonal antibodies 142-24E05 and Ras 11. ADP-ribosylation of LSR with [32P]NAD in the presence of C3 exoenzyme of Clostridium botulinum yielded a labeled band at about 23 kDa. Our results indicate the presence in rabbit LSR of a Gs alpha, the absence of Gi and G(o), and the presence of several low molecular weight GTP-binding proteins, distinct from p21 ras, one of which belongs to the rho or rac family.

Involvement of a pertussis toxin-sensitive G-protein in excitation-contraction coupling of intact and cut-end voltage-clamped skeletal muscle fibres

In voltage-clamped frog muscle fibres 10 ng/ml PTX induced a decrease (approximately 35%) of tension when applied externally. Internal application in cut-end fibres significantly depressed tension after 20 min. This effect increased with time to reach 65% after 60 min. PTX shifted the voltage-dependent inactivation curve of tension by 30 mV towards hyperpolarizations and this was counteracted by raising external calcium concentration. The toxin induced a parallel decrease in tension and voltage-sensitive charge movement (49 +/- 9% and 52 +/- 6% respectively; n = 6). This was not counteracted by prior impregnation with forskolin. Internally applied GTP gamma S (500 microM) induced a simultaneous increase in tension (57 +/- 5%) and charge amount displaced (40 +/- 7%). By contrast, GDP beta S decreased tension and charge movement by 35 +/- 5% and 36 +/- 6% respectively.

Excitation-contraction coupling in skeletal muscle fibres of rat and toad in the presence of GTP gamma S

1. Rapid force responses were elicited in single mechanically skinned fibres from extensor digitorum longus (EDL) muscle of the rat when the fibres were depolarized by substituting K+ in the bathing solution with Na+. The properties of these depolarization-induced responses, the responses to lowered [Mg2+], and the characteristics of the slow prolonged response ('second component') produced in 'loaded' fibres by choline chloride (ChCl) substitution, were virtually identical to those observed previously in skinned fibres from toad muscle. 2. At physiological levels of [Mg2+] (1 mM) and Ca2+ loading, application of 50 microM- to 1 mM-GTP gamma S (guanosine-5'-O-(3-thiotriphosphate), a non-hydrolysable analogue of GTP) did not produce a response in any mammalian or amphibian fibre, even though the depolarization-induced coupling was totally functional. Furthermore, the presence of GTP gamma S had no apparent effect on the size, the threshold or the maximum number of responses which could be elicited by depolarization. 3. GTP gamma S did not elicit any response when excitation-contraction coupling was abolished by prolonged depolarization or by chemically skinning the fibre with saponin or by 24 h exposure to low [Ca2+] (5 mM-EGTA). 4. GDP beta S (guanosine-5'-O-(2-thiodiphosphate), 250 microM or 1 mM) neither evoked a response nor affected the responses to depolarization or caffeine. 5. When the [Mg2+] was lowered to 0.2 mM and the fibres were heavily loaded with Ca2+, addition of GTP gamma S (250 microM or 1 mM) induced a small response in about 50% of fibres, but depolarization-induced responses were not affected in any fibres. 6. Asymmetric charge movement recorded in EDL fibres with the vaseline-gap voltage clamp was not affected by the application of 1 mM-GTP gamma S to the cut ends of the fibres for up to 1 h. 7. These data imply that GTP-binding proteins (G-proteins) are not involved in coupling the voltage sensors to Ca2+ release in skeletal muscle. Furthermore, there was no evidence that G-proteins play any role in modulating the voltage sensors, though this possibility could not be totally excluded.