G(o)-2 protein mediates the reduction in Ca2+ currents by somatostatin in cultured ovine somatotrophs

Somatostatin and dopamine receptor regulation of pituitary somatotroph adenomas

Somatostatin and dopamine receptors are expressed in normal and tumoral somatotroph cells. Upon receptor stimulation, somatostatin and the somatostatin receptor ligands octreotide, lanreotide, and pasireotide, and to a lesser extent, dopamine and the dopamine analogs bromocriptine and cabergoline, suppress growth hormone (GH) secretion from a GH-secreting pituitary somatotroph adenoma. Somatostatin and dopamine receptors are G_αi-protein coupled that inhibit adenylate cyclase activity and cAMP production and reduce intracellular calcium concentration and calcium flux oscillations. Although their main action on somatotroph cells is acute inhibition of GH secretion, they also may inhibit GH production and possibly somatotroph proliferation. These receptors have been reported to create complexes that exhibit functions distinct from that of receptor monomers. Somatostatin suppression of GH is mediated mainly by somatostatin receptor subtype 2 and to a lesser extent by SST5. Human somatostatin receptor subtype 5 has also been shown to harbor mutations associated with GH levels, somatotroph tumor behavior, and somatostatin receptor ligand (SRL) responsiveness. Reviewing current knowledge of somatostatin and dopamine receptor expression and signaling in normal and tumoral somatotroph cells offers insights into mechanisms underlying SRL and dopamine agonist effectiveness in patients with acromegaly.

Goldfish brain somatostatin-28 differentially affects dopamine- and pituitary adenylate cyclase-activating polypeptide-induced GH release and Ca(2+) and cAMP signals

Dopamine (DA) and pituitary adenylate cyclase-activating polypeptide (PACAP) stimulate goldfish growth hormone (GH) release via cAMP- and Ca(2+)-dependent pathways while DA also utilizes NO. In this study, identified goldfish somatotropes responded to sequential applications of PACAP and the DA D1 agonist SKF38393 with increased intracellular Ca(2+) levels ([Ca(2+)](i)), indicating that PACAP and DA D1 receptors were present on the same cell. A native goldfish brain somatostatin (gbSS-28) reduced SKF38393-stimulated cAMP production and PACAP- and NO donor-elicited GH and [Ca(2+)](i) increases, but not PACAP-induced cAMP production nor the GH and [Ca(2+)](i) responses to forskolin, 8-bromo-cAMP and SKF38393. gbSS-28 might inhibit PACAP-induced GH release by interfering with PACAP's ability to increase [Ca(2+)](i) in a non-cAMP-dependent manner. However, DA D1 receptor activation bypassed gbSS-28 inhibitory effects on cAMP production and NO actions via unknown mechanisms to maintain a normal [Ca(2+)](i) response leading to unhampered GH release.

Three native somatostatin isoforms differentially affect membrane voltage-sensitive ion currents in goldfish somatotrophs

Message encoding for three isoforms of somatostatin (SS) peptides, SS-14, goldfish brain (gb)SS-28 and [Pro²]SS-14, are expressed in goldfish hypothalamus and pituitary tissues. All three native goldfish SSs are active in reducing basal and stimulated growth hormone (GH) responses in cultured goldfish pituitary cells, although with different potencies and efficacies. In the present study, we examined the effects of these three endogenous SSs on electrophysiological properties of goldfish somatotrophs and their physiological relevance. Voltage-sensitive K+ , Ca²+ and Na+ channels in identified goldfish somatotrophs in primary culture were isolated using whole-cell, amphotericin B-perforated patch-clamp techniques. None of the three SSs affected Na+ currents but all three SSs increased maximal K+ current magnitude, with SS-14 being the most effective. [Pro²]SS14 did not affect Ba²+ currents through voltage-sensitive Ca²+ channels but SS14 decreased the magnitude of early and late Ba²+ currents, whereas gbSS-28 reduced that of the late Ba²+ current. Under current-clamp conditions, SS14 and gbSS28 attenuated evoked action potential magnitudes by 34% and 18%, respectively, although [Pro²]SS14 had no effects. However, all three SSs decreased basal intracellular Ca²+ levels ([Ca²+ ](i)) and suppressed basal GH release. These data suggest that, although the ability of SS-14 and gbSS-28 to decrease basal [Ca²+](i) and GH release can be explained, at least in part, by their attenuating effects on cell excitability and current flow through voltage-sensitive Ca²+ channels, [Pro²]SS14-induced reduction in GH responses and [Ca²+](i) cannot be explained by changes in Ca²+ channel properties.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Involvement of somatostatin receptor subtypes in membrane ion channel modification by somatostatin in pituitary somatotropes

1. Growth hormone (GH) secretion from pituitary somatotropes is mainly regulated by two hypothalamic hormones, GH-releasing hormone (GHRH) and somatotrophin releasing inhibitory factor (SRIF). 2. Somatotrophin releasing inhibitory factor inhibits GH secretion via activation of specific membrane receptors, somatostatin receptors (SSTRs) and signalling transduction systems in somatotropes. 3. Five subtypes of SSTRs, namely SSTR1, 2, 3, 4 and 5, have been identified, with the SSTR2 subtype divided into SSTR2A and SSTR2B. All SSTRs are G-protein-coupled receptors. 4. Voltage-gated Ca(2+) and K(+) channels on the somatotrope membrane play an important role in regulating GH secretion and SRIF modifies both channels to reduce intracellular free Ca(2+) concentration and GH secretion. 5. Using specific SSTR subtype-specific agonists, it has been found that reduction in Ca(2+) currents by SRIF is mediated by SSTR2 and an increase in K(+) currents is mediated by both SSTR2 and SSTR4 in rat somatotropes.

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

Somatostatin decreases voltage-gated Ca2+ currents in GH3 cells through activation of somatostatin receptor 2

The secretion of growth hormone (GH) is inhibited by hypothalamic somatostatin (SRIF) in somatotropes through five subtypes of the somatostatin receptor (SSTR1-SSTR5). We aimed to characterize the subtype(s) of SSTRs involved in the Ca2+ current reduction in GH3 somatotrope cells using specific SSTR subtype agonists. We used nystatin-perforated patch clamp to record voltage-gated Ca2+ currents, using a holding potential of -80 mV in the presence of K+ and Na+ channel blockers. We first established the presence of T-, L-, N-, and P/Q-type Ca2+ currents in GH3 cells using a variety of channel blockers (Ni+, nifedipine, omega-conotoxin GVIA, and omega-agatoxin IVA). SRIF (200 nM) reduced L- and N-type but not T- or P/Q-type currents in GH3 cells. A range of concentrations of each specific SSTR agonist was tested on Ca2+ currents to find the maximal effective concentration. Activation of SSTR2 with 10(-7) and 10(-8) M L-797,976 decreased the voltage-gated Ca2+ current and abolished any further decrease by SRIF. SSTR1, SSTR3, SSTR4, and SSTR5 agonists at 10(-7) M did not modify the voltage-gated Ca2+ current and did not affect the Ca2+ current response to SRIF. These results indicate that SSTR2 is involved mainly in regulating voltage-gated Ca2+ currents by SRIF, which contributes to the decrease in intracellular Ca2+ concentration and GH secretion by SRIF.

Regulation of vesicular monoamine and glutamate transporters by vesicle-associated trimeric G proteins: new jobs for long-known signal transduction molecules

Neurotransmitters of neurons and neuroendocrine cells are concentrated first in the cytosol and then in either small synaptic vesicles ofpresynaptic terminals or in secretory vesicles by the activity of specific transporters of the plasma and the vesicular membrane, respectively. In the central nervous system the postsynaptic response depends--amongst other parameters-on the amount of neurotransmitter stored in a given vesicle. Neurotransmitter packets (quanta) vary over a wide range which may be also due to a regulation of vesicular neurotransmitter filling. Vesicular filling is regulated by the availability of transmitter molecules in the cytoplasm, the amount of transporter molecules and an electrochemical proton-mediated gradient over the vesicular membrane. In addition, it is modulated by vesicle-associated heterotrimeric G proteins, Galphao2 and Galphaq. Galphao2 and Galphaq regulate vesicular monoamine transporter (VMAT) activities in brain and platelets, respectively. Galphao2 also regulates vesicular glutamate transporter (VGLUT) activity by changing its chloride dependence. It appears that the vesicular content activates the G protein, suggesting a signal transduction from the luminal site which might be mediated by a vesicular G protein-coupled receptor or as an alternative possibility by the transporter itself. Thus, G proteins control transmitter storage and thereby probablylink the regulation of the vesicular content to intracellular signal cascades.

Galphao2 regulates vesicular glutamate transporter activity by changing its chloride dependence

Classical neurotransmitters, including monoamines, acetylcholine, glutamate, GABA, and glycine, are loaded into synaptic vesicles by means of specific transporters. Vesicular monoamine transporters are under negative regulation by alpha subunits of trimeric G-proteins, including Galpha(o2) and Galpha(q). Furthermore, glutamate uptake, mediated by vesicular glutamate transporters (VGLUTs), is decreased by the nonhydrolysable GTP-analog guanylylimidodiphosphate. Using mutant mice lacking various Galpha subunits, including Galpha(o1), Galpha(o2), Galpha(q), and Galpha11, and a Galpha(o2)-specific monoclonal antibody, we now show that VGLUTs are exclusively regulated by Galpha(o2). G-protein activation does not affect the electrochemical proton gradient serving as driving force for neurotransmitter uptake; rather, Galpha(o2) exerts its action by specifically affecting the chloride dependence of VGLUTs. All VGLUTs show maximal activity at approximately 5 mm chloride. Activated Galpha(o2) shifts this maximum to lower chloride concentrations. In contrast, glutamate uptake by vesicles isolated from Galpha(o2-/-) mice have completely lost chloride activation. Thus, Galpha(o2) acts on a putative regulatory chloride binding domain that appears to modulate transport activity of vesicular glutamate transporters.

Somatostatin increases voltage-gated K+ currents in GH3 cells through activation of multiple somatostatin receptors

The secretion of GH by somatotropes is inhibited by somatostatin (SRIF) through five specific membrane receptors (SSTRs). SRIF increases both transient outward (IA) and delayed rectifying (IK) K+ currents. We aim to clarify the subtype(s) of SSTRs involved in K+ current enhancement in GH3 somatotrope cells using specific SSTR subtype agonists. Expression of all five SSTRs was confirmed in GH3 cells by RT-PCR. Nystatin-perforated patch clamp was used to record voltage-gated K+ currents. We first established the presence of IA and IK type K+ currents in GH3 cells using different holding potentials (-40 or -70 mV) and specific blockers (4-aminopirimidine and tetraethylammonium chloride). SRIF (200 nM) increased the amplitude of both IA and IK in a fully reversible manner. Various concentrations of each specific SRTR agonist were tested on K+ currents to find the maximal effective concentration. Activation of SSTR2 and SSTR4 by their respective agonists, L-779,976 and L-803,087 (10 nM), increased K+ current amplitude without preference to IA or IK, and abolished any further increase by SRIF. Activation of SSTR1 and SSTR5 by their respective agonists, L-797,591 or L-817,818 (10 nM), increased K+ current amplitude, but SRIF evoked a further increase. The SSTR3 agonist L-797,778 (10 nM) did not affect the K+ currents or the response to SRIF. These results indicate that SSTR1, -2, -4, and -5 may all be involved in the enhancement of K+ currents by SRIF but that only the activation of SSTR2 or -4 results in the full activation of K+ current caused by SRIF.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

Light response of retinal ON bipolar cells requires a specific splice variant of Galpha(o)

Glutamate released onto retinal ON bipolar neurons binds to a metabotropic receptor to activate a heterotrimeric G-protein (G(o)) that ultimately closes a nonspecific cation channel. Signaling requires the alpha subunit (Galpha(o)), but its effector is unknown. Because Galpha(o) is transcribed into two splice variants (alpha(o1) and alpha(o2)) that differ in the key GTPase domain, the next step in elucidating this pathway was to determine which splice variant carries the signal. Here we show by reverse transcription-PCR and Western blots that retina expresses both splice variants. Furthermore, in situ hybridization and immunostaining on mouse retina deficient in one splice variant or the other show that both alpha(o1) and alpha(o2) are expressed by ON bipolar cells but that alpha(o1) is much more abundant. Finally, electroretinography performed on mice deficient for one splice variant or the other shows that the positive b-wave (response of ON bipolar cells to rod and cone input) requires alpha(o1) but not alpha(o2). Thus, the light response of the ON bipolar cell is probably carried by its strongly expressed splice variant, Galpha(o1).

Light response of retinal ON bipolar cells requires a specific splice variant of Galpha(o)

Glutamate released onto retinal ON bipolar neurons binds to a metabotropic receptor to activate a heterotrimeric G-protein (G(o)) that ultimately closes a nonspecific cation channel. Signaling requires the alpha subunit (Galpha(o)), but its effector is unknown. Because Galpha(o) is transcribed into two splice variants (alpha(o1) and alpha(o2)) that differ in the key GTPase domain, the next step in elucidating this pathway was to determine which splice variant carries the signal. Here we show by reverse transcription-PCR and Western blots that retina expresses both splice variants. Furthermore, in situ hybridization and immunostaining on mouse retina deficient in one splice variant or the other show that both alpha(o1) and alpha(o2) are expressed by ON bipolar cells but that alpha(o1) is much more abundant. Finally, electroretinography performed on mice deficient for one splice variant or the other shows that the positive b-wave (response of ON bipolar cells to rod and cone input) requires alpha(o1) but not alpha(o2). Thus, the light response of the ON bipolar cell is probably carried by its strongly expressed splice variant, Galpha(o1).

G protein specificity: traffic direction required

This review focuses on the coupling specificity of the Galpha and Gbetagamma subunits of pertussis toxin (PTX)-sensitive G(i/o) proteins that mediate diverse signaling pathways, including regulation of ion channels and other effectors. Several lines of evidence indicate that specific combinations of G protein alpha, beta and gamma subunits are required for different receptors or receptor-effector networks, and that a higher degree of specificity for Galpha and Gbetagamma is observed in intact systems than reported in vitro. The structural determinants of receptor-G protein specificity remain incompletely understood, and involve receptor-G protein interaction domains, and perhaps other scaffolding processes. By identifying G protein specificity for individual receptor signaling pathways, ligands targeted to disrupt individual pathways of a given receptor could be developed.

Most central nervous system D2 dopamine receptors are coupled to their effectors by Go

We reported previously that Go-deficient mice develop severe neurological defects that include hyperalgesia, a generalized tremor, lack of coordination, and a turning syndrome somewhat reminiscent of unilateral lesions of the dopaminergic nigro-striatal pathway. By using frozen coronal sections of serially sectioned brains of normal and Go-deficient mice, we studied the ability of several G protein coupled receptors to promote binding of GTPgammaS to G proteins and the ability of GTP to promote a shift in the affinity of D2 dopamine receptor for its physiologic agonist dopamine. We found a generalized, but not abolished reduction in agonist-stimulated binding of GTPgammaS to frozen brain sections, with no significant left-right differences. Unexpectedly, the ability of GTP to regulate the binding affinity of dopamine to D2 receptors (as seen in in situ [(35)S]sulpiride displacement curves) that was robust in control mice, was absent in Go-deficient mice. The data suggest that most of the effects of the Gi/Go-coupled D2 receptors in the central nervous system are mediated by Go instead of Gi1, Gi2, or Gi3. In agreement with this, the effect of GTP on dopamine binding to D2 receptors in double Gi1 plus Gi2- and Gi1 plus Gi3-deficient mice was essentially unaffected.

Diverse intracellular signalling systems used by growth hormone-releasing hormone in regulating voltage-gated Ca2+ or K channels in pituitary somatotropes

Influx of Ca2+ via Ca2+ channels is the major step triggering exocytosis of pituitary somatotropes to release growth hormone (GH). Voltage-gated Ca2+ and K+ channels, the primary determinants of the influx of Ca2+, are regulated by GH-releasing hormone (GHRH) through G-protein-coupled intracellular signalling systems. Using whole-cell patch-clamp techniques, the changes of the Ca2+ and K+ currents in primary cultured ovine and human somatotropes were recorded. Growth hormone-releasing hormone (10 nmol/L) increased both L- and T-type voltage-gated Ca2+ currents. Inhibition of the cAMP/protein kinase A (PKA) pathway by either Rp-cAMP or H89 blocked this increase in both L- and T-type Ca2+ currents. Growth hormone-releasing hormone also decreased voltage-gated transient (IA) and delayed rectified (IK) K+ currents. Protein kinase C (PKC) inhibitors, such as calphostin C, chelerythrine or downregulation of PKC, blocked the effect of GHRH on K+ currents, whereas an acute activation of PKC by phorbol 12, 13-dibutyrate (1 micromol/L) mimicked the effect of GHRH. Intracellular dialysis of a specific PKC inhibitor (PKC19-36) also prevented the reduction in K+ currents by GHRH. It is therefore concluded that GHRH increases voltage-gated Ca2+ currents via cAMP/PKA, but decreases voltage-gated K+ currents via the PKC signalling system. The GHRH-induced alteration of Ca2+ and K+ currents augments the influx of Ca2+, leading to an increase in [Ca2+]i and the GH secretion.

VIP and PACAP potentiation of nicotinic ACh-evoked currents in rat parasympathetic neurons is mediated by G-protein activation

The effects of vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP27 and PACAP38) on isolated parasympathetic neurons of rat intracardiac and submandibular ganglia were examined under voltage clamp using whole-cell patch-clamp recording techniques. VIP and PACAP (</= 10 nM) selectively and reversibly increased the affinity of nicotinic acetylcholine receptor channels (nAChRs) for their agonists resulting in a potentiation of acetylcholine (ACh)-evoked whole-cell currents at low agonist concentrations. VIP-induced potentiation was observed with either ACh or nicotine as the cholinergic agonist. The VIP- but not the PACAP-induced potentiation of ACh-evoked currents was inhibited by [Ac-Tyr1, D-Phe2]-GRF 1-29, amide (100 nM), a selective antagonist of VPAC1 and VPAC2 receptors; whereas the PACAP38- but not the VIP-induced potentiation was inhibited by 100 nM PACAP6-38, a PAC1 and VPAC2 receptor antagonist. The signal transduction pathway mediating VIP- and PACAP-induced potentiation of nicotinic ACh-evoked currents involves a pertussis toxin (PTX)-sensitive G-protein. Intracellular application of 200 microM GTPgammaS or GDPbetaS inhibited VIP-induced potentiation of ACh-evoked whole-cell currents. GTPgammaS alone potentiated ACh- and nicotine-evoked currents and the magnitude of these currents was not further increased by VIP or PACAP. The G-protein subtype modulating the neuronal nAChRs was examined by intracellular dialysis with antibodies directed against alphao, alphai-1,2, alphai-3 or beta G-protein subunits. Only the anti-Galphao and anti-Gbeta antibodies significantly inhibited the effect of VIP and PACAP on ACh-evoked currents. The potentiation of ACh-evoked currents by VIP and PACAP may be mediated by a membrane-delimited signal transduction cascade involving the PTX-sensitive Go protein.

Somatostatin-induced control of cytosolic free calcium in pituitary tumour cells

1. In rat pituitary tumour cells (GC cells), spontaneous oscillations of the intracellular concentration of Ca2+ ([Ca2+]i) induce growth hormone (GH) secretion that is inhibited by octreotide, a somatostatin (SRIF) agonist which binds to SRIF subtype (sst) receptor 2. The effects of its functional activation on the control of [Ca2+]i were investigated using fluorimetric measurements of [Ca2+]i. 2. SRIF decreases the basal [Ca2+]i and the [Ca2+]i rise in response to forskolin (FSK) through the inhibition of L-type voltage-dependent Ca2+ channels. 3. Pretreatment with octreotide or with L-Tyr8++ Cyanamid 154806, a sst2 receptor antagonist, abolishes the SRIF-induced inhibition of [Ca2+]i. Octreotide is known to operate through agonist-induced desensitization, while the antagonist operates through receptor blockade. 4. sst1 and sst2 receptor-immunoreactivities (-IRs) are localized to cell membranes. sst2, but not sst1 receptor-IR, internalizes after cell exposure to octreotide. 5. SRIF-induced inhibition of basal [Ca2+]i or FSK-induced Ca2+ entry is blocked by pertussis toxin (PTX). 6. FSK-induced cyclic AMP accumulation is only partially decreased by SRIF or octreotide, indicating that sst2 receptors are coupled to intracellular pathways other than adenylyl cyclase (AC) inhibition. 7. In the presence of H-89, an inhibitor of cyclic AMP-dependent protein kinase (PKA), SRIF-induced inhibition of basal [Ca2+]i is still present, although reduced in amplitude. 8. SRIF inhibits [Ca2+]i by activating sst2 receptors. Inhibition of AC activity is only partly responsible for this effect, and other transduction pathways may be involved.

Human GHRH reduces voltage-gated K+ currents via a non-cAMP-dependent but PKC-mediated pathway in human GH adenoma cells

1. Whole-cell voltage-gated K+ currents and the K+ current response to growth hormone-releasing hormone (GHRH) were characterised in primary cultures of human acromegalic somatotropes. 2. Both delayed rectifier (IK) and transient (IA) K+ currents were recorded from human somatotropes held at -80 mV and bathed in a solution containing Cd2+ (1 mM), TTX (1 microM) and a low concentration of Ca2+ (0.5 mM). Only IK was recorded, however, when a holding potential of -40 mV was used. 3. GHRH (10 nM) immediately and significantly reduced the amplitude of both IA and IK. While the reduction in the amplitude of IA was fully reversed following the removal of GHRH, the amplitude of IK had only partially recovered 10 min after GHRH removal. In addition, GHRH shifted the voltage-dependent inactivation curve of IA by 13.5 mV in the negative direction. 4. In a low Ca2+ and Cd2+-containing solution, the Ca2+-activated K+ channel blockers apamin (100 nM and 1 microM) and charybdotoxin (1 microM) did not alter K+ currents or the effect of GHRH on the recorded K+ currents. 5. The whole-cell K+ currents and their responses to GHRH were unaffected by the application of 8-bromo-cAMP (100 microM), Rp-cAMP (100 microM) or the protein kinase A (PKA) inhibitor H89 (1 microM). In addition, intracellular dialysis of the PKA inhibitory peptide PKI (10 microM) had no effect on the K+ current response to GHRH. 6. While the application of protein kinase C (PKC) inhibitors calphostin C (100 nM) or chelerythrine (1 microM) did not affect the amplitude of the K+ currents, the K+ current response to GHRH was significantly attenuated. Downregulation of PKC with phorbol 12,13-dibutyrate (PDBu, 0.5 microM for 16 h) also abolished the K+ current response to GHRH. In addition, intracellular dialysis of somatotropes with the PKC inhibitory peptide PKC19-36 (1 microM) prevented the GHRH-induced decrease in the amplitude of the voltage-gated K+ currents. Local application of PDBu (1 microM) significantly reduced the amplitude of the voltage-gated K+ currents in a similar manner to that induced by GHRH, but without clear recovery upon removal. 7. This study demonstrates that GHRH decreases voltage-gated K+ currents via a PKC-mediated pathway in human adenoma somatotropes, rather than by the cAMP-PKA pathway that is usually implicated in the actions of GHRH.

Mechanisms mediating somatostatin-induced reduction of cytosolic free calcium in PC12 cells

In PC12 cells, somatostatin (SRIF) decreases the Ca2+ influx in response to high K+ through the inhibition of voltage-dependent Ca2+ channels. In the present study, we measured the intracellular Ca2+ concentration ( [Ca2+]i) following the application of SRIF-14 or its analogues which bind to different SRIF subtype (sst) receptors. Their application differentially reduces [Ca2+]i (octreotide > SRIF-14 > CGP23996 > BIM23056) in a dose-dependent manner. Pretreatment with a SRIF antagonist prevents the SRIF-induced inhibition of [Ca2+]i. These results suggest the involvement of specific membrane receptors and are in line with recent findings indicating the presence of receptor mRNAs on PC12 cells. Our results also exclude the possibility that SRIF interacts with opioid receptors and suggest that the SRIF-induced inhibition of [Ca2+]i is mediated by mechanisms involving PTX-sensitive G-proteins and PK-dependent pathways.

Gi-3 protein mediates the increase in voltage-gated K+ currents by somatostatin on cultured ovine somatotrophs

Voltage-gated K+ currents in rat somatotrophs are increased by somatostatin (SRIF) through unidentified G protein. In this experiment, somatotroph-enriched cells (up to 85%) were obtained from ovine pituitary glands and further identified by the increase in K+ currents by SRIF. The whole cell recording was employed to study the voltage-gated K+ currents. A reversible increase in K+ currents (up to 150% of control) was obtained in response to local application of SRIF (10 nM) but not vehicle. When the guanosine 5'-O-(3-thiotriphosphate) was included in the pipette solution (200 microM), the recovery phase of K+ current response to SRIF was abolished. Inclusion of guanosine 5'-O-(2-thiodiphosphate) (200 microM) in pipette solution blocked the K+ current response to SRIF. Intracellular dialysis of antibodies against alphao-, alphai-, alphai-1-2-, or alphai-3-subunits of G proteins via patch pipettes was confirmed by immunofluorescent staining of the antibodies. Antibody dialysis alone did not modify voltage-gated K+ currents. Dialysis of anti-alphai or anti-alphai-3 antibodies significantly attenuated the increase in K+ currents that was obtained after application of 10 or 100 nM SRIF. Dialysis with anti-alphao, anti-alphai-1-2, or heat-inactivated (60 degreesC for 10 min) anti-alphai antibodies did not diminish the effect of SRIF on K+ currents. We conclude that the Gi-3 protein mediates the effect of SRIF on voltage-gated K+ currents in ovine somatotrophs.

Ryanodine- and cyclopiazonic acid-sensitive components in human vas deferens contractions to noradrenaline

1. The role of calcium stores in noradrenaline- (NA) and caffeine-induced contractions of human vas deferens were investigated using ryanodine and cyclopiazonic acid (CPA) in the presence of the calcium antagonist, nifedipine (1 microM) or in calcium-free/EGTA (1 mM) medium. 2. In either media, NA (100 microM) evoked biphasic contractions of longitudinal muscle and tonic circular muscle contractions. Caffeine (20 mM) evoked longitudinal but not circular muscle contractions. 3. Ryanodine (1-30 microM) or CPA (1-30 microM) inhibited contractions of circular muscle, and the initial but not secondary component of longitudinal muscle contraction to NA. 4. In the presence of nifedipine, pre-exposure to caffeine caused a potentiation of circular muscle, and the initial but not secondary longitudinal muscle contractions to NA. The presence of ryanodine or CPA during the caffeine pre-exposures effectively blocked the potentiation of the initial component and reduced the secondary component of the subsequent responses to NA in longitudinal muscle. 5. In calcium-free media, caffeine pre-exposures had little effect on subsequent NA-induced contractions in circular muscle, but reduced both components in longitudinal muscle. The presence of ryanodine or CPA during caffeine pre-exposures produced no further effects on either component of the subsequent NA-induced contraction in longitudinal muscle. 6 In the presence of nifedipine or in calcium-free media, repeated applications of caffeine evoked contractions in longitudinal muscle which were not blocked by either ryanodine or CPA. 7. These results suggest that circular muscle contraction by NA and the initial component of longitudinal muscle to NA both utilize an intracellular pool of calcium that is triggered via a ryanodine-sensitive mechanism and replenished via a CPA-sensitive Ca2+-ATPase. 8. In longitudinal muscle, both the secondary component of its response to NA and contraction to caffeine appear to involve an unusual but pharmacologically distinct (ryanodine- and CPA-insensitive) pathway. 9. The quiescence to caffeine of circular muscle may be caused by a relative absence of the ryanodine- and CPA-insensitive pathway.

Electromechanical coupling in human vas deferens: effects of agents that modulate intracellular release of calcium

1. The effects of ryanodine, cyclopiazonic acid (CPA) and caffeine on electromechanical coupling in human vas deferens were investigated. 2. High [K+]o (120 mM) evoked nifedipine-sensitive contractions of longitudinal and circular muscle which consisted of initial and secondary components. 3. Exposures to ryanodine (< or =10 microM) or CPA (< or = 3 microM) induced a change of basal tension, and higher doses (30 microM) induced intermittent rhythmic contractions of both muscle types in the quiescent tissue. In the presence of the drugs, contraction to high [K+]o was preceded by marked rhythmic activity. 4. In circular muscle, ryanodine (1-30 microM) or CPA (1-30 microM) reduced both components of contractions to high [K+]o. In longitudinal muscle, the drugs enhanced the initial component and prolonged the secondary component. High doses (> or = 10 microM) produced variable effects on the initial component. 5. Caffeine (20 mM) reliably contracted longitudinal, but not circular muscle. Pre-exposures to caffeine enhanced both components in the post-caffeine contractions of circular muscle to high [K+]o. In longitudinal muscle, only the initial component (post-caffeine) was enhanced. 6. Contractions evoked in longitudinal muscle by caffeine were not blocked by ryanodine (30 microM) or CPA (30 microM). However, the enhancement of post-caffeine contractions to high [K+]o was inhibited. 7. These results show that ryanodine and CPA produced comparable effects on the excitability of longitudinal and circular muscle in the quiescent tissue, but electromechanical coupling was affected differently. The findings suggest that the muscle types utilize different mechanisms to regulate elevations in cytosolic Ca2+ during stimulation. 8. Electromechanical coupling in both muscle types involves Ca2+ influx via nifedipine-sensitive voltage-operated calcium channels and activation of ryanodine-sensitive calcium-induced calcium release from the sarcoplasmic reticulum (SR). In longitudinal muscle, the SR also buffers increases in cytosolic Ca2+ via a pharmacologically distinct Ca2+ compartment (caffeine releasable but ryanodine/CPA-insensitive). In circular muscle, the SR (ryanodine/CPA-sensitive) serves mainly in the regulation of excitability of the quiescent tissue.

Multiple neurological abnormalities in mice deficient in the G protein Go

The G protein Go is highly expressed in neurons and mediates effects of a group of rhodopsin-like receptors that includes the opioid, alpha2-adrenergic, M2 muscarinic, and somatostatin receptors. In vitro, Go is also activated by growth cone-associated protein of Mr 43,000 (GAP43) and the Alzheimer amyloid precursor protein, but it is not known whether this occurs in intact cells. To learn about the roles that Go may play in intact cells and whole body homeostasis, we disrupted the gene encoding the alpha subunits of Go in embryonic stem cells and derived Go-deficient mice. Mice with a disrupted alphao gene (alphao-/- mice) lived but had an average half-life of only about 7 weeks. No Goalpha was detectable in homogenates of alphao-/- mice by ADP-ribosylation with pertussis toxin. At the cellular level, inhibition of cardiac adenylyl cyclase by carbachol (50-55% at saturation) was unaffected, but inhibition of Ca2+ channel currents by opioid receptor agonist in dorsal root ganglion cells was decreased by 30%, and in 25% of the alphao-/- cells examined, the Ca2+ channel was activated at voltages that were 13.3 +/- 1.7 mV lower than in their counterparts. Loss of alphao was not accompanied by appearance of significant amounts of active free betagamma dimers (prepulse test). At the level of the living animal, Go-deficient mice are hyperalgesic (hot-plate test) and display a severe motor control impairment (falling from rotarods and 1-inch wide beams). In spite of this deficiency, alphao-/- mice are hyperactive and exhibit a turning behavior that has them running in circles for hours on end, both in cages and in open-field tests. Except for one, all alphao-/- mice turned only counterclockwise. These findings indicate that Go plays a major role in motor control, in motor behavior, and in pain perception and also predict involvement of Go in Ca2+ channel regulation by an unknown mechanism.

Direct interaction of gbetagamma with a C-terminal gbetagamma-binding domain of the Ca2+ channel alpha1 subunit is responsible for channel inhibition by G protein-coupled receptors

Several classes of voltage-gated Ca2+ channels (VGCCs) are inhibited by G proteins activated by receptors for neurotransmitters and neuromodulatory peptides. Evidence has accumulated to indicate that for non-L-type Ca2+ channels the executing arm of the activated G protein is its betagamma dimer (Gbetagamma). We report below the existence of two Gbetagamma-binding sites on the A-, B-, and E-type alpha1 subunits that form non-L-type Ca2+ channels. One, reported previously, is in loop 1 connecting transmembrane domains I and II. The second is located approximately in the middle of the ca. 600-aa-long C-terminal tails. Both Gbetagamma-binding regions also bind the Ca2+ channel beta subunit (CCbeta), which, when overexpressed, interferes with inhibition by activated G proteins. Replacement in alpha1E of loop 1 with that of the G protein-insensitive and Gbetagamma-binding-negative loop 1 of alpha1C did not abolish inhibition by G proteins, but the exchange of the alpha1E C terminus with that of alpha1C did. This and properties of alpha1E C-terminal truncations indicated that the Gbetagamma-binding site mediating the inhibition of Ca2+ channel activity is the one in the C terminus. Binding of Gbetagamma to this site was inhibited by an alpha1-binding domain of CCbeta, thus providing an explanation for the functional antagonism existing between CCbeta and G protein inhibition. The data do not support proposals that Gbetagamma inhibits alpha1 function by interacting with the site located in the loop I-II linker. These results define the molecular mechanism by which presynaptic G protein-coupled receptors inhibit neurotransmission.

G(o)2 and Gi3 proteins mediate the action of somatostatin on membrane Ca2+ and K+ currents in ovine pituitary somatotrophs

1. Growth hormone (GH) secretion from the anterior pituitary gland is mainly regulated by hypothalamic GH-releasing hormone (GHRH) and somatostatin (SRIF). Somatostatin reduces both spontaneous and GHRH-stimulated GH secretion. 2. Exocytosis of GH is mainly determined by the intracellular free Ca2+ concentration ([Ca2+]i), which is regulated by the influx of Ca2+ via membrane Ca2+ channels. Somatostatin reduces the influx of Ca2+ through two separate mechanisms, namely a direct action on Ca2+ channels and an indirect action on membrane potentials through the activation of K+ channels. 3. In the present experiments, somatotroph-enriched cells were obtained from the ovine pituitary gland by means of collagenase dissociation and Percoll-gradient centrifugation. Further identification was based on the effect of SRIF (10 nmol/L) on Ca2+ or K+ currents. 4. A significant reduction in Ca2+ currents and an increase in K+ currents was obtained in response to local application of SRIF (10 nmol/L), but vehicle application had no effect. The responses of Ca2+ and K+ currents to SRIF were reversible after removal of SRIF. 5. Dialysis of GTP-gamma-s (200 mumol/L) abolished the recovery phase of K+ current response to SRIF after its removal, whereas GDP-beta-s (200 mumol/L) totally blocked the response. Pretreatment of the cells with pertussis toxin (100 nmol/L) overnight abolished the Ca2+ current response to SRIF. 6. Intracellular dialysis of antibodies to alpha o, alpha i1-3, alpha i1-2 and alpha i3 subunits of the G-proteins into cells via whole-cell patch-clamp pipettes was confirmed by immunofluorescent staining of the antibodies. 7. Dialysis of anti-alpha i1-3 or anti-alpha i3 antibodies significantly attenuated the increase in the K+ current in response to 10 nmol/L SRIF, whereas neither anti-alpha o nor anti-alpha i1-2 antibodies diminished the effect of SRIF on the K+ current. 8. Dialysis of anti-alpha o antibodies significantly attenuated the reduction in the Ca2+ current that was obtained upon application of 10 nmol/L SRIF. Neither anti-alpha i1-2 nor anti-alpha i3 antibody dialysis diminished the effect of SRIF on the Ca2+ current. 9. Dialysis of the alpha o common antisense oligonucleotides (ASm) but not the alpha i3 AS significantly diminished the inhibitory effect of SRIF on the Ca2+ current. This effect of alpha o ASm dialysis occurred at 12 h incubation after dialysis, reaching a maximal level at 48 h and partially recovering at 72 h incubation. Antisense oligonucleotides specific for alpha o1 (alpha o1 AS) or alpha o2 (alpha o2 AS) were dialysed into somatotrophs and only alpha o2 AS significantly attenuated the inhibition of SRIF on the Ca2+ current. 10. It is concluded that the Gi3 protein mediates the effect of SRIF on the K+ current and that the G(o)2 protein mediates the effect of SRIF on the Ca2+ current in primary cultured ovine somatotrophs.

Different G proteins mediate somatostatin-induced inward rectifier K+ currents in murine brain and endocrine cells

1. Types of G proteins (G protein alpha-subunit subtypes) which mediate the activation of inward rectifier K+ currents by somatostatin (somatotrophin release-inhibiting factor, SRIF) were determined in cultured locus coeruleus neurones from newborn rats and in AtT-20 cells (a mouse pituitary cell line). 2. The whole-cell patch clamp technique was used together with injection of antibodies against pertussis toxin (PTX)-sensitive G protein alpha-subunits or with injection of antisense (or sense) oligonucleotides against these G proteins. 3. In locus coeruleus neurones, the SRIF-induced activation of inward rectifier K+ currents was inhibited by anti-G alpha i1/G alpha i2 antibody injection, but not by anti-G alpha i3 or by anti-G alpha o/G alpha i3 antibody injection, suggesting that the SRIF response is mediated through G alpha i1 and/or G alpha i2. 4. The SRIF-induced activation of the inward rectifier was suppressed in locus coeruleus neurones after injection of antisense oligonucleotides against G alpha i2, but not by injection of sense oligonucleotides against G alpha i2. Injection of antisense (or sense) oligonucleotides against G alpha i1, G alpha i3 and G alpha O (common) had no effect. These results suggest that G alpha i2 is involved in this SRIF response. 5. In AtT-20 cells, the SRIF-induced activation of inward rectifier K+ currents was suppressed by injection of anti-G alpha i3 antibody, but not by injection of anti-G alpha i1/G alpha i2 antibody. 6. The above results indicate that Gi mediates the SRIF effects on inward rectifier K+ currents. However, different subtypes of Gi are involved in the brain neurones and in the endocrine cells: Gi2 in locus coeruleus neurones and Gi3 in AtT-20 cells.

Role of subunit diversity in signaling by heterotrimeric G proteins

The heterotrimeric G proteins are extensively involved in the regulation of cells by extracellular signals. The receptors that control them are often the targets of drugs. There are many isoforms of each of the three subunits that make up these proteins. Thus far, genes for at least sixteen alpha subunits, five beta subunits, and eleven gamma subunits have been identified. In addition, some of these proteins have splice variants or are differentially modified. Based upon what is already known, there are well over a thousand possible G protein heterotrimer combinations. The role of subunit diversity in heterotrimer formation and its effect on signaling by G proteins are still not well understood. However, many current lines of research are leading toward an understanding of these roles. The functional significance of subunit heterogeneity is related to the mechanisms used by G proteins to transmit and integrate the many signals coming into cells through this system. Described here are the basic mechanisms by which G proteins integrate cellular responses, the possible role of subunit heterogeneity in these mechanisms, and the evidence for and against their physiological significance. Recent studies suggest the likely possibility that subunit heterogeneity plays an important role in signaling by G proteins. This role has the potential to extend substantially the flexibility of G proteins in mediating cellular responses to extracellular signals. However, the details of this are yet to be worked out, and they are the subject of many different avenues of research.

Gi3 mediates somatostatin-induced activation of an inwardly rectifying K+ current in human growth hormone-secreting adenoma cells

SRIF activates an inwardly rectifying K+ current in human GH-secreting adenoma cells. Activation of this K+ current induces hyperpolarization of the membrane and abolishment of action potential firing. This mechanism is an essential mechanism for SRIF-induced decrease in intracellular Ca2+ concentration and inhibition of GH secretion. The activation of the inwardly rectifying K+ current is mediated by a pertussis toxin-sensitive G protein. In this article, the expression of the pertussis toxin-sensitive G protein alpha-subunits in the human GH-secreting adenoma cells were analyzed by RT-PCR, and the G protein transducing the SRIF-induced activation of this inwardly rectifying K+ current was investigated. RT-PCR of the messenger RNA from two human GH-secreting adenomas revealed that all G alpha(i1), G alpha(i2), G alpha(i3), and G alpha(o) were expressed in these adenomas. Primary cultured cells from these two adenoma cells were investigated under the voltage clamp of the whole-cell mode. Specific antibodies against the carboxyl terminus of G protein alpha-subunits were microinjected into the cells. Microinjection of antibody against the carboxyl terminal sequence of G alpha(i3) attenuated the SRIF-induced activation of the inwardly rectifying K+ current, whereas antibody against the common carboxyl terminal sequence of G alpha(i1) and G alpha(i2) did not. These data indicate that the G protein transducing the SRIF-induced activation of the inwardly rectifying K+ current is Gi3.