Heparin uncouples the muscarinic receptors from GK protein in the atrial cell membrane of the guinea-pig heart

Voltage-dependent Ca2+ release in rat megakaryocytes requires functional IP3 receptors

Using simultaneous whole-cell patch-clamp and fluorescence measurements of [Ca2+]i in rat megakaryocytes we have investigated the requirement for functional inositol 1,4,5-trisphosphate (IP3) receptors in Ca2+ release induced by membrane depolarization during agonist stimulation. Voltage-dependent Ca2+ release was observed during application of the IP3-generating agonists U46619 (a thromboxane A2 analogue) and ADP. Furthermore, voltage-dependent Ca2+ release was observed in the absence of exogenous agonist following sensitization of IP3 receptors with thimerosal. Depolarization-induced Ca2+ release was not detected during depletion of intracellular Ca2+ stores by thapsigargin. Thus, depletion of stores alone is not sufficient to confer voltage dependence upon the Ca2+ release mechanism. Block of IP3 receptors by carbacyclin-stimulated elevations in cAMP, uncaging of cAMP or exposure to a high concentration of caffeine reversibly abolished Ca2+ increases stimulated by both ADP and depolarization. The cAMP-dependent block was prevented by a peptide inhibitor of protein kinase A, indicating that an alteration of adenylate cyclase activity leading to modulation of protein kinase A activity does not underlie the control of Ca2+ release by voltage. These results are consistent with the requirement for functional IP3 receptors for voltage control of Ca2+ release from intracellular stores during inositol lipid signalling. The data also indicate the involvement of a voltage sensor downstream of surface membrane receptors in the depolarization-evoked Ca2+ response.

New Ca2+-releasing messengers: are they important in the nervous system?

In the nervous system, Ca2+ signalling is determined primarily by voltage-gated Ca2+-selective channels in the plasma membrane, but there is increasing evidence for involvement of intracellular Ca2+ stores in such signalling. It is generally assumed that neurotransmitter-elicited release of Ca2+ from internal stores is primarily mediated by Ins(1,4,5)P3, as originally discovered in pancreatic acinar cells. The more-recently discovered Ca2+-releasing messenger, cyclic ADP-ribose (cADPR), which activates ryanodine receptors, has so far only been implicated in a few cases, and the possible importance of another Ca2+-releasing molecule, nicotinic acid adenine dinucleotide phosphate (NAADP), has been ignored. Recent investigations of the action of the brain-gut peptide cholecystokinin on pancreatic acinar cells have indicated that NAADP and cADPR receptors are essential for Ca2+ release. Tools are available for testing the possible involvement of NAADP and cADPR in neurotransmitter-elicited intracellular Ca2+ release, and such studies could reveal complex mechanisms that control this release in the nervous system.

Polarity in intracellular calcium signaling

The concentration of free calcium ions (Ca(2+)) in the cytosol is precisely regulated and can be rapidly increased in response to various types of stimuli. Since Ca(2+) can be used to control different processes in the same cell, the spatial organization of cytosolic Ca(2+) signals is of considerable importance. Polarized cells have advantages for Ca(2+) studies since localized signals can be related to particular organelles. The pancreatic acinar cell is well-characterized with a clearly polarized structure and function. Since the discovery of the intracellular Ca(2+)-releasing function of inositol 1,4,5-trisphosphate (IP(3)) in the pancreas in the early 1980s, this cell has become a popular study object and is now one of the best-characterized with regard to Ca(2+) signaling properties. Stimulation of pancreatic acinar cells with the neurotransmitter acetylcholine or the hormone cholecystokinin evokes Ca(2+) signals that are either local or global, depending on the agonist concentration and the length of the stimulation period. The nature of the Ca(2+) transport events across the basal and apical plasma membranes as well as the involvement of the endoplasmic reticulum (ER), the nucleus, the mitochondria, and the secretory granules in Ca(2+) signal generation and termination have become much clearer in recent years.

Impairment of muscarinic stimulation of adenylyl cyclase by heparin in rat olfactory bulb

In rat olfactory bulb membranes, the stimulation of adenylyl cyclase by the cholinergic agonist carbachol (CCh) was markedly inhibited by heparin at concentrations (0.3-10 microM) that had smaller or no effects on the enzyme stimulations elicited by vasoactive intestinal peptide, pituitary adenylate cyclase activating polypeptide (PACAP), 1-isoproterenol and corticotropin releasing hormone. Heparin did not significantly affect the binding of [3H]N-methylscopolamine ([3H]NMS) to muscarinic receptors, but decreased the potency of CCh in displacing the bound radioligand in a manner similar to that of the GTP analogue guanosine-5'-O-(3'-thio)triphosphate (GTPgammaS). Heparin inhibited the binding of [35S]GTPgammaS to membrane G proteins stimulated by CCh more potently than that elicited by PACAP. Moreover, at the same concentrations, heparin inhibited the muscarinic inhibition of adenylyl cyclase in rat striatal membranes. These data indicate that heparin impairs the muscarinic stimulation of olfactory bulb adenylyl cyclase likely by interfering with the receptor-induced activation of G proteins. The higher sensitivity to heparin of this response as compared to those displayed by Gs-mediated enzyme stimulations provides further evidence that Gi/Go, rather than Gs, mediate the muscarinic stimulation of adenylyl cyclase in rat olfactory bulb.

Effect of trypan blue on the action of acetylcholine, histamine and salbutamol in the isolated guinea-pig ileum

It has been reported that trypan blue, a diazo dye with polyamphipatic structure, can inhibit the coupling of receptors to G-proteins. This inhibition of G-protein coupling has been investigated in isolated guinea-pig ileum. It was found that trypan blue could elicit a slight but dose-dependent contractile response in isolated guinea-pig ileum (4.5% of maximum contractile response induced by acetylcholine). While trypan blue potentiated the effect of histamine and shifted its dose-response curve to the left, it did not affect the contractile effects of acetylcholine. Furthermore, the relaxation which has been induced by salbutamol, a beta 2 agonist, was inhibited by trypan blue. It is concluded that trypan blue, as shown in biochemical studies, act selectively and can uncouple Gs-protein from beta 1 receptors. However, the effect of trypan blue on the whole tissue preparation depends on the type of G-protein involved and post G-protein processes which are stimulated after receptor activation. Trypan blue and similar agents could provide useful tools for further investigations of the mechanism of receptor-G protein coupling in the whole tissue preparation.

Extracellular heparin inhibits Ca2+ transients and contraction in mammalian cardiac myocytes

The effect of heparin on Ca2+ transients and cell shortening was studied in isolated cardiac myocytes from rat and guinea-pig ventricles. Ca2+ signals were measured with the fluorescent indicator fura-2. Heparin reversibly decreased Ca2+ transients and cell shortening in a dose-dependent manner. Half and complete blockade were obtained with 50microg/ml and 200microg/ml heparin, respectively. The dihydropyridine agonist BAY K 8644 (50nM) antagonized the effects of heparin. However, Ca2+ release elicited by caffeine (10mM) was not affected by heparin. The actions of heparin were also studied in multicellular preparations. In papillary muscle, heparin (5mg/ml) reversibly reduced the amplitude of the plateau of the action potential and the associated peak tension. BAY K 8644 (500nM) also antagonized these effects. It is proposed that heparin interacts with dihydropyridine-sensitive Ca2+ channels to cause a decrease of Ca2+ transients and contractility in heart.

Effects of noradrenaline on membrane currents and action potential shape in smooth muscle cells from guinea-pig ureter

1. The effects of noradrenaline (NA) on action potential shape and underlying membrane currents were examined in single smooth muscle cells freshly isolated from the ureter of the guinea-pig. 2. The voltage-dependent Ca2+ current (ICa) elicited upon depolarization from -50 to 0 mV was reduced by 27% upon application of 10 microM NA. This reduction was inhibited or converted to potentiation by internal application of low molecular weight heparin or 5 mM EGTA, indicating that it may be mediated by Ca(2+)-dependent Ca2+ channel inactivation via inositol 1,4,5-trisphosphate production and subsequent Ca2+ release from intracellular Ca2+ storage sites. 3. In contrast, Ba2+ current (IBa) through Ca2+ channels was potentiated by 36% in the presence of 10 microM NA. Internal application of GTP gamma S made it difficult to remove potentiation of IBa by wash-out; internal application of GDP beta S abolished potentiation. 4. NA caused a greater reduction in the transient Ca(2+)-dependent K+ current (IK(Ca)) upon depolarization than it did in ICa. This reduction was inhibited by internally applied heparin, suggesting that the amount of releasable Ca2+ in the storage sites was markedly reduced in the presence of NA. The sustained component of IK(Ca) which gradually increased during depolarization was also reduced by NA. 5. Action potential duration, which was recorded in a standard solution containing Ca2+, was prolonged by the application of NA. 6. It can be concluded that Ca2+ channel activity in ureter smooth muscle cells is regulated by a dual mechanism: Ca(2+)-dependent inhibition and GTP-binding protein-mediated potentiation. Under physiological conditions, both ICa and IK(Ca) were reduced by NA but the reduction of IK(Ca) was much larger than that of ICa; this results in an increase in net inward current during the action potential plateau and prolongs the action potential.

Differential time course for desensitization to muscarinic effects on K+ and Ca2+ channels

The time course of muscarinic effects on K and Ca currents was investigated at 22-24 degrees C in guinea-pig atrial myocytes, using the whole-cell voltage clamp. At a holding potential of -40 or -50 mV, short exposures to 100 microM acetylcholine (ACh) or carbachol (CCh) reproducibly induced outward K currents (IK,ACh). During long exposures to these agonists, IK,ACh faded with time. In cells not dialysed with guanosine triphosphate (GTP), IK,ACh could dissipate completely following 15-20 min of agonist exposure. After agonist washout, lost sensitivity was not recovered. In cells dialysed with GTP (0.2-1 mM), IK,ACh still faded but normal sensitivity to agonists was restored with washout. Fade of IK,ACh was not prevented by intracellular heparin or dextran, excluding the involvement of either beta-adrenergic or muscarinic receptor kinase. IK,ACh induced by bethanechol or adenosine also faded, and subsequent CCh application after washout revealed a diminished response. Intracellular guanosine-5'-o-(3-thiotriphosphate (GTP gamma S) induced IK,ACh which faded, and subsequent exposure to CCh was without effect. Equally, after full desensitization with CCh, GTP gamma S failed to induce IK,ACh. The Ca current (ICa) was activated by voltage steps to 0 mV and increased with 1-3 microM isoproterenol. This increase could be reversed by CCh, even when IK,ACh had completely faded. Prolonged muscarinic agonist exposure sometimes also caused fade of the effect on ICa, which always occurred after loss of IK,ACh. The results show that desensitization is heterologous and may involve the guanine nucleotide-binding (G) protein. The differential desensitization to the effects on IK,ACh and ICa suggests the involvement of two different signalling pathways in the muscarinic control of K and Ca channels.

Ca2+ oscillations in pancreatic acinar cells: spatiotemporal relationships and functional implications

The pancreatic acinar cells are of particular interest for the study of cytosolic Ca2+ signals, since they are morphologically polarized and generate agonist-specific Ca2+ oscillation patterns. Recent data obtained by combining digital video imaging of Fura-2 fluorescence with patch-clamp whole-cell current recording have provided new information on the spatiotemporal relationships of the cytosolic Ca2+ signals and the Ca(2+)-activated ionic currents. Low agonist concentrations evoke repetitive short-lasting local Ca2+ spikes in the secretory pole region that activate shortlasting current spikes. In the case of acetylcholine stimulation the spikes are confined to this region. When cholecystokinin is used the shortlasting local spikes precede longer Ca2+ transients that spread to the whole of the cell. Infusion of non-metabolizable inositol trisphosphate analogues can mimic these responses. The shortlasting local Ca2+ spikes are particularly sensitive to blockade by the inositol trisphosphate receptor antagonist heparin. These results show that the secretory pole region has a particularly high sensitivity to inositol trisphosphate probably due to clustering of high affinity receptors.

Ca2+ channel modulating effects of heparin in mammalian cardiac myocytes

1. The effect of heparin on L-type Ca2+ channels in rabbit, rat and guinea-pig cardiac myocytes was studied using the whole-cell patch clamp method. 2. Sodium salts of heparin uniformly suppressed the Ca2+ current, ICa, independent of their molecular weight, in the rat and guinea-pig ventricular and rabbit atrial myocytes. The suppression of ICa by heparin was dose dependent and reached its maximum, about 30%, around 10 microM. Heparin did not alter the voltage-dependence or the steady-state inactivation properties of ICa. These effects were specific to heparin as another polysaccharide, dextran, failed to have any effect on ICa. 3. The suppressive effect of heparin was not diminished when [Ca2+]o was increased to 10 mM, or when Ba2+ was the charge carrier through the Ca2+ channel. 4. Spectrophotometric assays showed that heparin-induced changes in [Ca2+]o generally were too small to alter ICa significantly. 5. In myocytes buffered with 0.1 mM EGTA, the suppressive effect of heparin was more prominent on the inactivating than on the maintained component of ICa. 6. When extracellular Na+ was replaced by Cs+, the heparin suppressive effect was accompanied by a 10 mV shift of both the voltage dependence of activation and the steady-state inactivation parameters toward more negative potentials. 7. When both Mg2+ and Na+ were omitted from the bathing solutions, the suppressive effect of heparin was significantly enhanced such that almost 80% of the current was blocked. 8. In Cs(+)-based solutions 10 mM [Mg2+]o suppressed ICa by about 70% and heparin partially relieved this block. Heparin, however, did not counteract the Mg(2+)-induced suppression of ICa in Na(+)-based solution. 9. Extracellularly applied heparin did not alter the isoprenaline-induced enhancement of ICa or interfere with the blocking effect of phorbol esters on ICa. 10. Heparin thus appears to interfere with the permeation of Ca2+ through the channel by a mechanism regulating the Ca(2+)-induced inactivation of the Ca2+ channel. Na+ and Mg2+ appear to alter the kinetics and the magnitude of the suppressive effect of heparin on the Ca2+ channel, suggesting an interaction of these cations with either the Ca2+ or the heparin-binding sites of the channel.

Roles of inositol trisphosphate and protein kinase C in the spontaneous outward current modulated by calcium release in rabbit portal vein

We examined the effects of heparin, guanosine nucleotides, protein kinase C (PKC) modulators, such as phorbol 12,13-dibutyrate (PDBu) and H-7 on Ca(2+)-dependent K+ currents in smooth muscle cells of the rabbit portal vein using the whole-cell patch-clamp technique, to explore the effects of PKC on the oscillatory outward current (Ioo). Neomycin (30 microM), an inhibitor of phospholipase C, and intracellular applications of heparin (10 micrograms/ml) and guanosine 5'-O-(2-thiodiphosphate) (GDP[beta S]; 1 mM) partly but consistently inhibited the generation of Ioo, whereas a higher concentration of heparin (100 micrograms/ml) transiently enhanced then suppressed the generation of Ioo. Inhibition of Ioo generation by heparin was more powerful at the holding potential of +20 mV than at -20 mV. Inositol 1,4,5-trisphosphate (InsP3; 30 microM) continuously generated Ioo at holding potentials more positive than -60 mV. Noradrenaline (10 microM) and caffeine (3-20 mM) transiently augmented, then reduced the generation of Ioo. Heparin (10 micrograms/ml) completely inhibited responses induced by InsP3 and noradrenaline, but not those induced by caffeine. Intracellular application of guanosine 5'-triphosphate (GTP; 200 microM) or low concentrations of guanosine 5'-O-(3-thiotriphosphate) (GTP[gamma S]; < or = 3 microM) continuously augmented the generation of Ioo. High concentrations of GTP[gamma S] (> or = 10 microM) transiently augmented, then inhibited Ioo. Neither GTP[gamma S] nor noradrenaline induced the transient augmentation or the subsequent inhibition of Ioo when applied in the presence of GDP[beta S] (1 mM), neomycin (30 microM) or heparin (10 micrograms/ml). PDBu (0.1 microM) reduced the generation of Ioo but failed to produce an outward current following application of caffeine (3-5 mM). This action of PDBu was inhibited by pretreatment with H-7 (20 microM). In the presence of H-7, GTP[gamma S] continuously enhanced the generation of Ioo. The suppression of the generation of Ioo during application of noradrenaline (10 microM) was reduced by pretreatment with H-7. Thus both InsP3 and protein kinase C contribute to the generation of Ioo in smooth muscle cells of the rabbit portal vein and heparin is not a specific InsP3 antagonist on the InsP3-induced Ca(2+)-release channel (PIRC). InsP3 opens PIRC and protein kinase C may deplete the stored Ca2+ by either inhibiting the reuptake of Ca2+ or by enhancement of the releasing actions of InsP3.

Effects of anions on the G protein-mediated activation of the muscarinic K+ channel in the cardiac atrial cell membrane. Intracellular chloride inhibition of the GTPase activity of GK

The effects of various intracellular anions on the G protein (GK)-mediated activation of the muscarinic K+ (KACh) channel were examined in single atrial myocytes isolated from guinea pig hearts. The patch clamp technique was used in the inside-out patch configuration. With acetylcholine (ACh, 0.5 microM) in the pipette, 1 microM GTP caused different magnitudes of KACh channel activation in internal solutions containing different anions. The order of potency of anions to induce the KACh channel activity at 0.5 microM ACh and 1 microM GTP was Cl- greater than or equal to Br- greater than 1-. In the SO4(2-) or aspartic acid internal solution, no channel openings were induced by 1 microM GTP with 0.5 microM ACh. In both the Cl- and SO4(2-) internal solutions (with 0.5 microM ACh) the relationship between the concentration of GTP and the channel activity was fit by the Hill equation with a Hill coefficient of approximately 3-4. However, the concentration of GTP at the half-maximal activation (Kd) was 0.2 microM in the Cl- and 10 microM in the SO4(2-) solution. On the other hand, the quasi-steady-state relationship between the concentration of guanosine-5'-o-(3-thiotriphosphate) and the channel activity did not differ significantly between the Cl- and SO4(2-) solutions; i.e., the Hill coefficient was approximately 3-4 and the Kd was approximately 0.06-0.08 microM in both solutions. The decay of channel activity after washout of GTP in the Cl- solution was much slower than that in the SO4(2-) solution. These results suggest that intracellular Cl- does not affect the turn-on reaction but slows the turn-off reaction of GK, resulting in higher sensitivity of the KACh channel for GTP. In the Cl- solution, even in the absence of agonists, GTP (greater than 1 microM) or ATP (greater than 1 mM) alone caused activation of the KACh channel, while neither occurred in the SO4(2-) solution. These observations suggest that the activation of the KACh channel by the basal turn-on reaction of GK or by phosphate transfer to GK by nucleoside diphosphate-kinase may depend at least partly on the intracellular concentration of Cl-.

Ca-dependent K channels in smooth muscle cells permeabilized by beta-escin recorded using the cell-attached patch-clamp technique

Using the cell-attached patch-clamp technique, the activity of single, Ca-dependent K channels was recorded in single smooth muscle cells permeabilized by beta-escin. The conductance and the relationship between the open probability of the channels and pCa recorded in permeabilized cells were very similar to those obtained in excised inside-out patches. At pCa 7, application of 30 microM acetylcholine (ACh) or 0.1 microM substance P (SP) together with 1 mM guanosine 5'-trisphosphate to permeabilized cells elicited transient bursts of channel openings similar to those which occur in intact cells. Transient activation was also observed when 2-30 microM inositol trisphosphate (IP3) was applied to permeabilized cells. This single channel activity was inhibited by pretreatment with low-molecular-weight heparin at 50-100 micrograms/ml. Channel activity at pCa 7.0 was greatly enhanced by 200 microM cyclic adenosine monophosphate. These results provide direct evidence that single Ca-dependent K channel activity is regulated by the transmitters ACh and SP, as well as a second messenger, IP3, via the release of intracellular Ca from intracellular sites which are blocked by heparin. This novel approach is valuable in elucidating second messenger mechanisms involved in the regulation of single channel activity by transmitters and autocoids, since permeabilization by beta-escin preserves the entire system of receptor-operated signal transduction and allows intracellular application of second messengers at fixed concentrations.

Heparin and other polyanions uncouple alpha 1-adrenoceptors from G-proteins

Several polyanionic compounds antagonize the interaction between receptors and the G-proteins that regulate adenylate cyclase or K+ channels, possibly by binding to a basic stretch of the receptor that is proposed to mediate its interaction with the G-proteins. We have studied the effects of polyanions on the interaction between the liver alpha 1-adrenoceptor and the G-protein through which it stimulates polyphosphoinositide turnover. Heparin [concn. causing 50% of maximal effect (EC50) = 0.5 microM], Trypan Blue (EC50 7.1 microM) or suramin (EC50 2.1 microM) prevented formation of the high-affinity adrenaline-receptor-G-protein complex without affecting antagonist binding. After alkaline treatment of the membranes, previously reported to cause G-protein removal, binding of agonists was insensitive to both guanine nucleotides and heparin. We conclude that these polyanions uncouple the alpha 1-adrenoceptor from its G-protein, suggesting that similar coupling mechanisms may underlie receptor activation of the G-proteins that activate polyphosphoinositide hydrolysis and those that regulate adenylate cyclase. This action of heparin severely limits its utility as a selective antagonist of the Ins(1,4,5)P3 receptor in intact cells.