ATP-dependent calcium transport in rat parotid basolateral membrane vesicles. Modulation by agents which elevate cyclic AMP

Neurotransmitter Control of Secretion

It is very well established that the principal control of salivary secretion is derived from autonomic innervation. Transmission of a neural signal to a salivary gland acinar cell occurs chemically via neurotransmitters, the first messengers of a secretory response. Neurotransmitters bind to specific cell surface receptor proteins, an event which activates precise transduction mechanisms which then transfer the neural signal to the inside of the cell. There are two major transduction mechanisms operative in salivary gland acinar cells. One involves the generation of cAMP, the other involves the breakdown of plasma membrane polyphosphoinositides. For both mechanisms, the appropriate stimulated receptor activates a second plasma membrane protein, termed an N (or G) protein. The N protein requires GTP to activate an enzyme (adenylate cyclase or phospholipase C), which then catalyzes the formation of a second messenger (cAMP and inositol trisphosphate/diacylglycerol, respectively). This action provides the intracellular signal for secretory events (protein, fluid, electrolyte secretion) to begin.

Voltage-dependent calcium channel and NMDA receptor antagonists augment anticonvulsant effects of lithium chloride on pentylenetetrazole-induced clonic seizures in mice

Although lithium is still a mainstay in the treatment of bipolar disorder, its underlying mechanisms of action have not been completely elucidated. Several studies have shown that lithium can also modulate seizure susceptibility in a variety of models. In the present study, using a model of clonic seizures induced with pentylenetetrazole (PTZ) in male Swiss mice, we investigated whether there is any interaction between lithium and either calcium channel blockers (CCBs: nifedipine, verapamil, and diltiazem) or N-methyl-D-aspartate (NMDA) receptor antagonists (ketamine and MK-801) in modulating seizure threshold. Acute lithium administration (5-100mg/kg, ip) significantly (P<0.01) increased seizure threshold. CCBs and NMDA receptor antagonists also exerted dose-dependent anticonvulsant effects on PTZ-induced seizures. Noneffective doses of CCBs (5mg/kg, ip), when combined with a noneffective dose of lithium (5mg/kg, ip), exerted significant anticonvulsant effects. Moreover, co-administration of a noneffective dose of either MK-801 (0.05mg/kg, ip) or ketamine (5mg/kg, ip) with a noneffective dose of lithium (5mg/kg, ip) significantly increased seizure threshold. Our findings demonstrate that lithium increases the clonic seizure threshold induced by PTZ in mice and interacts with either CCBs or NMDA receptor antagonists in exerting this effect, suggesting a role for Ca(2+) signaling in the anticonvulsant effects of lithium in the PTZ model of clonic seizures in mice.

Effect of lithium on endothelium-dependent and neurogenic relaxation of rat corpus cavernosum: Role of nitric oxide pathway

INTRODUCTION

Some studies have reported erectile dysfunction in patients receiving lithium through a mechanism that has not yet been defined. The aim of the present study was to verify the effect of acute lithium administration on the nonadrenergic noncholinergic (NANC)- and endothelium-mediated relaxation of rat isolated corpus cavernosum.

MATERIALS AND METHODS

The isolated rat corporeal strips were precontracted with phenylephrine hydrochloride (7.5 microM) and electrical field stimulation (EFS) was applied at different frequencies (2, 5, 10, and 15 Hz) to obtain NANC-mediated relaxation or relaxed by adding cumulative doses of acetylcholine (10nM-1mM) to obtain endothelium-dependent relaxation in the presence or absence of lithium (0.3, 0.5, 1, and 5mM). Also, effects of combining lithium (0.3mM) with 30 nM and 0.1 nM L-NAME (an NO synthase inhibitor) on NANC- and acetylcholine-mediated relaxation was investigated, respectively. Moreover, effects of combining lithium (1mM) with 0.1mM and 10 microM L-arginine (a precursor of NO) on NANC- and endothelium-mediated relaxation was assessed, respectively. Also, the effect of lithium (1mM) on relaxation to sodium nitroprusside (SNP; 1nM-1mM), an NO donor, was investigated.

RESULTS

The NANC-mediated relaxation was significantly (P<0.001) reduced by 1 and 5mM, but not by 0.3 and 0.5mM lithium. Lithium significantly (P<0.001) attenuated the maximum response to acetylcholine in a concentration-dependent manner. Combination of lithium (0.3mM) with 30 and 0.1 nM L-NAME, which separately had a minimum effect on NANC- and endothelium-mediated relaxation, significantly (P<0.001) reduced the NANC- and endothelium-mediated relaxation, respectively. Although L-arginine at 10 microM and 0.1mM did not alter the relaxant responses to acetylcholine and EFS, it improved the inhibition by lithium (1mM) of relaxant responses to acetylcholine and EFS, respectively. Also, SNP produced similar concentration-dependent relaxations from both groups.

DISCUSSION

Our experiments indicated that lithium likely by interfering with NO pathway in both endothelium and nitrergic nerve can result in impairment of both the endothelium- and NANC-mediated relaxation of rat corpus cavernosum.

Chronic lithium treatment attenuates intracellular calcium mobilization

Elevated basal intracellular calcium (Ca(2+)) levels ([Ca(2+)](B)) in B lymphoblast cell lines (BLCLs) from bipolar I disorder (BD-I) patients implicate altered Ca(2+) homeostasis in this illness. Chronic lithium treatment affects key proteins modulating intracellular Ca(2+) signaling. Thus, we sought to determine if chronic exposure to therapeutic lithium concentrations also modifies intracellular Ca(2+) homeostasis in this surrogate cellular model of signal transduction disturbances in BD. BLCLs from BD-I (N=26) and healthy subjects (N=17) were regrown from frozen stock and incubated with 0.75 mM lithium or vehicle for 24 h (acute) or 7 days (chronic). [Ca(2+)](B), lysophosphatidic acid (LPA)-stimulated Ca(2+) mobilization ([Ca(2+)](S)), and thapsigargin-induced store-operated Ca(2+) entry (SOCE) were determined using ratiometric fluorometry with Fura-2. Compared with vehicle, chronic lithium exposure resulted in significantly higher [Ca(2+)](B) (F=8.47; p=0.006) in BLCLs from BD-I and healthy subjects. However, peak LPA-stimulated [Ca(2+)](S) and SOCE were significantly reduced (F=11.1, p=0.002 and F=8.36, p=0.007, respectively). Acute lithium exposure did not significantly affect measured parameters. In summary, the effect of chronic lithium to elevate [Ca(2+)](B) in BLCLs while attenuating both receptor-stimulated and SOCE components of intracellular Ca(2+) mobilization in BLCLs suggests that modulation of intracellular Ca(2+) homeostasis may be important to the therapeutic action of lithium.

Associated disturbances in calcium homeostasis and G protein-mediated cAMP signaling in bipolar I disorder

BACKGROUND

Evidence of extensive cross-talk between calcium (Ca(2+))- and cAMP-mediated signaling systems suggests that previously reported abnormalities in Ca(2+) homeostasis in bipolar I (BP-I) patients may be linked to disturbances in the function of G proteins that mediate cAMP signaling.

METHODS

To test this hypothesis, the beta-adrenergic agonist, isoproterenol, and the G protein activator, sodium fluoride (NaF), were used to stimulate cAMP production in B lymphoblasts from healthy and BP-I subjects phenotyped on basal intracellular calcium concentration ([Ca(2+)](B)). cAMP was measured by radioimmunoassay and [Ca(2+)](B) by ratiometric fluorometry with fura-2.

RESULTS

Isoproterenol- (10 microM) stimulated cAMP formation was lower in intact B lymphoblasts from BP-I patients with high [Ca(2+)](B) (>/= 2 SD above the mean concentration of healthy subjects) compared with patients having normal B lymphoblast [Ca(2+)](B) and with healthy subjects. Although basal and NaF-stimulated cAMP production was greater in B lymphoblast membranes from male BP-I patients with high versus normal [Ca(2+)](B), there were no differences in the percent stimulation. This suggests the differences in NaF response resulted from higher basal adenylyl cyclase activity.

CONCLUSIONS

These findings suggest that trait-dependent disturbances in processes regulating beta-adrenergic receptor sensitivity and G protein-mediated cAMP signaling occur in conjunction with altered Ca(2+) homeostasis in those BP-I patients with high B lymphoblast [Ca(2+)](B).

Evidence that both Ca(2+)-ATPase and (Ca2+ + Mg2+)-ATPase activities in the plasma membrane-rich fraction from bovine parotid gland reside on the same enzyme molecule

1. Evidence was obtained that activities of both low-affinity Ca(2+)-ATPase and high-affinity (Ca2+ + Mg2+)-ATPase in the plasma membrane-rich fraction from bovine parotid gland reside on the same enzyme. 2. Two solubilized ATPases were purified by four steps of HPLC; and both activities eluted at the same fractions from each column, and the specific activity ratio of the two enzymes at each step was constant. 3. By non-denaturing PAGE, the final preparation gave a single band for both protein staining and activity staining for the two ATPases; and the Ca(2+)-ATPase activity comigrated with that of (Ca2+ + Mg2+)-ATPase. 4. In SDS-PAGE, each activity staining for the ATPases also gave a single band, and both activities comigrated. 5. These findings suggest that Ca(2+)-ATPase and (Ca2+ + Mg2+)-ATPase are a single enzyme.

Characterization of high affinity GTPase activity correlated to beta-adrenergic receptor stimulation of adenylyl cyclase in rat parotid membranes

beta-Adrenergic receptor stimulation of adenylyl cyclase involves the activation of a GTP-binding regulatory protein (G-protein, termed here Gs). Inactivation of this G-protein is associated with the hydrolysis of bound GTP by an intrinsic high affinity GTPase activity. In the present study, we have characterized the GTPase activity in a Gs-enriched rat parotid gland membrane fraction. Two GTPase activities were resolved; a high affinity GTPase activity displaying Michaelis-Menten kinetics with increasing concentrations of GTP, and a low affinity GTPase activity which increased linearly with GTP concentrations up to 10 mM. The beta-adrenergic agonist isoproterenol (10 microM) increased the Vmax of the high affinity GTPase component approx. 50% from 90 to 140 pmol/mg protein per min, but did not change its Km value (approximately 450 nM). Isoproterenol also stimulated adenylyl cyclase activity in parotid membranes both in the absence or presence of GTP. In the presence of a non-hydrolyzable GTP analogue, guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), isoproterenol increased cAMP formation to the same extent as that observed with AlF-4. Cholera toxin treatment of parotid membranes led to the ADP-ribosylation of two proteins (approximately 45 and 51 kDa). Cholera toxin also specifically decreased the high affinity GTPase activity in membranes and increased cAMP formation induced by GTP in the absence or the presence of isoproterenol. These data demonstrate that the high affinity GTPase characterized here is the 'turn-off' step for the adenylyl cyclase activation seen following beta-adrenergic stimulation of rat parotid glands.

ATP-dependent Ca2+ transport in the rat parotid basolateral plasma membrane is regulated by calmodulin

Calmodulin regulation of ATP-dependent Ca2+ transport activity was assessed in inverted basolateral plasma membrane vesicles (BLMV) isolated from rat parotid glands. The initial rate of Ca2+ transport in media containing 100 nM Ca2+ was stimulated by approximately 60% at maximal concentrations (300 nM) of exogenously added calmodulin (CAM). Half-maximal activation was obtained at 50 and 175 nM CAM in KCl and mannitol containing assay media, respectively. In the KCl medium, addition of 300 nM CAM increased the affinity of the BLMV Ca2+ transport activity for Ca2+ from approximately 70 nM, in the absence of added CAM, to approximately 50 nM. Vmax was consistently increased by approximately 20% under these conditions. When BLMV were treated with ethylene glycol bis(beta-aminoethylether) N,N'-tetraacetic acid (EGTA) (200 microM), the affinity of the transporter for Ca2+ decreased by 50% to approximately 150 nM, with no change in Vmax. When CAM was added to the EGTA-treated membranes, Ca2+ transport activity was comparable to that obtained when CAM was added directly to control, untreated BLMV. The CAM antagonists, trifluoperazine (TFP), W-7, and calmidazolium, inhibited Ca2+ transport in the presence of CAM. Half-maximal inhibition of transport was achieved by 12 microM TFP and 20 microM W-7. Calmidazolium (1 microM) inhibited Ca2+ transport by 75%. The inhibitory effects on ATP-dependent Ca2+ transport exerted by these agents were not due to an increase in the passive permeability of the membranes to Ca2+. Furthermore, in the absence of added CAM, the inhibitory effects of these agents on initial Ca2+ transport rate was decreased. The data presented suggest that the Ca2+-dependent interaction of CAM with the ATP-dependent Ca2+ transporter in rat parotid BLMV modifies the kinetic properties of this Ca2+ transporting mechanism.

Enhancement of cyclic AMP modulated salivary amylase secretion by protein kinase C activators

The effect of protein kinase C activators on isoproterenol-induced amylase secretion were investigated in isolated rat parotid cells. Pretreatment with phorbol dibutyrate potentiated isoproterenol-induced amylase secretion. This effect of phorbol dibutyrate was mimicked by dioctanoylglycerol or carbachol. Phorbol dibutyrate also potentiated secretion evoked by the adenylate cyclase activator forskolin and by dibutyryl cAMP. Neither phorbol dibutyrate nor carbachol enhanced isoproterenol-induced cAMP accumulation. The present study reveals a coordinate interaction between cAMP and protein kinase C at a step in the secretory mechanism distal to cAMP generation.

Characterisation of a high affinity Ca2+-stimulated, Mg2+-dependent ATPase in the rat parotid plasma membrane

Two Ca2+-stimulated ATPase activities have been identified in the plasma membrane of rat parotid: (a) a (Ca2+ + Mg2+)-ATPase with high affinity for free Ca2+ (apparent Km = 208 nM, Vmax = 188 nmol/min per mg) and requiring micromolar concentration of Mg2+ and (b) a (Ca2+ or Mg2+)-ATPase with relatively low affinity for free Ca2+ (K0.5 = 23 microM) or free Mg2+ (K0.5 = 26 microM). The low-affinity (Ca2+ or Mg2+)-ATPase can be maximally stimulated by Ca2+ alone or Mg2+ alone. The high-affinity (Ca2+ + Mg2+)-ATPase exhibits sigmoidal kinetics with respect to ATP concentration with K0.5 = 0.4 mM and a Hill coefficient of 1.91. It displays low substrate specificity with respect to nucleotide triphosphates. Although trifluoperazine inhibits the activity of the high affinity (Ca2+ + Mg2+)-ATPase only slightly, it inhibits the activity of the low-affinity (Ca2+ or Mg2+)-ATPase quite potently with 22 microM trifluoperazine inhibiting the enzymic activity by 50%. Vanadate, inositol 1,4,5-trisphosphate, phosphatidylinositol 4,5-bisphosphate, Na+,K+ and ouabain had no effect on the activities of both ATPases. Calmodulin added to the plasma membranes does not stimulate the activities of both ATPases. The properties of the high-affinity (Ca2+ + Mg2+)-ATPase are distinctly different from those of the previously reported Ca2+-pump activity of the rat parotid plasma membrane.

ATP-dependent calcium transport in rat parotid basolateral membrane vesicles is modulated by membrane potential

The ATP-dependent Ca2+ transport activity (T. Takuma, B.L. Kuyatt and B.J. Baum, Biochem. J. 227:239-245, 1985) exhibited by inverted basolateral membrane vesicles isolated from rat parotid gland was further characterized. The activity was dependent on Mg2+. Phosphate (5 mM), but not oxalate (5 mM), increased maximum Ca2+ accumulation by 50%. Half-maximal Ca2+ transport was achieved at approximately 70 nM Ca2+ in EGTA-buffered medium while maximal activity required greater than 1 microM Ca2+ (Vmax = 54 nmol/mg protein/min). Optimal rates of Ca2+ transport were obtained in the presence of KCl, while in a KCl-free medium (mannitol or sucrose) approximately 40% of the total activity was achieved, which could not be stimulated by FCCP. The initial rate of Ca2+ transport could be significantly altered by preimposed membrane potentials generated by K+ gradients in the presence of valinomycin. Compared to the transport rate in the absence of membrane potential, a negative (interior) potential stimulated uptake by approximately 30%, while a positive (interior) potential inhibited uptake. Initial rates of Ca2+ uptake could also be altered by imposing pH gradients, in the absence of KCl. When compared to the initial rate of Ca2+ transport in the absence of a pH gradient, pHi = 7.5/pHo = 7.5; the activity was approximately 60% higher in the presence of an outwardly directed pH gradient, pHi = 7.5/pHo = 8.5; while it was approximately 80% lower when an inwardly directed pH gradient was imposed, pHi = 7.5/pHo = 6.2. The data show that the ATP-dependent Ca2+ transport in BLMV can be modulated by the membrane potential, suggesting therefore that there is a transfer of charge into the vesicle during Ca2+ uptake, which could be compensated by other ion movements.

Adrenoreceptor mobilization of calcium in rat submandibular cells

The stimulation of adrenoreceptors by epinephrine leads to an alteration in the Ca2+ homeostasis of rat submandibular cells. The relative contributions of alpha- and beta-adrenoreceptors were assessed with specific adrenergic agents. Stimulation of alpha 1-adrenoreceptors resulted in enhanced unidirectional 45Ca2+ fluxes, while such effects following beta-adrenoreceptor stimulation, though suggestive, were equivocal. When the cytosolic Ca2+ level was followed in cells pre-loaded with the Ca2+-sensitive fluorescent indicator Quin 2, clear results were observed. Both alpha 1- and beta-adrenoreceptor stimulation were capable of mobilizing intracellular and extracellular Ca2+ pools, as assessed by differential responses of cells incubated in media (+/-)-ethylene-glycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid. However, Ca2+ mobilization following alpha 1-adrenoreceptor stimulation was more rapid (initial rate approximately 4-5 X) and to a greater extent (approximately 2 X) than seen with beta-adrenoreceptor stimulation.

Neurotransmitter control of secretion

It is very well established that the principal control of salivary secretion is derived from autonomic innervation. Transmission of a neural signal to a salivary gland acinar cell occurs chemically via neurotransmitters, the first messengers of a secretory response. Neurotransmitters bind to specific cell surface receptor proteins, an event which activates precise transduction mechanisms which then transfer the neural signal to the inside of the cell. There are two major transduction mechanisms operative in salivary gland acinar cells. One involves the generation of cAMP, the other involves the breakdown of plasma membrane polyphosphoinositides. For both mechanisms, the appropriate stimulated receptor activates a second plasma membrane protein, termed an N (or G) protein. The N protein requires GTP to activate an enzyme (adenylate cyclase or phospholipase C), which then catalyzes the formation of a second messenger (cAMP and inositol trisphosphate/diacylglycerol, respectively). This action provides the intracellular signal for secretory events (protein, fluid, electrolyte secretion) to begin.