Effect of intracellular Na+ on platelet aggregation and Ca2+ mobilization

The effects of ionophores, which can carry alkali metal cations, on platelet aggregation were examined. At an alkaline extracellular pH, alkali metal cation/H+ exchanger nigericin accelerated aggregation in K+-enriched medium, whereas it rather inhibited aggregation in Na+-enriched medium, even though the intracellular pH was only slightly alkaline. The inhibitory effect of Na+ on platelet aggregation was more clearly shown with the alkali metal cation exchanger gramicidin D. The ionophore had no effect or a slightly accelerative effect on aggregation in K+-enriched medium, whereas it significantly inhibited aggregation induced by thrombin, ADP and platelet activating factor in Na+-enriched medium. Fluorescence studies on fura-2-labeled platelets revealed that in Na+-enriched medium gramicidin D inhibited agonist-induced Ca2+ mobilization both in the presence and absence of extracellular Ca2+. These results suggest that the intracellular Na+ inhibits platelet aggregation by inhibiting Ca2+ mobilization.     

Effect of Bcl-2 on oxidant-induced cell death and intracellular Ca2+ mobilization

The mechanism by which Bcl-2 inhibits cell death is unknown. Ithas been suggested that Bcl-2 functions as an antioxidant. BecauseBcl-2 is localized mainly to the membranes of the endoplasmic reticulum(ER) and the mitochondria, which represent the main intracellularstorage sites for Ca²⁺, wehypothesized that Bcl-2 might protect cells against oxidant injury byaltering intracellular Ca²⁺homeostasis. To test this hypothesis, we examined the effect of oxidanttreatment on viability in normal rat kidney (NRK) cells and in NRKcells stably transfected with Bcl-2 in the presence or absence ofintracellular Ca²⁺, and wecompared the effect of Bcl-2 expression on oxidant-induced intracellular Ca²⁺ mobilizationand on ER and mitochondrial Ca²⁺pools. NRK cells transfected with Bcl-2 (NRK-Bcl-2) were significantly more resistant toH₂O₂-inducedcytotoxicity than control cells. EGTA-AM, an intracellularCa²⁺ chelator, as well as theabsence of Ca²⁺ in the medium,reducedH₂O₂-inducedcytotoxicity in both cell lines. Compared with controls, cellsoverexpressing Bcl-2 showed a delayed rise in intracellularCa²⁺ concentration([Ca²⁺]_i)afterH₂O₂treatment. After treatment with theCa²⁺ ionophore ionomycin,Bcl-2-transfected cells showed a much quicker decrease after themaximal rise than control cells, suggesting stronger intracellularCa²⁺ buffering, whereas treatmentwith thapsigargin, an inhibitor of the ERCa²⁺-ATPases, transientlyincreased[Ca²⁺]_iin control and in Bcl-2-transfected cells. Estimates of mitochondrial Ca²⁺ stores using an uncoupler ofoxidative phosphorylation show that NRK-Bcl-2 cells have a highercapacity for mitochondrial Ca²⁺storage than control cells. In conclusion, Bcl-2 may prevent oxidant-induced cell death, in part, by increasing the capacity ofmitochondria to store Ca²⁺.

     

Effects of thyroid hormone on Ca2+ efflux and Ca2+ transport capacity in rat skeletal muscle

The present study was undertaken to investigate the effects of 3,5,3'-triiodothyronine (T3) treatment on passive Ca2+ efflux, Ca2(+)-dependent Mg2(+)-ATPase (Ca2(+)-ATPase) concentration and active Ca2+ transport in isolated rat skeletal muscle. In addition, the question was examined whether changes in Ca2+ efflux at rest and during electrical stimulation in the hyperthyroid state were accompanied by parallel changes in 3-O-methylglucose efflux. The resting Ca2+ efflux from rat soleus muscle was increased by 25% after 8 days of treatment with T3 (20 micrograms/100 g body weight). This was associated with a 78% increase in the basal efflux of 3-O-methylglucose. Electrical stimulation resulted in a rapid stimulation of Ca2+ efflux and 3-O-methylglucose efflux in the two groups of rats, and the levels obtained were significantly higher in the T3-treated group. The stimulating effect of the alkaloid veratridine on Ca2+ efflux was 60% larger in 8-day hyperthyroid rats. Within 24 h after the start of T3 treatment, a significant (21%) increase in Ca2(+)-ATPase concentration was detected. Significant increases in active Ca2+ uptake and passive Ca2+ efflux were not observed until after 2 and 3 days of T3 treatment, respectively. It is concluded that T3 stimulates the synthesis of Ca2+ ATPase and augments the intracellular Ca2+ pools (sarcoplasmic reticulum and mitochondria). The latter results in enhancement of the passive Ca2+ leak, which in turn, may lead to activation of substrate transport systems. The suggested increase in intracellular Ca2+ cycling after T3 treatment may, at least partly, explain the T3-induced stimulation of energy metabolism.     

Possible involvement of eicosanoids in the zymosan and arachidonic-acid-induced oxygen uptake, glycogenolysis and Ca2+ mobilization in the perfused rat liver

Exposure of perfused rat livers to zymosan, arachidonic acid and phenylephrine, but not to latex particles, induces pronounced oxygen uptake, glycogenolysis and Ca2+ mobilization. The oxygen uptake induced by arachidonic acid and by zymosan remains elevated even after the agents have been removed. NaN3 was found to be much more effective in inhibiting the oxygen uptake induced by phenylephrine than that induced by zymosan or arachidonic acid. Glucose release induced by zymosan and by arachidonic acid reaches a maximum after about 2 min and then declines very rapidly even while the agents are still being infused. In contrast, glucose release induced by phenylephrine remains elevated for the duration of the infusion. Ca2+ fluxes induced by arachidonic acid are similar to those induced by phenylephrine in that efflux occurs when the agent is administered and influx occurs only when the agent is removed. This contrasts to the Ca2+ flux changes induced by zymosan where both Ca2+ efflux and Ca2+ influx occur even while zymosan is still being infused. Glucose release induced by zymosan is inhibited by bromophenacylbromide and nordihydroguaiaretic acid, but not by indomethacin. Indomethacin, however inhibits the arachidonic-acid-induced glucose release which is also inhibited by nordihydroguaiaretic acid but not by bromophenacylbromide. Indomethacin inhibits also the arachidonic-acid-induced Ca2+ flux changes whereas the zymosan- and the phenylephrine-induced Ca2+ flux changes are not inhibited by the cyclooxygenase inhibitor. The data presented in this paper suggest that in the perfused rat liver the zymosan-induced glycogenolysis, as well as the Ca2+ flux changes and glycogenolysis induced by arachidonic acid, are mediated by eicosanoids.     

Effect of hypothyroidism on the cytosolic free Ca2+ concentration in rat hepatocytes during rest and following stimulation by noradrenaline or vasopressin

The mean resting concentration of cytosolic free Ca2+ [( Ca2+]i) in parenchymal liver cells, as determined with the intracellular Ca2+ indicator quin2, was lowered by about 30% in hypothyroidism (0.17 microM vs. 0.27 microM in normal cells). The [Ca2+]i level in hypothyroid cells at 10 s following stimulation by noradrenaline (1 microM) was about 64% lower than in normal cells (0.33 microM vs. 1.0 microM). The response to noradrenaline in hypothyroid cells was slower in onset (significant at 5 s vs. 3 s in euthyroid cells), and the maximum of the initial [Ca2+]i increase was reached later (14 s vs. 8 s in normal cells). In hypothyroid hepatocytes the initial increase was followed by a slow but prolonged secondary increase in [Ca2+]i. With vasopressin similar results were found. Chelation of extracellular Ca2+ with EGTA immediately prior to stimulation had no effect on the initial [Ca2+]i increase. Treatment with T3 in vivo (0.5 micrograms/100 g body weight daily during 3 days) completely restored the basal and stimulated [Ca2+]i in hypothyroid cells. The half-maximally effective dose of noradrenaline was the same in euthyroid and hypothyroid liver cells (1.8 X 10(-7) M). Hypothyroidism had no significant effect on the number of alpha 1-receptors determined by [3H]prazosin labeling in crude homogenate fractions, while the Kd for [3H]prazosin was 21% lower than in the euthyroid group. These results show that thyroid hormone has a general stimulating effect on intracellular Ca2+ mobilization by Ca2+-mobilizing hormones, probably at a site distal to the binding of the agonist to its receptor. The results also support our idea that thyroid hormone may control metabolism during rest and activation, at least partially, by altering Ca2+ homeostasis.     

Influence of thyroid status on Ca2+ mobilization and amylase secretion in rat pancreatic acini

Thyroid hormones influence Ca2+ homeostasis in both skeletal and cardiac muscle. Since secretory cells, like muscle cells, store and use Ca2+ in stimulus-response coupling, we have studied the effects of thyroid status on Ca2+ mobilization and secretion in a model secretory tissue, the pancreatic acinar cell. Hyperthyroidism was induced by rats by daily, subcutaneous injections of triiodothyronine for 8 days and hypothyroidism by adding 6-n-propyl-2-thiouracil to the drinking water for 14 days. Pancreatic acini were prepared by collagenase digestion of pancreatic tissue from hyper- and hypo-thyroid animals and from euthyroid controls. Ca2(+)-mobilization was assessed using Quin-2 fluorescence and secretion by assaying amylase release. The data indicate that the amount of Ca2+ mobilized by the muscarinic agonist carbachol or by cholecystokinin octapeptide increases with increasing thyroid hormone concentrations. Only in hypothyroidism was this change in Ca2+ homeostasis reflected by a parallel change in amylase secretion. This implies the existence of some compensatory mechanism which stabilizes secretory rate in the face of stimulus-evoked increases in intracellular Ca2+ concentration.     

Mechanism of arachidonic acid-induced Ca2+ mobilization from rat liver microsomes

Recent evidence indicates that unesterified arachidonic acid functions as a mediator of intracellular Ca2+ mobilization by inducing Ca2+ release from the endoplasmic reticulum of pancreatic islet beta cells in a manner closely similar to that of inositol 1,4,5-trisphosphate. To test the generality and explore the mechanism of this phenomenon we have examined the effects of arachidonic acid on calcium accumulation and release by hepatocyte subcellular fractions enriched in endoplasmic reticulum (microsomes). At concentrations above 0.017 mumol/mg microsomal protein, arachidonate induced rapid (under 2 min) 45Ca2+ release from microsomes that had been preloaded with 45Ca2+. Arachidonate also suppressed microsomal 45Ca2+ accumulation when present during the loading period, as reflected by reduction both of 45Ca2+ accumulation at steady state and of the rate of uptake. Neither the cyclooxygenase inhibitor indomethacin nor the lipoxygenase/cyclooxygenase inhibitor BW755C suppressed arachidonate-induced 45Ca2+ release, indicating that this effect was not dependent upon oxygenation of the fatty acid to metabolites. The long-chain unsaturated fatty acids oleate and linoleate were less potent than arachidonate in inducing 45Ca2+ release, and the saturated fatty acid stearate did not exert this effect. Albumin prevented 45Ca2+ release by arachidonate, presumably by binding the fatty acid. As is the case for inositol 1,4,5-trisphosphate, the ability of arachidonate to induce 45Ca2+ release was dependent on the ambient free Ca2+ concentration. Arachidonate did not influence microsomal membrane permeability or Ca2+-ATPase activity and may exert its effects on microsomal Ca2+ handling by activation of a Ca2+ extrusion mechanism or by dissociating Ca2+ uptake from Ca2+-ATPase activity.     

Modifications of Ca2+ mobilization and noradrenaline release by S-nitroso-cysteine in PC12 cells

The effects of nitrogen monoxide (NO)-related compounds on cytosolic free Ca2+ concentrations ([Ca2+]i) and noradrenaline (NA) release in neurosecretory PC12 cells were investigated. The addition of S-nitroso-cysteine (SNC) stimulated [Ca2+]i increases from an intracellular Ca2+ pool continuously in a concentration-dependent manner. Other NO donors, which stimulate cyclic GMP accumulation, did not cause [Ca2+]i increases. After treatment with 0.2 mM SNC, transient increases in [Ca2+]i from the Ca2+ pool induced by caffeine were completely abolished. The addition of N-ethylmaleimide (NEM) caused sustained [Ca2+]i increases from the intracellular Ca2+ pool. Furthermore, caffeine did not stimulate further [Ca2+]i increases in PC12 cells pretreated with NEM. These findings suggest that SNC and NEM predominantly interact with a caffeine-sensitive Ca2+ pool. The addition of dithiothreitol (DTT) to 0.4 mM SNC-stimulated cells reduced [Ca2+]i to basal levels, and the addition of DTT to NEM-stimulated cells locked [Ca2+]i at high levels. The stimulatory effects of SNC but not NEM were not abolished by pretreatment with DTT. These findings suggest that modification of the oxidation status of the sulfhydryl groups on the caffeine-sensitive receptors by SNC or NEM regulates Ca2+ channel activity in a reversible manner. SNC did not stimulate NA release by itself but did inhibit ionomycin-stimulated NA release. In contrast, NEM stimulated NA release in the absence of extracellular CaCl2 and further enhanced ionomycin-stimulated NA release. Ca2+ mobilization by SNC from the caffeine-sensitive pool was not a sufficient factor, and other factors stimulating NA release may be negatively regulated by SNC.     

The effect of thyroid hormone on the high affinity Ca2+-ATPase in rat liver plasma membrane

The effect of thyroid hormone on the high affinity Ca2+-ATPase activity in rat liver plasma membrane was studied. The high affinity Ca2+-ATPase activity in plasma membrane was activated by 10(-7)-10(-5) M of Ca2+ and was inhibited by 70 microM trifluoperazine. Thyroidectomy of rats was associated with an increase in the activity of high affinity Ca2+-ATPase. The increased enzyme activity was normalized by T4 administration to the animals. On the other hand, Na+-K+-ATPase activity in the membrane was decreased by thyroidectomy and the decreased enzyme activity was normalized by T4 administration. The results suggest that thyroid hormone inhibits the Ca2+ extrusion system by inhibiting calmodulin-independent high affinity Ca2+-ATPase in liver plasma membrane.     

Regulation of cardiac sarcolemmal Ca2+ channels and Ca2+ transporters by thyroid hormone

In order to examine the regulatory role of thyroid hormone on sarcolemmal Ca²⁺-channels, Na⁺–Ca²⁺ exchange and Ca²⁺-pump as well as heart function, the effects of hypothyroidism and hyperthyroidism on rat heart performance and sarcolemmal Ca²⁺-handling were studied. Hyperthyroid rats showed higher values for heart rate (HR), maximal rates of ventricular pressure development+(dP/dt)max and pressure fall–(dP/dt)max, but shorter time to peak ventricular pressure (TPVP) and contraction time (CT) when compared with euthyroid rats. The left ventricular systolic pressure (LVSP) and left ventricular end-diastolic pressure (LVEDP), as well as aortic systolic and diastolic pressures (ASP and ADP, respectively) were not significantly altered. Hypothyroid rats exhibited decreased values of LVSP, HR, ASP, ADP, +(dP/dt)max and –(dP/dt)max but higher CT when compared with euthyroid rats; the values of LVEDP and TPVP were not changed. Studies with isolated-perfused hearts showed that while hypothyroidism did not modulate the inotropic response to extracellular Ca²⁺ and Ca²⁺ channel blocker verapamil, hyperthyroidism increased sensitivity to Ca²⁺ and decreased sensitivity to verapamil in comparison to euthyroid hearts. Studies of [³H]-nitrendipine binding with purified cardiac sarcolemmal membrane revealed decreased number of high affinity binding sites (B_max) without any change in the dissociation constant for receptor-ligand complex (K_d) in the hyperthyroid group when compared with euthyroid sarcolemma; hypothyroidism had no effect on these parameters. The activities of sarcolemmal Ca²⁺-stimulated ATPase, ATP-dependent Ca²⁺ uptake and ouabain-sensitive Na⁺–K⁺ ATPase were decreased whereas the Mg²⁺-ATPase activity was increased in hypothyroid hearts. On the other hand, sarcolemmal membranes from hyperthyroid samples exhibited increased ouabain-sensitive Na⁺–K⁺ ATPase activity, whereas Ca²⁺-stimulated ATPase, ATP-dependent Ca²⁺ uptake, and Mg²⁺-ATPase activities were unchanged. The V_max and K_a for Ca²⁺ of cardiac sarcolemmal Na⁺–Ca²⁺ exchange were not altered in both hyperthyroid and hypothyroid states. These results indicate that the status of sarcolemmal Ca²⁺-transport processes is regulated by thyroid hormones and the modification of Ca²⁺-fluxes across the sarcolemmal membrane may play a crucial role in the development of thyroid state-dependent contractile changes in the heart.     

Effect of pH and Ca2+ on the retention of Ca2+ by rat liver mitochondria.

A transient Ca²⁺ release from preloaded mitochondria can be induced by a sudden decrease in the pH of the outer medium from 8.0 or 7.4 to 6.8. In the presence of inorganic phosphate the released Ca²⁺ is not taken up again. Upon Ca²⁺ addition to respiring mitochondria the mitochondrial membrane potential (Δ♀) decreases to a new resting level. A further decrease in Δ♀ occurs after the decrease in pH from 7.4 to 6.8, concomitant with the reuptake phase of the Ca²⁺ release. Phosphate, EGTA, and ruthenium red restore Δ♀ to its initial level. If phosphate is present initially, only transient changes in Δ♀ occur upon addition of Ca²⁺ or H⁺ ions. Only a small transient change in Δ♀ upon H⁺ ion addition is seen in the absence of accumulated Ca²⁺. La³⁺, a competitive inhibitor of Ca²⁺ transport, prevents the H⁺ ion-induced Ca²⁺ efflux, whereas this is not the case in the presence of the noncompetitive inhibitor ruthenium red. Ruthenium red, however, prevents the reuptake phase. Mg²⁺, an inhibitor of the surface binding of Ca²⁺, has no or only a slight effect on the H⁺ ion-induced Ca²⁺ release. Mitochondria preloaded with Ca²⁺ release a small fraction of Ca²⁺ during the subsequent uptake of another pulse of Ca²⁺. The results indicate that at least one pool of mitochondrial Ca²⁺ exists in a mobile state. The possible existence of a $\text{H}{}^{\mathrm{+}}\text{Ca}{}^{\mathrm{2+}}$ exchanger in the mitochondrial membrane is discussed.     

Ca2+-dependent activation of the malate-aspartate shuttle by norepinephrine and vasopressin in perfused rat liver

The role of Ca2+ in stimulation of the malate-aspartate shuttle by norepinephrine and vasopressin was studied in perfused rat liver. Shuttle capacity was indexed by measuring the changes in both the rate of production of glucose from sorbitol and the ratio of lactate to pyruvate during the oxidation of ethanol. (T. Sugano et al. (1986) Amer. J. Physiol. 251, E385-E392). Asparagine (0.5 mM), but not alanine (0.5 mM) decreased the ethanol-induced responses. Norepinephrine and vasopressin had no effect on the ethanol-induced responses when the liver was perfused with sorbitol or glycerol. In the presence of 0.25 mM alanine, norepinephrine, vasopressin, and A23187 decreased the ethanol-induced responses that occurred with the increase of flux of Ca2+. In liver perfused with Ca2+-free medium, asparagine also decreased the ethanol-induced responses, but norepinephrine and vasopressin had no effect. Aminooxyacetate inhibited the effects of norepinephrine, A23187, and asparagine. Regardless of the presence or absence of perfusate Ca2+, the combination of glucagon and alanine had no effect on the ethanol-induced responses. Norepinephrine caused a decrease in levels of alpha-ketoglutarate, aspartate, and glutamate in hepatocytes incubated with Ca2+. The present data suggest that the redistribution of cellular Ca2+ may activate the efflux of aspartate from mitochondria in rat liver, resulting in an increase in the capacity of the malate-aspartate shuttle.     

Caffeine does not inhibit substance P-evoked intracellular Ca2+ mobilization in rat salivary acinar cells

We used theCa²⁺-sensitive fluorescent dyefura 2, together with measurements of intracellularD-myo-inositol1,4,5-trisphosphate [Ins(1,4,5)P₃],to assess the inhibitory effects of caffeine on signal transduction viaG protein-coupled receptor pathways in isolated rat mandibular salivaryacinar cells. ACh, norepinephrine (NE), and substance P (SP) all evokedsubstantial increases in the intracellular freeCa²⁺ concentration([Ca²⁺]_i).Responses to ACh and NE were markedly inhibited by prior application of20 mM caffeine. The inhibitory effect of caffeine was not reproduced byphosphodiesterase inhibition with IBMX or addition of cell-permeantdibutyryl cAMP. In contrast to the ACh and NE responses, the[Ca²⁺]_iresponse to SP was unaffected by caffeine. Despite this, SP and AChappeared to mobilize Ca²⁺ from acommon intracellular pool. Measurements of agonist-induced changes inIns(1,4,5)P₃levels confirmed that caffeine inhibited the stimulus-response couplingpathway at a point beforeIns(1,4,5)P₃ generation. Caffeine did not, however, inhibit[Ca²⁺]_iresponses evoked by direct activation of G proteins with 40 mMF. These data show thatcaffeine inhibits G protein-coupled signal transduction in these cellsat some element that is common to the muscarinic and -adrenergicsignaling pathways but is not shared by the SP signaling pathway. Wesuggest that this element might be a specific structural motif on the Gprotein-coupled muscarinic and -adrenergic receptors.     

Effect of endothelin on Ca2+ influx, intracellular free Ca2+ levels and ligand binding to N and L type Ca2+ channels in rat brain

The actions of endothelin, an endogenous vasoconstrictor compound with potent effects on various parameters of Ca2+ metabolism in peripheral tissue, were studied in several neuronal preparations. Endothelin, by itself, did not alter resting intracellular free Ca2+ levels or Ca2+ influx in either rat or chicken brain preparations; nor did it affect depolarization (K+) induced changes in these parameters. Endothelin also had no effect on the binding of [3H]-nitrendipine or [125I]-omega-conotoxin to "L " or "N" type channels respectively nor did it induce the release of endogenous acetylcholine from brain slices. The results show that, despite the proposed role of endothelin on voltage sensitive Ca2+ channels in peripheral tissue and despite the existence of endothelin binding sites on both smooth muscle and neurons, endothelin has no measurable effects on Ca2+ metabolism in neural tissue of central origin.     

Crosslinking by ligands to surface immunoglobulin triggers mobilization of intracellular 45Ca2+ in B lymphocytes

Detailed studies of steady-state ion fluxes in murine lymphocytes were used to examine for possible ionic changes generated by surface Ig, the antigen receptor of B lymphocytes. When bound by ligands, surface Ig triggered the mobilization and release of 45Ca2+ from the cell interior by a transmembrane process requiring crosslinking of the bound receptors. This ionic event was unique for two reasons: (a) it did not occur when other common lymphocyte surface macromolecules were bound with rabbit anti-lymphocyte antibodies; and (b) it was not accompanied by a general perturbation of lymphocyte ionic properties such as a change in 42K+ fluxes nor did it depend on the presence of extracellular ions. Capping of surface Ig shares the same time sequence, dose response, requirement for crosslinking, and lack of dependence on extracellular ions. These correlations suggest that mobilization of intracellular Ca2+ may represent an early ionic signal for the contractile activation of lymphocytes that generates capping of surface Ig.     

Effect of intracellular free Ca2+ on leucine transport in Chang liver cells

Addition of Ca2+ ionophore (A23187) to the medium stimulated the Na+-independent leucine transport in Chang liver cells, increasing the cytoplasmic free Ca2+ concentration, irrespective of the presence or absence of extracellular Ca2+. Anticalmodulin drugs, such as chlorpromazine, trifluoperazine, and W-7, significantly inhibited the leucine transport in the cells. The stimulatory effect of A23187 on leucine transport was completely blocked in the presence of the anticalmodulin drug. Two microtubule disrupting drugs, colchicine and colcemid, significantly stimulated leucine transport. On the other hand, taxol, a microtubule stabilizing agent, decreased the stimulatory effect of colchicine on the leucine transport. These results strongly suggest the involvement of Ca2+ and calmodulin in regulation of Na+-independent leucine transport, possibly through control of assembly and disassembly of the microtubule network.     

Regulation of Ca2+ fluxes in rat liver mitochondria by Ca2+. Effects on Ca2+ distribution

Rat liver mitochondria are able to temporarily lower the steady-state concentration of external Ca²⁺ after having accumulated a pulse of added Ca²⁺. This has been attributed to inhibition of a putative -modulated efflux pathway [Bernardi, P. (1984)Biochim. Biophys. Acta 766, 277–282]. On the other hand, the rebounding could be due to stimulation of the uniporter by Ca²⁺ [Kröner, H. (1987)Biol. Chem. Hoppe-Seyler 369, 149–155]. By measuring unidirectional Ca²⁺ fluxes, it was found that the uniporter was stimulated during the rebounding peak both under Bernardi's and Kröner's conditions, while no effects on the efflux could be demonstrated. The rate of unidirectional efflux of Ca²⁺ was not affected by inhibition of the uniporter. It appears likely that the rebounding is due to stimulation of the uniporter rather than inhibition of efflux.     

Noradrenaline, vasopressin and angiotensin increase Ca2+ influx by opening a common pool of Ca2+ channels in isolated rat liver cells.

The effects of the Ca2+-mobilizing hormones noradrenaline, vasopressin and angiotensin on the unidirectional influx of Ca2+ were investigated in isolated rat liver cells by measuring the initial rate of 45Ca2+ uptake. The three hormones increased Ca2+ influx, with EC50 values (concentrations giving half-maximal effect) of 0.15 microM, 0.44 nM and 0.8 nM for noradrenaline, vasopressin and angiotensin respectively. The actions of noradrenaline and angiotensin were evident within seconds after their addition to the cells, whereas the increase in Ca2+ influx initiated by vasopressin was slightly delayed (by 5-15s). The activation of Ca2+ influx was maintained as long as the receptor was occupied by the hormone. The measurement of the resting and hormone-stimulated Ca2+ influxes at different external Ca2+ concentrations revealed Michaelis-Menten-type kinetics compatible with a saturable channel model. Noradrenaline, vasopressin and angiotensin increased both Km and Vmax. of Ca2+ influx. It is proposed that the hormones increase the rate of translocation of Ca2+ through a common pool of Ca2+ channels without changing the number of available channels or their affinity for Ca2+.