Possible regulation of caffeine-induced intracellular Ca2+ mobilization by intracellular free Na+

Caffeine stimulates Ca(2+) entry through store-operated channels to activate tyrosine hydroxylase in bovine chromaffin cells

The ability of caffeine-induced store Ca(2+) mobilization to activate tyrosine hydroxylase was studied in bovine adrenal chromaffin cells. Caffeine increased tyrosine hydroxylase activity over 10 min with an EC(50) of 3 mm and maximum effect at 20 mm. The maximum response to caffeine was substantial, being almost one third that of the strongest agonists acetylcholine and PACAP-27, about half that for K(+) and similar to that for histamine. In contrast, catecholamine secretion evoked by caffeine was small, being less than 10% of the response to strong agonists. Caffeine-induced tyrosine hydroxylase activation was not mimicked or prevented by phosphodiesterase inhibition with isobutylmethylxanthine, nor was it mimicked by an equimolar concentration of sucrose. However, the effect of caffeine was prevented by depleting intracellular Ca(2+) stores by thapsigargin pretreatment, and reduced substantially by removing extracellular Ca(2+), by blocking Ca(2+) channels with Co(2+) or Ni(2+), or by inhibiting store-operated channels with 2-aminoethyl diphenylborate. It was not affected by inhibiting Ca(2+) entry through voltage-operated Ca(2+)-channels or by tetrodotoxin. The effect of caffeine was mimicked by acute thapsigargin treatment or by depleting intracellular Ca(2+) stores in Ca(2+)-free buffer and then reintroducing extracellular Ca(2+). The results indicate that mobilizing store Ca(2+) with caffeine is a very effective mechanism for activating tyrosine hydroxylase and that the majority of this response depends on extracellular Ca(2+) entry through store-operated channels. They also suggest that extracellular Ca(2+) entry through such channels regulates cellular responses differently to Ca(2+) entry through voltage-operated Ca(2+) channels.

T-477, a novel Ca(2+)- and Na(+) channel blocker, prevents veratridine-induced neuronal injury

To evaluate the effect of (R)-(+)-2-(4-chlorophenyl)-2, 3-dihydro-4-diethyl aminoacetyl-4H-1,4-benzothiazine hydrochloride (T-477), a novel Na(+)- and Ca(2+) channel blocker, on neuronal injury in vitro, we studied veratridine-induced injury in cultured rat hippocampal neurons. Neurons swelled extensively 10 min after the addition of veratridine, and returned to their initial size within 2 h. Intracellular Na(+) and Ca(2+) concentrations and amino acid release from the cells, in particular, that of glutamate, increased after the treatment with veratridine. Approximately 70% of neurons died within 24 h. T-477 inhibited both veratridine-induced swelling and death in a concentration-dependent manner. Moreover, T-477 concentration dependently reduced the increases in Na(+) and Ca(2+) influx and amino acid release. These results suggest that T-477 prevented the veratridine-induced influx of Na(+) and, thereby, reduced neuronal swelling. This, combined with the effects of T-477 on the inhibition of Ca(2+) influx and glutamate release, possibly by the blockade of Na(+) channels, may be the mechanism by which T-477 protects neurons from death induced by veratridine.

Excitatory effect of Cd2+ on cat adrenal chromaffin cells

To understand the mechanism(s) underlying the Cd2+- and Co2+-induced increases in the cytosolic free Ca2+ concentration ([Ca]i) in cat adrenal chromaffin cells, we used nystatin-perforated patch recording method and fura-2 microfluorometry. Under the current-clamp conditions, the external application of 5x10(-7) M Cd2+ slowly depolarized the cells resulting in the bursting of action potentials. Under the voltage-clamp conditions, Cd2+ evoked a slow inward current accompanied by a decrease of K+ conductance at a holding potential of -40 mV, and Co2+ mimicked Cd2+ action. In some cells (16%), Cd2+ evoked an additional rapid transient outward current associated with an increased K+ conductance and a successive slow inward current. The Cd2+-induced inward current was activated in a concentration-dependent manner with a half-maximum concentration of 9.3x10(-8) M. The Cd2+- and Co2+-induced [Ca]i increases measured with fura-2 microfluorometry were maximal at 10(-6) and 10(-5) M, respectively, and the higher concentrations of both cations caused the smaller responses. Additional transient increase in [Ca]i was often evoked upon the removal of relatively higher concentrations of these metals. It was concluded that the Cd2+-induced membrane depolarization due to the decrease in K+ conductances evoked the bursting firings resulting in the increase in [Ca]i, and consequently might stimulate the catecholamine secretion.

Functional characterization of thapsigargin and agonist-insensitive acidic Ca2+ stores in Drosophila melanogaster S2 cell lines

The role of acidic intracellular calcium stores in calcium homeostasis was investigated in the Drosophila Schneider cell line 2 (S2) by means of free cytosolic calcium ([Ca2+]i) and intracellular pH (pHi) imaging together with measurements of total calcium concentrations within intracellular compartments. Both a weak base (NH4Cl, 15 mM) and a Na+/H+ ionophore (monensin, 10 microM) evoked cytosolic alkalinization followed by Ca2+ release from acidic intracellular Ca2+ stores. Pretreatment of S2 cells with either thapsigargin (1 microM), an inhibitor of endoplasmic reticulum Ca(2+)-ATPases, or with the Ca2+ ionophore ionomycin (10 microM) was without effect on the amplitude of Ca2+ release evoked by alkalinization. Application of the cholinergic agonist carbamylcholine (100 microM) to transfected S2-DM1 cells expressing a Drosophila muscarinic acetylcholine receptor (DM1) emptied the InsP3-sensitive Ca2+ store but failed to affect the amplitude of alkalinization-evoked Ca2+ release. Glycyl-L-phenylalanine-beta-naphthylamide (200 microM), a weak hydrophobic base known to permeabilize lysosomes by osmotic swelling, triggered Ca2+ release from internal stores, while application of brefeldin A (10 microM), an antibiotic which disperses the Golgi complex, resulted in a smaller increase in [Ca2+]i. These results suggest that the alkali-evoked calcium release is largely attributable to lysosomes, a conclusion that was confirmed by direct measurements of total calcium content of S2 organelles. Lysosomes and endoplasmic reticulum were the only organelles found to have concentrations of total calcium significantly higher than the cytosol. However, NH4Cl (15 mM) reduced the level of total calcium only in lysosomes. Depletion of acidic Ca2+ stores did not elicit depletion-operated Ca2+ entry. They were refilled upon re-exposure of cells to normal saline ([Ca2+]o = 2 mM), but not by thapsigargin-induced [Ca2+]i elevation in Ca(2+)-free saline.

Cd2+ and Co2+ at micromolar concentrations mobilize intracellular Ca2+ via the generation of inositol 1,4,5-triphosphate in bovine chromaffin cells

To understand the mechanisms underlying the Cd2+- and Co2+-induced intracellular Ca2+ mobilization, we measured the levels of inositol phosphates using bovine chromaffin cells. Studies using HPLC indicated that Cd2+, Co2+ and methacholine significantly increased the generation of 1,4,5-IP3. The results suggest that Cd2+ and Co2+ mobilize Ca2+ from IP3-sensitive Ca2+ stores, possibly through the presumptive Cd2+ receptor.

Measurement of intracellular Na+ concentration by a Na+-sensitive fluorescent dye, sodium-binding benzofuran isophthalate, in porcine adrenal chromaffin cells--usage of palytoxin as a Na+ ionophore

Palytoxin was found to equilibrate sodium ions (Na+) across the cell membrane much faster than dose gramicidin, which has been frequently used to calibrate the intracellular Na+ concentration ([Na+]in), in cells treated with a Na+-sensitive fluorescent dye, sodium-binding benzofuran isophthalate (SBFI). Palytoxin was capable of equilibrating Na+ in cells treated with SBFI-acetoxymethyl ester (SBFI-AM) and in voltage-clamped cells loaded with SBFI through a patch pipette. Nicotine caused a dose-dependent increase in ([Na+]in) in porcine adrenal chromaffin cells treated with SBFI-AM and caused a simultaneous increase in [Na+]in and inward current in the voltage-clamped cells loaded with SBFI. Palytoxin has an advantage of calibrating ([Na+]in) in a shorter time than dose gramicidin because of its powerful ionophoretic activity.

Role of [Na+]i and [Ca2+]i in nicotine-induced norepinephrine release from bovine adrenal chromaffin cells

Intracellular free sodium ([Na+]i) and calcium ([Ca2+]i) concentrations were determined by sodium-binding benzofuran isophthalate (SBFI) and fura 2 microfluorimetry, respectively, in bovine adrenal chromaffin cells (BCC). Validation of SBFI microfluorimetry by in vitro and in vivo calibration revealed a reliable assessment of [Na+]i within a range of 1-30 mM in single BCC. Nicotine (0.1-10 microM) induced concentration-dependent increases of both [Na+]i (from 3.3 +/- 0.1 to 25.6 +/- 0.4 mM, n = 76, P < 0.001) and [Ca2+]i (from 64 +/- 1 to 467 +/- 16 nM, n = 87, P < 0.001), which were accompanied by an increase in [3H]norepinephrine (NE) release. Consistent with an exocytotic release mechanism, nicotine-induced increments of [Ca2+]i and [3H]NE release were reduced under calcium-free conditions and by gadolinium chloride (40 microM), whereas [Na+]i was not affected. In contrast, a parallel attenuation of nicotine-evoked changes in [Na+]i, [Ca2+]i, and [3H]NE release was observed during reduction of the extracellular sodium concentration. The nicotine-evoked responses were neutralized by the nicotinic receptor antagonist hexamethonium (100 microM) but not by blockade of voltage-dependent sodium channels (1 microM tetrodotoxin). In conclusion, the nicotine-induced exocytotic release of [3H]NE is triggered by an increase in [Ca2+]i, which is facilitated by sodium influx through the nicotinic receptor ionophore.

Cd2+ and Co2+ at micromolar concentrations stimulate catecholamine secretion by increasing the cytosolic free Ca2+ concentration in cat adrenal chromaffin cells

Cd2+ and Co2+ at micromolar concentrations increased the cytosolic free Ca2+ concentration, which was measured by fura-2 microfluorometry, in cat adrenal chromaffin cells. Simultaneously, these divalent cations stimulated catecholamine secretion from the perfused adrenal. The present findings suggest that these cations increase the Ca2+ influx by depolarizing the cell membrane and consequently stimulate catecholamine secretion.