RESUMEN
The relationship between histamine-induced Ca2+ mobilization and Ca2+ entry in bovine adrenal chromaffin cells has been investigated. Stopped-flow fluorimetry of fura-2-loaded chromaffin cell populations revealed that 10 microM histamine promoted entry of Ca2+ or Mn2+ without measurable delay (< or = 20 ms), through a pathway that was insensitive to the dihydropyridine antagonist nifedipine. In the absence of extracellular Ca2+, or in the presence of 100 microM La3+, a blocker of receptor-mediated Ca2+ entry, 10 microM histamine triggered an elevation in intracellular calcium concentration ([Ca2+]i), but only after a delay of approx. 200 ms, which presumably represented the time required to mobilize intracellular Ca2+. These data suggested that histamine-induced bivalent-cation entry precedes extensive Ca2+ mobilization in chromaffin cells. In order to confirm that histamine can promote Ca2+ entry largely independently of mobilizing intracellular Ca2+, the ability of histamine to promote Ca2+ entry into cells whose intracellular Ca2+ store had been largely depleted was assessed. Fura-2-loaded chromaffin cells were treated with 10 microM ryanodine together with 40 mM caffeine, to deplete the hormone-sensitive Ca2+ store. This resulted in an approx. 95% inhibition of histamine-induced Ca2+ release. Under these conditions, histamine was still able to promote an entry of Ca2+ that was essentially indistinguishable from that promoted in control cells. In single cells, introduction of heparin (100 mg/ml), but not de-N-sulphated heparin (100 mg/ml), abolished the histamine-induced rise in [Ca2+]i. All these data suggest that histamine can induce G-protein- or inositol phosphate-dependent rapid (< or = 20 ms) Ca2+ entry without an extensive intracellular mobilization response in chromaffin cells, which points to activation of an entry mechanism distinct from the Ca(2+)-release-activated Ca2+ channel found in non-excitable cells.
Asunto(s)
Glándulas Suprarrenales/metabolismo , Calcio/metabolismo , Sistema Cromafín/metabolismo , Histamina/farmacología , Glándulas Suprarrenales/efectos de los fármacos , Animales , Cationes Bivalentes , Bovinos , Sistema Cromafín/efectos de los fármacos , Ácido Egtácico/farmacología , Fura-2 , Proteínas de Unión al GTP/fisiología , Fosfatos de Inositol/farmacología , Cinética , Lantano/farmacología , Manganeso/metabolismo , Potasio/farmacología , Espectrometría de FluorescenciaRESUMEN
Low caffeine concentrations were unable to completely release the caffeine- and ryanodine-sensitive intracellular Ca2+ pool in intact adrenal chromaffin cells. This 'quantal' Ca2+ release is the same as that previously observed with inositol Ins(1,4,5)P3-induced Ca2+ release. The molecular mechanism underlying quantal Ca2+ release from the ryanodine receptor was investigated using fura-2 imaging of single chromaffin cells. Our data indicate that the intracellular caffeine-sensitive Ca2+ pool is composed of functionally discrete stores, that possess heterogeneous sensitivities to caffeine. These stores are mobilized by caffeine in a concentration-dependent fashion, and, when stimulated, individual stores release their Ca2+ in an 'all-or-none' manner. Such quantal Ca2+ release may be responsible for graded Ca2+ responses in single cells.
Asunto(s)
Médula Suprarrenal/metabolismo , Canales de Calcio/metabolismo , Calcio/metabolismo , Sistema Cromafín/metabolismo , Proteínas Musculares/metabolismo , Médula Suprarrenal/efectos de los fármacos , Animales , Cafeína/administración & dosificación , Cafeína/farmacología , Bovinos , Sistema Cromafín/efectos de los fármacos , Colorantes Fluorescentes , Fura-2 , Inositol 1,4,5-Trifosfato/farmacología , Rianodina/farmacología , Canal Liberador de Calcio Receptor de RianodinaRESUMEN
In populations of fura-2-loaded chromaffin cells, caffeine caused a concentration-dependent increase in the intracellular Ca2+ concentration ([Ca2+]i), in the presence or absence of external Ca2+ ([Ca2+]o), that was saturable, reversible, and inhibited in a use-dependent fashion by ryanodine. These data confirm that caffeine mobilizes Ca2+ from the ryanodine-sensitive intracellular stores in chromaffin cells. In nominally Ca(2+)-free medium, sustained stimulation of cell populations or single cells with low caffeine concentrations failed to completely empty the caffeine-sensitive stores. In each case, there was a transient [Ca2+]i elevation, but a subsequent challenge with a higher caffeine concentration evoked a further [Ca2+]i rise, indicating that Ca2+ stores within individual cells were heterogeneous in their sensitivities to caffeine and that caffeine-induced Ca2+ release was quantal. The heterogeneous sensitivity was also demonstrated using ryanodine; pretreatment of cell populations with increasing caffeine concentrations with a constant ryanodine concentration, caused a dose-dependent irreversible inhibition of the response to the subsequent addition of a maximal caffeine concentration. We conclude that, within single chromaffin cells, intracellular Ca2+ stores are heterogeneous in their sensitivity to caffeine and the fraction of Ca2+ stores mobilized by caffeine increases in direct proportion to the caffeine concentration.
Asunto(s)
Médula Suprarrenal/metabolismo , Cafeína/farmacología , Calcio/metabolismo , Médula Suprarrenal/citología , Médula Suprarrenal/efectos de los fármacos , Animales , Bovinos , Células Cultivadas , Gránulos Cromafines , Histamina/farmacología , Rianodina/farmacologíaRESUMEN
The role of a Ca(2+)-induced Ca2+ release (CICR) mechanism in the generation of agonist-induced increases of intracellular free Ca2+ concentration ([Ca2+]i) was studied in bovine adrenal chromaffin cells. In single cells, repetitive stimulations with caffeine at 200-s intervals evoked reproducible spikes of [Ca2+]i. Ryanodine, an agent that interacts with the CICR channel of muscle, inhibited the caffeine-induced spikes of [Ca2+]i in a "use-dependent" way. High affinity binding sites for [3H]ryanodine (Kd 3.3 nM, Bmax 26 fmol/mg protein) were also detected in membranes from chromaffin cells, supporting the presence of a caffeine- and ryanodine-sensitive CICR channel. Pretreatment of single cells with caffeine + ryanodine to reduce the size of the caffeine-sensitive Ca2+ compartment inhibited a subsequent spike of [Ca2+]i evoked by histamine, a D-myo-inositol 1,4,5-trisphosphate-forming agonist. This demonstrates that a significant portion of the Ca2+ released by histamine comes from a caffeine- and ryanodine-sensitive pool. Ryanodine inhibited by 50% the size of [Ca2+]i spikes evoked by repetitive stimulation with histamine and did so in a use-dependent manner. These data suggest that, in addition to D-myoinositol 1,4,5-trisphosphate, activation of a caffeine- and ryanodine-sensitive CICR channel participates in the generation of histamine-induced release of intracellular Ca2+.
Asunto(s)
Gránulos Cromafines/efectos de los fármacos , Histamina/metabolismo , Inositol 1,4,5-Trifosfato/metabolismo , Rianodina/farmacología , Potenciales de Acción , Animales , Sitios de Unión , Cafeína/farmacología , Calcio/metabolismo , Bovinos , Gránulos Cromafines/metabolismo , Gránulos Cromafines/fisiologíaRESUMEN
In single bovine adrenal chromaffin cells loaded with fura-2, histamine, angiotensin II (AII) and caffeine elicited large transient increases of intracellular free Ca2+ concentration [( Ca2+]i) in the absence of external Ca2+, with peak amplitudes averaging 726 +/- 138 (n = 14), 710 +/- 102 (n = 21) and 830 +/- 100 nM (n = 30) respectively. A substantial portion of the agonist-induced rise in [Ca2+]i depended on Ca2+ release from caffeine-sensitive stores, as pretreatment with caffeine diminished subsequent agonist responses by 90-95%. Conversely, pretreatment with histamine or AII decreased subsequent caffeine responses by 100% and 90% respectively. The effects of caffeine most likely resulted from activation of a Ca(2+)-induced Ca(2+)-release (CICR) process, whereas histamine and AII initially acted through generation of Ins(1,4,5)P3. The relationship of Ins(1,4,5)P3- and caffeine-sensitive Ca2+ pools was studied by using alpha-toxin-permeabilized chromaffin cells. Evidence was found for three non-mitochondrial, ATP-dependent, Ca2+ pools: one exclusively sensitive to Ins(1,4,5)P3 (pool 1), a second sensitive to both Ins(1,4,5)P3 and caffeine (pool 2), and a third exclusively sensitive to caffeine (pool 3). The existence of pools 1 and 3, and the ability of agonists such as histamine to discharge pool 3 completely, supports a two-pool model in which a caffeine-sensitive CICR mechanism plays a major role in the generation of agonist-induced Ca2+ spikes in bovine chromaffin cells.
Asunto(s)
Glándulas Suprarrenales/metabolismo , Cafeína/farmacología , Calcio/metabolismo , Sistema Cromafín/metabolismo , Inositol 1,4,5-Trifosfato/farmacología , Glándulas Suprarrenales/efectos de los fármacos , Angiotensina II/administración & dosificación , Angiotensina II/farmacología , Animales , Cafeína/administración & dosificación , Calcio/farmacología , Bovinos , Permeabilidad de la Membrana Celular , Sistema Cromafín/efectos de los fármacos , Colorantes Fluorescentes , Fura-2 , Histamina/administración & dosificación , Histamina/farmacología , Fosfatos de Inositol/metabolismo , Rianodina/farmacologíaRESUMEN
The patterns of agonist-induced elevations of cytosolic free Ca2+ ([Ca2+]i) were characterized and compared by the use of single adrenal chromaffin cells. Initial histamine- or angiotensin II (AII)-induced elevations of [Ca2+]i were equal in magnitude (peaks 329 +/- 20 [SE] and 338 +/- 46 nM, respectively). These initial increases of [Ca2+]i were transient, insensitive to either Gd3+ or removing external Ca2+, and were primarily the result of Ca2+ release from intracellular stores. After the initial peak(s) of [Ca2+]i, a second phase of moderately elevated [Ca2+]i was observed, and this response was sensitive to either Gd3+ or removing external Ca2+, supporting a role for Ca2+ entry. In most cases, the second phase of elevated [Ca2+]i was sustained during histamine stimulation but transient during AII stimulation. Maintenance of the second phase was a property of the agonist rather than of the particular cell being stimulated. Thus, individual cells exposed sequentially to histamine and AII displayed distinct patterns of [Ca2+]i changes to each agonist, regardless of the order of addition. Histamine also stimulated twice as much [3H]catecholamine release as AII, and release was completely dependent on external Ca2+. Therefore, the ability of histamine and AII to sustain (or promote) Ca2+ entry appears to underlie their efficacy as secretagogues. These data provide evidence linking agonist-dependent patterns of [Ca2+]i changes in single cells with agonist-dependent functional responses.
Asunto(s)
Médula Suprarrenal/metabolismo , Calcio/metabolismo , Gránulos Cromafines/metabolismo , Norepinefrina/metabolismo , Médula Suprarrenal/citología , Médula Suprarrenal/efectos de los fármacos , Angiotensina II/farmacología , Animales , Tampones (Química) , Bovinos , Células Cultivadas , Gránulos Cromafines/efectos de los fármacos , Citosol/metabolismo , Conductividad Eléctrica , Histamina/farmacologíaRESUMEN
Under conditions minimizing the contribution of Na+/Ca2+ exchange to calcium entry in synaptosomes, the K+ depolarization-dependent calcium influx (JCa) is a single exponential function of time. JCa activates and slowly inactivates at membrane potentials positive to -50 mV, a result indicating the involvement of moderate voltage-activating, slowly inactivating calcium channels. Calcium channels in synaptosomes are characterized by stronger sensitivity to blockage by Cd2+ than Co2+, insensitivity to dihydropyridine calcium antagonists or the agonist Bay K 8644, and weak, partial sensitivity to the peptide toxin omega-conotoxin GVIA. These characteristics suggest that voltage-sensitive calcium channels in rat cerebrocortical synaptosomes are dissimilar from the somatic T, N, or L channel types. JCa is not affected by treatment of synaptosomes with the adenylate cyclase activator forskolin, the membrane permeant dibutyryl-cyclic AMP, or the kinase C activator phorbol 12-myristate 13-acetate diester, results suggesting that calcium channels in synaptosomes are not directly modulated by protein kinase A- or C-mediated phosphorylation.
Asunto(s)
Canales de Calcio/clasificación , Calcio/metabolismo , Corteza Cerebral/metabolismo , Terminaciones Nerviosas/metabolismo , Sinaptosomas/metabolismo , Animales , Agonistas de los Canales de Calcio/farmacología , Bloqueadores de los Canales de Calcio/farmacología , Canales de Calcio/efectos de los fármacos , Canales de Calcio/fisiología , Corteza Cerebral/ultraestructura , Electrofisiología , Cinética , Venenos de Moluscos/farmacología , Ratas , Ratas Endogámicas , omega-Conotoxina GVIARESUMEN
The inhibition of K+-depolarization dependent Ca influx by omega-conotoxin GVIA was compared in the frog, chick, and rat brain synaptosomes. The toxin at concentrations greater than or equal to 0.3 microM completely inhibited Ca entry in the frog and chick preparations, but was only partly effective in blocking Ca influx in the rat brain synaptosomes. In chick synaptosomes the toxin's effect was biphasic: a small component (approximately equal to 15%) of total Ca influx was inhibited by the toxin with high affinity (I50 less than 0.002 microM); a major component (approximately equal to 80%) of Ca influx was inhibited with a moderate affinity (I50 approximately equal to 0.05 microM). In rat brain synaptosomes 40% of Ca influx was inhibited by the toxin with low affinity (I50 approximately equal to 0.3 microM), and 60% of Ca influx was unaffected by the toxin concentration of up to 10 microM. These data suggest a heterogeneity of voltage-sensitive Ca channels in vertebrate brain synaptosomes.
Asunto(s)
Calcio/metabolismo , Canales Iónicos/metabolismo , Venenos de Moluscos/farmacología , Sinapsis/metabolismo , Sinaptosomas/metabolismo , Animales , Encéfalo/metabolismo , Radioisótopos de Calcio , Pollos , Canales Iónicos/efectos de los fármacos , Cinética , Potasio/farmacología , Rana pipiens , Ratas , Ratas Endogámicas , Especificidad de la Especie , Sinapsis/efectos de los fármacos , Sinaptosomas/efectos de los fármacos , omega-Conotoxina GVIARESUMEN
Fast-mixing and rapid-filtration techniques were used to analyze the kinetics of potassium-depolarization-dependent (delta K+ = 47.5 mM) influx of 45Ca into synaptosomes, in the time range from 50 msec to 5 sec. The results are consistent with the presence in synaptosomes of a homogeneous population of voltage-sensitive Ca channels. With 1 mM Cao in the medium, the delta K+-dependent Ca influx has a single-exponential time course with the half-life, t1/2 approximately 0.5-0.7 sec. Ca influx, measured between 0.1 and 10 mM Cao, shows half-saturation (KCa) at 1.5 mM Cao and has the limiting value (JCamax) of 5.9 nmol/sec/mg protein, or a current of approximately 0.06 pA/micron2 surface area. The estimated density of functional Ca channels is 0.6-6 micron-2. Voltage- and time-dependent inactivation of Ca channels was measured in synaptosomes predepolarized in 52.5 mM Ko+ with Ca omitted from the medium. Channel inactivation is a single-exponential process with a half-life of t1/2 approximately 2.3 sec. Channel recovery in 5 mM Ko+ media is likewise a single-exponential process with a half-life of t1/2 approximately 4.3 sec. The slower rate of voltage-dependent channel inactivation than of decay of Ca influx suggests that Ca entry into synaptosomes terminates by a mechanism that depends on Ca influx itself. Synaptosomes contain 200 fmol/mg protein, or approximately 6 micron-2 high-affinity (KD = 0.12 nM) 3H-nitrendipine binding sites; however, nitrendipine at concentrations greater than 10(4) X KD is without effect on the phasic influx of Ca measured at 215 msec with either 1.0 or 0.1 mM Cao. This suggests that Ca channels characterized in this study belong to a class of dihydropyridine-insensitive channels.