The role of caffeine-sensitive Ca2+ stores in agonist- and inositol 1,4,5-trisphosphate-induced Ca2+ release from bovine adrenal chromaffin cells

Spatial heterogeneity of caffeine- and inositol 1,4,5-trisphosphate-induced Ca2+ transients in isolated snail neurons

Inositol 1,4,5-trisphosphate- and caffeine-induced Ca2+ release was examined in neurons isolated from the mollusc Helix pomatia using Ca2+ indicator fura-2 and fluorescent digital-imaging microscopy technique. Extracellular application of caffeine caused a fast and pronounced augmentation of [Ca2+]i whose amplitude and kinetics differ in the centre of the cell and near its membrane. Mean values of caffeine-induced increase of [Ca2+]i were 0.97 +/- 0.11 microM at the periphery and 0.53 +/- 0.13 microM in the centre. The rates of rise and relaxation of caffeine-evoked [Ca2+]i transients were faster near the membrane. Pressure injection of inositol, 1,4,5-trisphosphate into the same neurons produced an abrupt and significant increase of [Ca2+]i in the centre (mean value of inositol 1,4,5-trisphosphate-induced elevation = 0.55 +/- 0.11 microM) while the response was smaller or even absent near the cellular membrane. Inositol 1,4,5-trisphosphate- and caffeine-induced Ca2+ transients did not affect each other. The data obtained indicate that in snail neurons these two calcium pools are not overlapping and at least some part of the caffeine-sensitive store is located close to the cellular membrane and that the inositol 1,4,5-trisphosphate-sensitive one is located in the centre of the cell.

Ca2+ influx induced by the Ca(2+)-ATPase inhibitors 2,5-di-(t-butyl)-1,4-benzohydroquinone and thapsigargin in bovine adrenal chromaffin cells

We have characterized the effect of the Ca(2+)-ATPase inhibitors 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBHQ) and thapsigargin on the concentration of cytosolic Ca2+ in single bovine adrenal chromaffin cells by video-imaging of fura-2-loaded cells. Addition of either inhibitor released Ca2+ from internal stores in the absence of external Ca2+. tBHQ was unable to stimulate further Ca2+ release after addition of thapsigargin, but thapsigargin could do so after release by tBHQ, indicating that the tBHQ-sensitive stores are a sub-set of those sensitive to thapsigargin. Angiotensin II was able to elicit Ca2+ release after application of tBHQ, indicating that at least part of the tBHQ-sensitive stores were distinct from those discharged by Ins(1,4,5)P3. In the presence of external Ca2+, both Ca(2+)-ATPase inhibitors produced a more prolonged rise in cytosolic Ca2+ consistent with stimulated Ca2+ entry. The ability of the inhibitors to activate a Ca(2+)-entry pathway was confirmed by monitoring quenching of fura-2 after stimulated entry of the Ca2+ surrogate Mn2+. These findings indicate that bovine adrenal chromaffin cells possess a mechanism by which Ca2+ entry can be activated, following emptying of certain internal stores, independently of receptor occupation.

Simultaneous presence of two distinct endoplasmic-reticulum-type calcium-pump isoforms in human cells. Characterization by radio-immunoblotting and inhibition by 2,5-di-(t-butyl)-1,4-benzohydroquinone

Phosphorylation, immunoblotting, limited proteolysis and drug-sensitivity analysis were used to characterize the sarcoendoplasmic-reticulum Ca2+ ATPases in a variety of human cell types. In platelets, several megakaryoblastoid and lymphoblastoid cell lines two distinct autophosphorylated forms of these ATPases with molecular mass of 100 and 97 kDa could be observed, whereas in several other cell types the 97 kDa form was absent. On immunoblots the 97 kDa species was specifically recognized by an inhibitory monoclonal antibody raised against the Ca2+ pump of platelet internal membranes, yielded on trypsinolysis a major fragment of 80 kDa, exhibited a distinct electrophoretic migration pattern as compared with the skeletal-, cardiac- and smooth-muscle Ca2+ pumps, and its autophosphorylation was strongly inhibited by the Ca(2+)-mobilizing agent 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBHQ). The 100 kDa species reacted with an antibody specific for the cardiac- and smooth-muscle Ca2+ pumps, yielded on trypsinolysis fragments of 55 and 35 kDa, and its autophosphorylation was much less sensitive to tBHQ inhibition. These findings indicate the simultaneous presence of two different endoplasmic-reticulum Ca2+ pumps in a variety of human cell types, and may explain the previously observed differences in the Ca(2+)-handling characteristics of different intracellular Ca2+ pools and cell types.

Effect of thapsigargin and caffeine on Ca2+ homeostasis in HeLa cells: implications for histamine-induced Ca2+ oscillations

Several studies have already established that the stimulation of H1 receptors by exogenous histamine induces intracellular Ca2+ oscillations in HeLa cells. The molecular mechanism underlying this oscillatory process remains, however, unclear. A series of fura-2 experiments was undertaken in which the nature of the Ca2+ pools involved in the histamine-induced Ca2+ oscillations was investigated using the tumour promoter agent thapsigargin (TG) and the Ca(2+)-induced Ca(2+)-release promoter, caffeine. The results obtained indicate first that TG causes a gradual increase in cytosolic Ca2+ without inducing internal Ca2+ oscillations, and second that TG and histamine share common internal Ca2+ storage sites. The latter conclusion was derived from experiments performed in the absence of external Ca2+, where the addition of TG before histamine resulted in a total inhibition of the Ca2+ response linked to H1 receptor stimulation, whereas the addition of histamine before TG decreased by more than 90% the TG-induced Ca2+ release. Finally; TG was found to inhibit irreversibly histamine-induced Ca2+ oscillations when added to the bathing medium during the oscillatory process. The effect of caffeine at concentrations ranging from 1 mM to 10 mM on intracellular Ca2+ homeostasis was also investigated. The results obtained show that caffeine does not affect systematically the internal Ca2+ concentration in resting and TG-stimulated HeLa cells, but increases the Ca2+ sequestration ability of inositol-trisphosphate (InsP3)-related Ca2+ stores.(ABSTRACT TRUNCATED AT 250 WORDS)

Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects

Caffeine is the most widely consumed central-nervous-system stimulant. Three main mechanisms of action of caffeine on the central nervous system have been described. Mobilization of intracellular calcium and inhibition of specific phosphodiesterases only occur at high non-physiological concentrations of caffeine. The only likely mechanism of action of the methylxanthine is the antagonism at the level of adenosine receptors. Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, inducing a relative brain hypoperfusion. Caffeine activates noradrenaline neurons and seems to affect the local release of dopamine. Many of the alerting effects of caffeine may be related to the action of the methylxanthine on serotonin neurons. The methylxanthine induces dose-response increases in locomotor activity in animals. Its psychostimulant action on man is, however, often subtle and not very easy to detect. The effects of caffeine on learning, memory, performance and coordination are rather related to the methylxanthine action on arousal, vigilance and fatigue. Caffeine exerts obvious effects on anxiety and sleep which vary according to individual sensitivity to the methylxanthine. However, children in general do not appear more sensitive to methylxanthine effects than adults. The central nervous system does not seem to develop a great tolerance to the effects of caffeine although dependence and withdrawal symptoms are reported.

Calcium signalling and the triggering of secretion in adrenal chromaffin cells

The pivotal intracellular message for triggering catecholamine release from bovine adrenal chromaffin cells is an elevation in the concentration of cytosolic free Ca2+ ([Ca2+]i). Studies using video-imaging techniques have shown that a rise in [Ca2+]i at the cell periphery, that is due to Ca2+ entry, is the major activating signal for exocytosis. The cytoskeleton has been identified as a major regulatory site of exocytosis, with Ca(2+)-induced disruption of the cortical actin network being required in order that previously restrained granules may have access to their exocytotic sites. The Ca(2+)- and phospholipid-dependent annexin protein, calpactin, has been strongly implicated in a late stage of interaction between granules and the plasma membrane by both ultrastructural and biochemical studies.