Flash photolysis studies of the localization of calcium release sites in rat parotid isolated acinar cells

Cytosolic Ca(2+) and Ca(2+)-activated Cl(-) current dynamics: insights from two functionally distinct mouse exocrine cells

The dynamics of Ca(2+) release and Ca(2+)-activated Cl(-) currents in two related, but functionally distinct exocrine cells, were studied to gain insight into how the molecular specialization of Ca(2+) signalling machinery are utilized to produce different physiological endpoints: in this case, fluid or exocytotic secretion. Digital imaging and patch-clamp methods were used to monitor the temporal and spatial properties of changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) and Cl(-) currents following the controlled photolytic release of caged-InsP(3) or caged-Ca(2+). In parotid and pancreatic acinar cells, changes in [Ca(2+)](c) and activation of a Ca(2+)-activated Cl(-) current occurred with close temporal coincidence. In parotid, a rapid global Ca(2+) signal was invariably induced, even with low-level photolytic release of threshold amounts of InsP(3). In pancreas, threshold stimulation generated an apically delimited [Ca(2+)](c) signal, while a stronger stimulus induced a global [Ca(2+)](c) signal which exhibited characteristics of a propagating wave. InsP(3) was more effective in parotid, where [Ca(2+)](c) signals initiated with shorter latency and exhibited a faster time-to-peak than in pancreas. The increased potency of InsP(3) in parotid probably results from a four-fold higher number of InsP(3) receptors as measured by radiolabelled InsP(3) binding and western blot analysis. The Ca(2+) sensitivity of the Cl(-) channels in parotid and pancreas was determined from the [Ca(2+)]-current relationship measured during a dynamic 'Ca(2+) ramp' produced by the continuous, low-level photolysis of caged-Ca(2+). In addition to a greater number of InsP(3) receptors, the Cl(-) current density of parotid acinar cells was more than four-fold greater than that of pancreatic cells. Whereas activation of the current was tightly coupled to increases in Ca(2+) in both cell types, local Ca(2+) clearance was found to contribute substantially to the deactivation of the current in parotid. These data reveal specializations of common modules of Ca(2+)-release machinery and subsequent effector activation that are specifically suited to the distinct functional roles of these two related cell types.

Intracellular Ca2+ and Cl- channel activation in secretory cells

Molecular and functional evidence indicates that a variety of Ca(2+)-dependent chloride (Cl(Ca)) channels are involved in fluid secretion from secretory epithelial cells in different tissues and species. Most Cl(Ca) channels so far characterized have an I- permeability greater than Cl-, and most are sensitive to 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). Whole-cell Cl(Ca) currents show outward rectification. Single-channel current voltage relationships are linear with conductances ranging from 2 to 30 pS. Some Cl(Ca) channels are blocked by Ca(2+)-calmodulin-dependent protein kinase (CAMKII) inhibitors. Others, such as the Cl(Ca) channels of parotid and submandibular acinar cells, appear to be directly regulated by Ca2+. In native cells, the Cl(Ca) channels are located on the apical plasma membrane and activated by localized mechanisms of Ca2+ release. This positioning allows the Cl(Ca) channel to respond specifically to localized Ca2+ signals that do not invade other regions of the cell. The Cl(Ca) follows the rising phase of the Ca2+ signal, but in the falling phase hysteresis occurs where the Cl(Ca) current decays more rapidly than the underlying Ca2+. The future elucidation of the identity and mechanisms of regulation of Cl(Ca) channels will be critical to our understanding of stimulus-secretion coupling.

Calcium homeostasis and ultrastructural studies in a patient with limb girdle muscular dystrophy type 2C

There is increasing evidence that gamma-sarcoglycan is absent and other sarcoglycans are reduced in patients with the limb-girdle muscular dystrophy type 2C (LGMD2C) form of severe childhood autosomal recessive muscular dystrophy. In the present investigation, we combined microspectrofluorimetry and electron microscopy techniques to investigate the physiological function and the ultrastructure of control and LGMD2C myotubes. Results obtained from Ca2+ measurements showed that the resting level of the cytosolic free calcium ([Ca2+ ]i ) in control myotubes was 73+/-3.4 nmol/l (mean+/-se, n=35) and in LGMD2C myotubes was 69+/-4 nmol/l (n=44). Carbachol (CCh, 10 micromol/l ) induced a 335+/-10 nmol/l (n=8) rise in [Ca2+ ]i in control myotubes and 531.9+/-32 nmol/l (n=23) in LGMD2C myotubes. Similarly, elevations of [Ca2+ ]i by 35 mmol/l K+ were 324+/-32 nmol/l (n=8) in control myotubes and 442.8+/-24 nmol/l (n=22) in LGMD2C myotubes. Caffeine (10 mmol/l) activated similar [Ca2+]i peaks in control and LGMD2C myotubes but induced a biphasic response in LGMD2C in four out of 12 myotubes and only a monophasic response in control myotubes. The ultrastructural results showed that the plasma membrane was abnormally indented and convoluted in both the LGMD2C biopsy and the LGMD2C cultured myotubes. It is suggested that the reduction in components of the dystrophin-glycoprotein complex results in the instability and an increase in the surface area of the plasma membrane, which may result in a higher population of Ca2+ channels in the LGMD2C myotubes.

Intracellular calcium signalling in rat parotid acinar cells that lack secretory vesicles

Secretory vesicles from pancreatic acinar cells have recently been shown to release Ca2+ after stimulation with Ins(1,4,5)P3 [Gerasimenko, Gerasimenko, Belan and Petersen, (1996) Cell 84, 473-480]. These observations have been used in support of the hypothesis that Ca2+ release from secretory vesicles could be an important component of stimulus secretion coupling in exocrine acinar cells. In the rat, ligation of the parotid duct causes a reversible atrophy of the parotid gland. Most notably, after atrophy the acinar cells are reduced in size and no longer contain secretory vesicles [Liu, Smith, and Scott (1996) J. Dent. Res. 74, 900]. We have measured cytosolic free-Ca2+ concentration ([Ca2+]i) in single, acutely isolated, rat parotid acinar cells, and compared Ca2+ mobilization in response to acetylcholine (ACh) stimulation in cells obtained from control animals to that in cells lacking secretory vesicles obtained after atrophy of the parotid gland. Application of 50-5000 nM ACh to control cells gave rise to a typical, dose-dependent, biphasic increase in [Ca2+]i, of which the later, plateau, phase was acutely dependent on the extracellular Ca2+ concentration. An identical pattern of response was observed with cells obtained from atrophic glands. Low concentrations of ACh (10-100 nM) occasionally produced [Ca2+]i oscillations of a similar pattern in cells from both control and atrophic glands. We were able to show that Ca2+ rises first in the apical pole of the cell and the increase then spreads to the rest of the cell in cells from control glands but not in cells from atrophic glands. However, at present we are unable to determine whether this is due to the lack of secretory vesicles or whether the separation is too small to measure in the smaller acinar cells obtained from atrophic glands. We conclude therefore, that secretory vesicles make no significant contribution to overall Ca2+ mobilization in rat parotid acinar cells, nor are they required for oscillatory changes in [Ca2+]i to occur. However we are unable to eliminate completely any role for secretory vesicles in initiating Ca2+ mobilization at the apical pole of the cell.

Aging-related changes in calcium oscillations in fertilized mouse oocytes

Aging of oocytes, being not fertilized after ovulation for a prolonged time, considerably affects normal development of the fertilized oocyte. We examined effects of the aging on a series of highly repetitive Ca2+ transients commonly seen in fertilized mouse oocytes (Ca2+ oscillations). Frequency of Ca2+ oscillations in the aged oocyte [20 hrs after induction of superovulation by i.p. human chorionic gonadotropin (hCG)] was significantly higher (34.1 +/- 5.8 l/hr) than the fresh oocyte (14 hr post-hCG, 21.8 +/- 7.9 l/hr). Rates of rise and fall of individual Ca2+ transient in the aged oocyte were significantly slower than the fresh oocyte, whereas durations of individual Ca2+ transients were similar. When extracellular Ca2+ was raised from 2.04 mM to 5.00 mM, aged oocytes showed significant prolongation of the duration of individual Ca2+ transient, that resulted in a sustained elevation of intracellular Ca2+ ([Ca2+]i) in 33% of the aged oocyte. Transient increase in [Ca2+]i by photolysis of a caged Ca2+, Nitr-5, injected into cytoplasm was completely restored in the fresh oocyte [fluorescence intensity of [Ca2+]i indicator dye Fluo-3 (F480) returned to 97 +/- 2% of the control level, time constant = 37 +/- 9 sec]. In contrast, in the aged oocyte, restoration of F480 following Nitr-5 photolysis was incomplete (115 +/- 12% of the control) and slow (time constant = 64 +/- 23 sec). Because inhibition of the Ca2+ pump of the endoplasmic reticulum (ER) by 5 microM thapsigargin almost completely inhibited restoration of F480 following Nitr-5 photolysis in the fresh oocyte, we conclude that the aging-related changes in Ca2+ oscillations may be accounted for by dysfunction of intracellular Ca2+ regulation, presumably of the Ca2+ pump of the ER.

Caged compounds: tools for illuminating neuronal responses and connections

A number of new 'caged' intracellular second messengers and neurotransmitters have been developed using the photolabile o-nitrobenzyl group. This chemistry has also recently been exploited in novel ways, including the development of caged enzyme substrates and caged proteins. Although caged compounds continue to be used primarily for mechanistic (kinetic) studies of processes mediated by transmitters or second messengers, the spatial resolution afforded by the use of light to effect changes in transmitter concentrations has now been clearly demonstrated. The increased availability of caged compounds and of the technologies required to exploit them provides neurobiologists with powerful tools for probing neuronal response properties and connectivity patterns.

The control of chloride conductance in rat parotid isolated acinar cells investigated by photorelease of caged compounds

The control of Cl- conductance in rat parotid isolated acinar cells was studied by combined use of whole-cell recording and flash photolysis techniques. Cells were voltage-clamped either at a membrane potential of -40 mV or stepped between -85 mV and 0 mV. Bath-applied carbachol and noradrenaline evoked Cl- current at -85 mV and K+ current at 0 mV. Similar current activations resulted from the photolytic release of either inositol trisphosphate (InsP3) or Ca2+ by a brief near-UV flash. The peak amplitudes of the Cl- conductance (at -85 mV), measured relative to the K+ conductance (at 0 mV), evoked by application of carbachol, noradrenaline or direct manipulation of cytosolic free calcium ([Ca2+]i), were very similar, being 0.56 +/- 0.09 (mean +/- SEM, n = 9), 0.52 +/- 0.01 (n = 7) and 0.46 +/- 0.06 (n = 7). In contrast, the relative amplitude of the Cl- conductance evoked by InsP3 was much larger: 1.49 +/- 0.24 (n = 9). Neither bath application of isoprenaline nor photolysis of "caged" cAMP induced any detectable membrane current. The most probable interpretation of these results is that the observed activation of Cl- conductance by agonists can be explained by the elevation of [Ca2+]i alone. In addition, the present results provide further support for the previously reported suggestion that the Cl- channels and the Ca(2+)-release sites are co-localised [10].