Barium-induced exocytosis is due to internal calcium release and block of calcium efflux.

Abstract

The concentration of cytosolic free Ca2l ([Ca2+]J) and the release of tritiated norepinephrine ([3HJNE) were monitored during Ba2+ stimulation of sympathetic neurons cultured from chick embryos. Ba2+ (2.5 (17).Indo-1 Emission Spectra. Emission spectra of indo-1 were determined in the presence of increasing concentrations of Ca2+, Ba2+, and Ca2+ in the presence of Ba2+ by using a scanning spectrofluorometer (SPF-500C, SLM Aminco, Urbana, IL) with excitation at 355 nm and emission recorded from 360 to 550 nm (60 nm/min, slit width 4 nm). Solutions and calculations offree Ca2+ and Ba2+ were the same as those used for determining the standard curve of indo-1 emission ratios with the ACAS laser photometer (below).Indo-1 Imaging of [Ca2+];. Cultured sympathetic neurons were loaded with 0.25 ,uM indo-1 acetoxymethyl ester for monitoring [Ca2+]J (18) on an ACAS laser photometer (Meridian Instruments, Lansing, MI). Excitation was at 353-361 nm, and indo-1 fluorescence was recorded at 405 nm (Ca2+-bound) and 485 nm (Ca2+-free) for ratio determination (19). Simultaneous images at the two wavelengths were obtained over 10-sec periods at rest, coincident with electrical stimulation or following addition of Ba2+. To control for nonspecific charge effects, Ca2+-free Krebs solution was supplemented with an additional 2.5 mM MgCl2. In all experiments each cell body, neurite, or growth cone was used as its own control. A standard curve of indo-1 emission ratios (405 nm/485 nm) versus Ca2+ concentration (0-11.3 ,uM) and Ba2+ concentration (0-50.4 .uM) was determined in calibration buffer containing 100 mM KCI, 1 mM EGTA, 50 mM Hepes, and 1 ,uM indo-1 salt, pH 7.2 at 250C. 557The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Contributions of SERCA pump and ryanodine-sensitive stores to presynaptic residual Ca2+

“…Importantly though, Ba 2+ is cleared one to two orders of magnitude more slowly from synaptic terminals than is Ca 2+ [38] because it is ineffectively removed from the cytoplasm by transporters including SERCA pumps [39], the NCX [24], the PMCA [40] (but see: [41]), and the MCU [24]. The ability of Ba 2+ to bind to presynaptic cytoplasmic Ca 2+ -binding proteins is less clear, although it is ineffective in activating calmodulin-like [42] or troponin-like [43] binding sites.…”

Calcium- and barium-dependent exocytosis from the rat insulinoma cell line RINm5F assayed using membrane capacitance measurements and serotonin release

“…Furthermore, Ba 2+ was able to substitute for external Ca 2+ in the KCl-evoked release of [3H]5-HT. Some authors [38,48] have suggested that Ba 2+ acts directly to stimulate secretion whereas others [36] suggest that Ba 2+ acts by stimulating Ca 2+ release from intracellular stores. Our results show that unlike Ca 2+-stimulated [3H]5-HT release, Ba2+-stimulated release is only slightly affected by depletion of intracellular Ca 2+ stores with thapsigargin.…”

“…Ba 2+ can efficiently substitute for Ca 2+ in stimulating neurotransmitter and hormone release in several preparations such as the neuromuscular junction [43], sympathetic ganglia [26], adrenal chromaffin [12,22,23,36,38], pituitary [11] and pancreatic /3 [20,47,56] cells. In contrast to Ca 2+, Ba 2+ can also act as a secretagogue in the absence of other specific stimuli, an effect postulated to result from Ba 2+ blockade of K + channels, leading to subsequent depolarization, Ca 2+ influx and secretion [27,38].…”

Effect of Divalent Cations on the Contractile Response of Rat Aorta to Depolarization before and after Nifedipine Treatment

“…One could speculate that Ba2+ entering the cell might directly activate the contractile ma chinery in Ca2+-free medium, as has been shown in the glycerin-treated smooth muscle of guinea-pig tenia coli [24], although there are various effects that could also contribute to the contractile response to Ba2+: the release of Ca2+ from intracellular sites and an effec tive blocking of Ca2+ extrusion [25,26].…”