We have studied three divalent cations (Ca²⁺, Sr²⁺, Ba²⁺) capable of eliciting secretion in adrenal chromaffin and other secretory cells. These cations enter the cells through voltage-sensitive Ca²⁺ channels, yet Ba²⁺ and Sr²⁺ are poorly cleared from the intracellular space. In addition, Ba²⁺ blocks Ca²⁺-activated K⁺ channels, promoting cell depolarization and action potentials.

The amount of catecholamines released by intact depolarized cells depends on the cation applied extracellulary following order: Ba²⁺> Sr²⁺≥ Ca²⁺. Conversely, amperometric recordings on permeabilized cells (Fig. 1) show that Ca²⁺ promotes the longest lasting secretion, as Ba²⁺ only provokes secretion while present and Sr²⁺ induces intermediate-lasting secretion. Intracellular Ba²⁺ dialysis provokes exocytosis at concentrations 100-fold higher than those of Ca²⁺, whereas Sr²⁺ exhibits an intermediate sensitivity.

Fig. 1. Secretory responses induced by divalent cations in permeabilized chromaffin cells studied with single-cell amperometry. Superimposed, representative amperometric recordings obtained with 10 s application (arrow) of Ca²⁺, Sr²⁺ or Ba²⁺ at the estimated concentrations (22.5, 44.5 and 41.5 µM of free Ca²⁺, Sr²⁺ and Ca²⁺, respectively). Barium effect (green trace) lasts only during the cation application whereas Ca²⁺ lasts for minutes (black trace) and Sr²⁺ (red trace) exhibits an intermediate duration. Superimposed dashed traces show the temporal distribution of spike firing. Calibration bar on the left (in pA) for oxidation currents of secretory spike, on the right for the frequency of secretory events (expressed in Hz) are. Temporal calibration is shown in seconds.

Using Fura-2 on intact chromaffin cells, the extracellular application of Ba²⁺ increases cytosolic Ca²⁺ concentrations also inducing long-lasting catecholamine release. When secretion is triggered by pressure application of 5 mM BaCl₂, the substitution of Ca²⁺ by Sr²⁺ largely affects the kinetics of exocytosis elicited by Ba²⁺, whereas the replacement of Ca²⁺ by Mg²⁺ does not cause effects on secretory spikes (Fig. 2).

We also carried out an electrophysiological study under current-clamp configuration. This study shows that extracellular Sr²⁺ and Ba²⁺ cause membrane depolarization and action potentials, which are not blocked by Cd²⁺ but that can be mimicked by tetra-ethyl-ammonium. When applied intracellularly, only Ba²⁺ provokes action potentials. In addition, voltage-clamp monitoring of Ca²⁺-activated K⁺ channels (K_Ca) shows that Ba²⁺ reduces outward currents, which are enhanced by Sr²⁺.

Our results are compatible with the following sequence of events: Ba²⁺ blocks K_Ca channels from both the outside and inside of the cell, causing membrane depolarization that, in turn, opens voltage-sensitive Ca²⁺ channels and favours the entry of Ca²⁺ and Ba²⁺. Although Ca²⁺ is less permeable through its own channels, it is more efficient on triggering exocytosis. Strontium possesses both an intermediate permeability and an intermediate ability to induce secretion.

Fig. 2. The substitution of Ca²⁺ by Sr²⁺ largely affects the kinetics of exocytosis elicited by Ba²⁺ whereas the replacement of Ca²⁺ by Mg²⁺ does not cause effects on secretory spikes. Experiments are conducted using single cell amperometry. Secretion is triggered by pressure application of 5 mM BaCl2 during 5 s. A, kinetics parameters obtained from secretory vesicles I_max, maximal current caused by the catecholamines reaching the electrode; t_1/2, spike width at its half height; Q, the integrated area under the spike trace that indicates the total quantal catecholamines released during the exocytotic event; m, the ascending slope was linearized between the 25% and 75% of the I_max. B, Average spikes are built using real values.

We have provided further details of the mechanisms underlying the sustained, yet slower secretory effects of Ba²⁺ in chromaffin cells, and described some novel actions of the lesser studied cation Sr²⁺ on BK channel currents and cell excitability that contribute to increase its secretagogue capability in comparison with Ba²⁺ and Ca²⁺.

Andrés M. Baraibar, Ricardo de Pascual, Marcial Camacho, Natalia Domínguez, J. David Machado, Luis Gandía, Ricardo Borges
Unidad de Farmacología, Fac. Medicina, Univ. La Laguna, Tenerife and Instituto Teófilo Hernando, Dept. Farmacología y Terapéutica, Fac. Medicina, Univ. Autónoma de Madrid, Spain

Publication

Distinct patterns of exocytosis elicited by Ca2+, Sr2+ and Ba2+ in bovine chromaffin cells.
Baraibar AM, de Pascual R, Camacho M, Domínguez N, Machado JD, Gandía L, Borges R
Pflugers Arch. 2018 Oct

Read offline:     

Related Articles:

	Why photobiomodulation therapy is helpful for diabetic wound… It is an obvious well-known issue that diabetic wounds are poorly healing, but the molecular and cellular alterations leading to this impairment is not clear. Many studies concluded the dysfunction…
	If you are cold, what two cold-sensitive ion channels TRPM8… Human and animals are capable of perceiving temperature stimuli due to the presence in the body of specialized sensory neurons in the peripheral and central nervous system. The sense of…
	Recording in the emergency department: illegal or immoral? Patients and staff should expect privacy in emergency departments and should not be recorded without giving their consent in advance. Emergency departments (ED) use audiovisual recordings to document clinical information…
	Zinc promotes myoblast differentiation activating Zip7… Skeletal muscle is a heterogeneous, dynamic and plastic tissue, which comprises 40% of adult human body mass. Through contraction and relaxation, skeletal muscles provide movement and stability to the body.…
	Method to measure calcium signaling in nanometer-sized… Cilia are slender microtubule-based organelles that protrude from the apical membrane in most adherent cells. Studies looking into the mechanosensory function of the primary cilium have proved it to be…
	Novel genetics approach identifies GNAS gene as an important… Type 2 diabetes mellitus (T2D) is a major health burden in developing and industrialized countries (www.idf.org). Common genetic variations, susceptibility genes, epigenetic regulation, and miRNAs have been suggested to be…