Origins of the concept of store-operated calcium entry

Abstract

The concept of capacitative or store-operated calcium entry, a process by which the release of stored calcium signals the opening of plasma membrane calcium channels, has its roots in the late 1970's, and was formalized in 1986. This short introduction to the current volume of Frontiers in Bioscience briefly summarizes the early experimental work that led to the idea of store-operated calcium entry, and provided the initial proofs for it.

Figure 1. Primitive version of store-operated Ca²⁺ entry

The Ca²⁺ store and channel were seen as a common entity within the plasma membrane. Receptor activation would cause a conformational change, resulting in the release of bound Ca²⁺, permitting influx of Ca²⁺ through the channels. Based on a model originally published in (9).

Figure 2. First experimental evidence that refilling Ca²⁺ stores does not require receptor activation

Parotid acinar cells (slices) were pre-incubated with ⁸⁶Rb⁺ and unidirectional efflux of the nuclide monitored as an index of activity of Ca²⁺-activated K⁺ channels. The black trace shows the result adding epinephrine (to activate α₁-adrenergic receptors), in the absence of Ca²⁺, which releases stored Ca²⁺ causing a transient increase in ⁸⁶Rb⁺ efflux. This is followed by the addition of Ca²⁺, causing Ca²⁺ influx, again increasing ⁸⁶Rb⁺ efflux. This is reversed by the addition of the α-receptor blocking drug, phentolamine, followed by Ca²⁺ removal. The Ca²⁺ stores have refilled as shown by the large transient increase in efflux upon addition of the muscarinic receptor agonist, carbachol. In the experiment shown in gray, the additions are similar, except that the receptors are first blocked by phentolamine before addition of Ca²⁺. Nonetheless, the response to carbachol is the same in both cases, indicating that the stores refill from the outside with similar efficiency, whether or not the surface receptor is activated. Based on data originally published in (10).

Figure 3. Two key experiments validating the concept of store-operated Ca²⁺ entry

Top: In the experiment shown by the black trace, the muscarinic cholinergic receptor activating drug, methacholine, was applied to Fura-2-loaded parotid acinar cells in the absence of extracellular Ca²⁺. Atropine was added to block the muscarinic receptors where indicated, and subsequently extracellular Ca²⁺ (10 mM) was added. This resulted in a transient increase in [Ca²⁺]_i, indicative of Ca²⁺ influx occurring to refill the empty intracellular stores. In the control experiment, shown in gray, no carbachol was added such that stores remained full. Based on data originally published in (13). Bottom: Thapsigargin (2 μM) was applied to parotid acinar cells in the presence of extracellular Ca²⁺ resulting in a sustained elevation in [Ca²⁺]_i. Subsequent addition of methacholine caused a small transient elevation in [Ca²⁺]_i, probably because stores were not yet fully empty, but did not further elevate the sustained Ca²⁺ signal. This indicated that full activation of Ca²⁺ entry could be accomplished simply by depleting the intracellular Ca²⁺ stores. Based on data originally published in (15).