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Capacitative Ca²⁺ entry is graded with degree of intracellular Ca²⁺ store depletion in bovine vascular endothelial cells

Marina Sedova, Andrey Klishin, Jörg Hüser and Lothar A. Blatter

1. In endothelial cells, release of Ca²⁺ from endoplasmic reticulum (ER) Ca²⁺ stores activates Ca²⁺ influx via the capacitative Ca²⁺ entry (CCE) pathway. In cultured bovine pulmonary artery endothelial cells, we investigated the relationship between intracellular Ca²⁺ store load and CCE activity, as well as the kinetics of CCE activation and deactivation, by simultaneously measuring changes in [Ca²⁺]_i and unidirectional manganese (Mn²⁺) entry through the CCE pathway.

2. Submaximal concentrations of ATP caused quantal release of Ca²⁺ from the ER, resulting in a dose-dependent depletion of Ca²⁺ stores and acceleration of Mn²⁺ entry. Mn²⁺ entry rate, as a measure of CCE activity, was graded with the amount of released Ca²⁺. Maximal activation of CCE did not require complete store depletion.

3. Slow depletion of the ER by exposure to the ER Ca²⁺ pump inhibitor cyclopiazonic acid resulted in a delayed activation of CCE, revealing a temporal dissociation between release of Ca²⁺ from intracellular stores and activation of CCE.

4. During [Ca²⁺]_i oscillations, at frequencies higher than 0·5 spikes min^-1, each Ca²⁺ spike resulted in a progressive acceleration of CCE without leading to oscillations of Ca²⁺ entry. In contrast, low frequency [Ca²⁺]_i oscillations were paralleled by transient CCE that was activated and deactivated with each Ca²⁺ spike, resulting in an oscillatory pattern of Ca²⁺ entry.

5. It is concluded that CCE is a rapidly activating process which is graded with store depletion and becomes fully activated before complete depletion. The duration of CCE activation correlates with the degree of store depletion and the time that is required to refill depleted stores. Overall, a mechanism of graded CCE prevents exhaustion of intracellular Ca²⁺ reserves and provides an efficient way to respond to variable degrees of intracellular store depletion.

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