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SERCA Pump Optimizes Ca²⁺ Release by a Mechanism Independent of Store Filling in Smooth Muscle Cells

Departamento de Bioquímica and ^* Departamento de Fisiología, Biofísica y Neurociencias, CINVESTAV-IPN, México D.F., 07000, Mexico

Correspondence: Address reprint requests to Agustín Guerrero-Hernández, Departamento de Bioquímica, CINVESTAV-IPN, Apdo. Postal 14-740, México D. F., 07000, Mexico. Tel.: 525-55-747-3950; Fax: 52-555-747-7083; E-mail: aguerrer{at}mail.cinvestav.mx.

Thapsigargin-sensitive sarco/endoplasmic reticulum Ca²⁺ pumps (SERCAs) are involved in maintaining and replenishing agonist-sensitive internal stores. Although it has been assumed that release channels act independently of SERCA pumps, there are data suggesting the opposite. Our aim was to study the relationship between SERCA pumps and the release channels in smooth muscle cells. To this end, we have rapidly blocked SERCA pumps with thapsigargin, to avoid depletion of the internal Ca²⁺ stores, and induced Ca²⁺ release with either caffeine, to open ryanodine receptors, or acetylcholine, to open inositol 1,4,5-trisphosphate receptors. Blocking SERCA pumps produced smaller and slower agonist-induced [Ca²⁺]_i responses. We determined the Ca²⁺ level of the internal stores both indirectly, measuring the frequency of spontaneous transient outward currents, and directly, using Mag-Fura-2, and demonstrated that the inhibition of SERCA pumps did not produce a reduction of the sarco/endoplasmic reticulum Ca²⁺ levels to explain the decrease in the agonist-induced Ca²⁺ responses. It appears that SERCA pumps are involved in sustaining agonist-induced Ca²⁺ release by a mechanism that involves the modulation of Ca²⁺ availability in the lumen of the internal stores.

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