SERCA pump optimizes Ca2+ release by a mechanism independent of store filling in smooth muscle cells

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+], 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.