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The role of the SR in Ca2+ extrusion from venous smooth muscle

University of British Columbia

Ca²⁺ extrusion from rabbit inferior vena cava (IVC) smooth muscle (SM) was studied using ratiometric fura-2 fluorimetry. Concomitant blockade ofthe plasma membrane (PM) Ca²⁺-adenosinetriphosphatase (PMCA), Na⁺- Ca²⁺ exchanger, and sarco-endoplasmic reticulum Ca²⁺ -ATPase (SERCA) completely prevented the decline in the intracellular Ca²⁺ concentration ([Ca²⁺][sub i]) normally observed when Ca²⁺ is removed from the extracellular space (ECS) after stimulated Ca²⁺ influx. Blockade of the Na⁺- Ca²⁺ exchanger by removal of external Na⁺ reduced the rate of [Ca²⁺][sub i] decline by 47%. Blockade of SERCA with cyclopiazonic acid (CPA) reduced it by 23%, and this was not additive to the effects of Na⁺ removal. The loss of SR Ca²⁺ was determined by measuring the decay of caffeine (CAF)-induced fura-2 fluorescence transients. Removal of Ca²⁺ from the ECS caused a rapid loss of SR Ca²⁺ and a decline of [Ca²⁺][sub i] preventing the sarcoplasmic reticulum (SR) from reloading. Simultaneous removal of extracellular Na⁺ greatly inhibited the rate of this SR Ca²⁺ loss. A rapid loss of SR Ca²⁺ was induced by 20 μM CPA, regardless of the presence or absence of extracellular Na⁺ or Ca²⁺. These effects were not influenced by alterations in membrane potential owing to activity of Ca²⁺-activated K+ channels (K[sub ca] channels) since 3mM or higher of TEA (or 70 nM Iberiotoxin) had no effect on the rate of Ca²⁺ loss from the SR. These results indicate that when Ca²⁺ is removed from the ECS, it induces Ca²⁺ release from the SR towards the PM Na⁺- Ca²⁺ exchanger which subsequently translocates it from the junctional cytoplasmic space to the ECS. When the Na⁺- Ca²⁺ exchanger is arrested by removal of extracellular Na⁺ and Ca²⁺, Ca²⁺ released from the SR is re-sequestered by the SERCA. However, when both the Na⁺- Ca²⁺ exchanger, and the SERCA are blocked, Ca²⁺ released from the SR is extruded from the cells by the PMCA. From these results and the additional information provided by electron micrographs of the adult IVC, we conclude that the SR not only contributes to Ca²⁺ extrusion by way of coupling of the SERCA to the Na⁺- Ca²⁺ exchanger but also forms part of systematic hierarchy of interaction between the different Ca²⁺ transporters in the SR and cell membranes.

Text

2009-12-23

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http://hdl.handle.net/2429/17235

Pharmacology and Therapeutics

University of British Columbia

UBCV

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