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Abstract
In smooth muscle the superficial sarcoplasmic reticulum accumulates a portion of the
Ca2+ that enters cells through the plasmalemma and thus functions as a buffer barrier
to Ca2+ entry into the myoplasm (superficial buffer barrier or SBB). In this review
Cornelis van Breemen, Qian Chen and Ismail Laher summarize experimental support for
the SBB, and discuss data indicating that: (1) contraction is related more to the
rate than extent of Ca2+ entry; (2) refilling of sarcoplasmic reticulum from the extracellular
space is mediated by Ca2+ influx and Ca2+ pumping by the sarcoplasmic reticulum Ca2+
pump; (3) the superficial sarcoplasmic reticulum unloads Ca2+ to the extracellular
space by a multi step process that involves sequentially the opening of Ca2+ and inositol
1,4,5-trisphosphate [Ins(1,4,5,)P3] sensitive channels and Ca2+ extrusion by Na(+)-Ca2+
exchange; (4) the SBB generates a peripheral Ca2+ gradient; (5) Ca(2+)-mobilizing
receptor agonists generate Ins(1,4,5)P3 which short circuits the SBB to increase the
effectiveness of Ca2+ influx in raising [Ca2+]i and consequently increase smooth muscle
contraction. A physiologically regulated SBB is thought to enhance the informational
content of Ca2+ signalling and support variable reduction of smooth muscle tone. Pharmacological
modulation of Ca2+ transport in the superficial sarcoplasmic reticulum therefore presents
an alternative means of controlling smooth muscle tone dependent on Ca2+ entry.