SUPERFICIAL BUFFER BARRIER FUNCTION OF SMOOTH-MUSCLE SARCOPLASMIC-RETICULUM

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 (superf icial buffer barrier or SBB). In this review Cornelis van Breemen, Qia n Chen and Ismail Laher summarize experimental support for the SBB, an d discuss data indicating that: (1) contraction is related more to the rate than extent of Ca2+ entry; (2) refilling of sarcoplasmic reticul um from the extracellular space is mediated by Ca2+ influx and Ca2+ pu mping by the sarcoplasmic reticulum Ca2+ pump; (3) the superficial sar coplasmic reticulum unloads Ca2+ to the extracellular space by a multi step process that involves sequentially the opening of Ca2+ and inosi tol 1,4,5-trisphosphate [Ins(1,4,5,)P-3] sensitive channels and Ca2+ e xtrusion by Na+-Ca2+ exchange; (4) the SBB generates a peripheral Ca2 gradient; (5) Ca2+-mobilizing receptor agonists generate Ins(1,4,5)P- 3 which short circuits the SBB to increase the effectiveness of Ca2+ i nflux in raising [Ca2+](i) and consequently increase smooth muscle con traction. A physiologically regulated SBB is thought to enhance the in formational 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 alternat ive means of controlling smooth muscle tone dependent on Ca2+ entry.