Relationship between the sarcoplasmic reticulum and the plasma membrane.

Ionic interactions between the plasma membrane (PM) and the sarcoplasmic reticulum (SR) play a crucial role in smooth muscle activation and homeostasis. The most common form of Ca2+ signalling seen in vascular smooth musde of conduit arteries and capacitance veins consists of repetitive asynchronous Ca2+ waves. In the inferior vena cava of the rabbit these waves are initiated by Ca2+ release via InsP3 receptors (InsP3R) and propagated by regenerative Ca2+ release. Maintenance of the [Ca2+] oscillations is dependent on Ca2+ entry through the Na+/Ca2+-exchanger (NCX) which is driven in the reverse mode by Na+ entry through non-specific cation channels. The latter are also responsible for depolarization and activation of voltage-gated Ca2+ channels. The sarcoplasmic-endoplasmic reticulum Ca2+-ATPase (SERCA) in the sheet-like junctional SR is responsible for refilling and completing the cycle. Under resting conditions the interaction between the superficial SR and the NCX is reversed with Ca2+ release channels supplying Ca2+ to the NCX in the PM to be extruded in exchange of extracellular Na+. It is proposed that the above Ca2+ transport between the SR lumen and the extracellular space takes place at PM-SR junctions across a narrow junctional space.