Figure 10. Proposed schema for Ca²⁺ distribution in smooth muscle cells.

Shown is a section of a smooth muscle cell showing at the left the extracellular space, at the top the subplasmalemmal space (labelled [Ca²⁺]_pm), and the sarcoplasmic reticulum. At the bottom right is the cytosol with the contractile apparatus. The proposed movements of Ca²⁺ are shown as arrows numbered 1–4. Abbreviations: PMCA, plasma membrane calcium ATPase (calcium pump); Na⁺-Ca²⁺, sodium-calcium exchanger; IP₃R, the inositol trisphosphate receptor calcium release channel; Leak, the leak channel; VOC, voltage-operated L-type calcium channel; Trp, the trp channel; SERCA the sarco(endo)plasmic reticulum calcium ATPase (calcium pump); other abbreviations as in text.