Fig. 1.

Block diagram (1977 version) of Ca²⁺ regulation in arterial smooth muscle myocytes used to explain the hypothesis that Na⁺ pumps, Na⁺/Ca²⁺ exchangers (NCX) and an endogenous ouabain-like compound (OLC) help regulate myocyte Ca²⁺ and arterial tone (29). Some key Na⁺ and Ca²⁺ transport pathways are shown: 1) ouabain-sensitive Na⁺ pump; 2) NCX operating in the Ca²⁺ extrusion mode, although we now know that NCX usually operates in the Ca²⁺ entry mode in arteries with tone (31, 319); 3) sarcoplasmic reticulum (SR) Ca²⁺ pump (SERCA). Most of the Na⁺ and Ca²⁺ entry is through plasma membrane voltage-gated channels (k₁ and k₂, respectively) and Ca²⁺ is released from the SR through ryanodine receptors or inositol trisphosphate receptors (k₃). This diagram anteceded the discovery of Na⁺ pump isoforms and PLasmERosomes. [Modified from Blaustein (29).]