Fig. 7.

Proposed signal transduction pathways for specific α₁-AR subtype-mediated Ca²⁺-dependent and Ca²⁺-independent smooth muscle cell (SMC) contraction in adult and fetal CA. Left: α_1A-AR and α_1B-AR subtypes mediate production of Ins(1,4,5)P₃, with the result of increasing intracellular Ca²⁺ release, and effect contraction in the Ca²⁺-dependent pathway in both fetus and adult. In addition, α_1D-AR with dashed line indicates activation of Ins(1,4,5)P₃ pathway only in adult. Right: fetus, in contrast to adult, α_1B- and α_1D-AR activate PKC, which in turn activates the extracellular regulated kinases ERK1/2 to stimulate gene expression to result in SMC differentiation and vascular growth.