Figure 4. Vascular smooth muscle junctional calcium transients.

Neuronal action potentials (AP) promote the release of ATP and norepinephrine (NE) from perivascular nerve terminals. Norepinephrine stimulates α₁ adrenergic receptors (α1R) giving rise to Ca²⁺ waves as described in Figure 3 while ATP opens Ca²⁺-permeable purinergic receptors (P2XR) on the vascular smooth muscle cell plasma membrane (VSMC PM) producing localized junctional Ca²⁺ transients. Purinergic receptor activation also leads to plasma membrane depolarization (+ Vm) and increased opening of voltage-dependent Cav1.2 L-type Ca²⁺ channels (L). Ca²⁺ influx from junctional Ca²⁺ transients and L-type Ca²⁺ channels summate to increase contraction.