Model of the contribution of Ca_V channels to smooth muscle tone. Ca_V1.2 (dark blue) and Ca_V3.3 (purple) promote smooth muscle contraction by allowing Ca²⁺ entry. The relative contribution of each depends on intraluminal pressure (or membrane potential). Ca_V1.2 is the major contributor between −50 and −35 mV (higher pressure, >40 mm Hg) whereas Ca_V3.3 is the major contributor between −60 and −50 mV (lower pressure, 20–40 mm Hg). Ca_V3.2 (light blue) opposes contraction through activation of Ca²⁺ sparks/RyRs through BK channel–dependent membrane hyperpolarization. Model is based on the data of Harraz et al. (2015).