Despite recent concerns over their long-term safety, the calcium channel blockers remain a popular means to control hypertension. Smooth muscle contraction depends on calcium influx to control muscle tone. When the latter enzyme is activated it phosphorylates myosin which acts with actin to contract muscle. Blocking the channel statistically prevents calcium ion influx and decreases tension in blood vessel smooth muscle. Skeletal muscles rely on intracellular calcium ion, and are not affected by these drugs.
Cardiac muscle is highly dependent on calcium channel action.
Calcium channels also exist at presynaptic nerve terminals in adrenergic neurons. These are voltage-dependent ion channels embedded in nerve membranes at the ends of adrenergic neurons. This triggers binding of vesicles and release of vesicular adrenalin, noradrenalin or dopamine, along with cotransmitters such as peptides, ATP, etc. One of the acid tests as to whether a substance is a neurotransmitter is whether its release is calcium-dependent. Verapamil is the oldest and prototypical calcium channel blocker.