Role of calcium ion in maintenance of vascular smooth muscle tone.

Recent evidence indicates that vascular smooth muscle tone can be increased either by increasing cytoplasmic calcium ion (Ca++) levels or by increasing the sensitivity of the contractile apparatus to calcium. Data obtained with both luminescent and fluorescent calcium indicators indicate that the amount of force produced at a specific Ca++ is variable in intact vascular smooth muscle. Data obtained with the luminescent calcium indicator aequorin indicate that the calcium force curve is very steep in intact vascular smooth muscle cells; i.e., small changes in Ca++ can cause large changes in vascular tone. Therapeutic agents designed to directly decrease intrinsic myogenic tone could be designed to either decrease Ca++ or to decrease the sensitivity of the contractile apparatus to calcium. Considerable selectivity might be obtained by the latter approach.