Contractility of vascular smooth muscle: maximum ability to contract in response to a stimulus.

The reactivity of vascular smooth muscle observed under the usual steady-state conditions (termed the usual reactivity) is generally relatively labile to changes in the external environment (e.g., to changes in PO2 or [K]o). However, reactivity during brief exposure to ouabain is nearly always greater than the usual reactivity and is relatively stable. It is demonstrated that for any given stimulus (i.e., a given concentration of a particular agonist), this latter reactivity is always the maximum that can be produced under any given set of conditions. Accordingly, it is proposed that the usual reactivity may be expressed as the algebraic sum of two properties of vascular smooth muscle: contractility, relatively stable property, which is defined as the maximum ability to contract in response to a stimulus, and electrogenesis, a relatively labile property, which produces a sometimes dominating electrical influence that modulates reactivity by opposing contractility. It is further proposed that because of the fundamental importance of these two properties it is useful to categorize all changes in reactivity, depressing or enhancing, according to whether they involve changes in contractility or electrogenesis.