The relationship between phosphorylation of the 20-kDa myosin light chain, intracellular calcium levels ([Ca2+]i), and isometric force was studied during prolonged activation of arterial smooth muscle. Aequorin, preloaded into ferret aortic strips, was used as a [Ca2+]i indicator. Two dimensional polyacrylamide gel electrophoresis was used to determine the phosphorylation levels of the 20-kDa myosin light chain (LC20). During the 30-min depolarization of arterial smooth muscle by K+ (21 mM), both LC20 phosphorylation and [Ca2+]i increased significantly at all time points examined as did the steady state stress. A transient rise in LC20 phosphorylation and [Ca2+]i occurred within 30 s, followed by suprabasal levels through the 10-min period during a sustained alpha 1-mediated activation by 10(-5) M phenylephrine whereas a higher force was developed at a shorter time compared to K+. An active phorbol ester 12-deoxyphorbol 13-isobutyrate 20-acetate (DPBA, 10(-6) M) induced a slow contraction of similar magnitude to that induced by K+ without significantly changing either [Ca2+]i or LC20 phosphorylation over a 90-min period. These results demonstrate that the amount of LC20 phosphorylation correlates with the [Ca2+]i in all three types of activation. The initial levels of [Ca2+]i and LC20 phosphorylation correlate with the onset of force development but not the magnitude of steady state stress, suggesting a role for [Ca2+]i and LC20 phosphorylation in regulating the cross bridge cycling rate during tension development. The lack of a detectable increase in [Ca2+]i and LC20 phosphorylation during DPBA activation suggests that sites other than LC20, phosphorylated by protein kinase C, may be involved in regulating smooth muscle contraction.