Control of the contraction/relaxation cycle in vascular smooth muscle is regulated by Ca2+ and the cyclic nucleotides, cAMP and cGMP. For the most part, the effectors of these intracellular messengers are the protein kinases. Four major protein kinases (myosin light chain kinase, protein kinase C, cAMP dependent protein kinase, and cGMP dependent protein kinase) have been identified in vascular smooth muscle. Substantial biochemical and physiological evidence exists supporting the involvement of Ca2+/calmodulin-mediated activation of myosin light chain kinase and phosphorylation of the 20,000 dalton P-light chain of myosin in the regulation of vascular contractile activity. However, alternative hypotheses exist which suggest that additional Ca2+ dependent regulatory mechanisms reside at other contractile protein sites. Calcium also activates protein kinase C, which requires phospholipid and diacylglycerol as co-factors instead of calmodulin. Protein kinase C also phosphorylates smooth muscle myosin P-light chain; however, phosphorylation occurs at a different site on the P-light chain and represses ATPase activity which has been stimulated by myosin light chain kinase-catalyzed phosphorylation. The precise physiological role of protein kinase C in modulating vascular smooth muscle contractile activity remains to be elucidated. Relaxation of vascular smooth muscle by some different relaxants is linked to either cAMP or cGMP formation. Correlative evidence also links activation of cAMP dependent protein kinase with relaxation. Two isozymes of cAMP dependent protein kinase exist in arterial smooth muscle; potential specific roles for each isozyme have not been elucidated. Mechanistically, relaxation mediated by both cyclic nucleotide-regulated protein kinases most likely involves primary effects on Ca2+ ion flux regulation rather than direct effects on contractile protein interactions. Activation of cGMP dependent protein kinase may be important in mediating the relaxant effects of endothelium derived relaxant factor or atrial natriuretic factor. Direct pharmacological modulation of smooth muscle vascular protein kinase activity represents an approach towards developing novel vasodilator agents. Various classes of agents, including phenothiazine antipsychotics, antidepressants, naphthalene sulfonamides, and certain lipophilic Ca2+ antagonists, inhibit myosin light chain kinase activity primarily by competition with the enzyme for Ca2+-calmodulin. However, additional inhibition via binding to the myosin P-light chain may also occur with some of these agents.(ABSTRACT TRUNCATED AT 400 WORDS)