Inhibition of CaM kinase II activation and force maintenance by KN-93 in arterial smooth muscle

Ca(+)/calmodulin-dependent protein kinase II (CaM kinase II) has been implicated in the regulation of smooth muscle contractility. The goals of this study were to determine: 1) to what extent CaM kinase II is activated by contractile stimuli in intact arterial smooth muscle, and 2) the effect of a CaM kinase II inhibitor (KN-93) on CaM kinase II activation, phosphorylation of myosin regulatory light chains (MLC(20)), and force. Both histamine (1 microM) and KCl depolarization activated CaM kinase II with a time course preceding maximal force development, and suprabasal CaM kinase II activation was sustained during tonic contractions. CaM kinase II activation was inhibited by KN-93 pretreatment (IC(50) approximately 1 microM). KN-93 inhibited histamine-induced tonic force maintenance, whereas early force development and MLC(20) phosphorylation responses during the entire time course were unaffected. Both force development and maintenance in response to KCl were inhibited by KN-93. Rapid increases in KCl-induced MLC(20) phosphorylation were also inhibited by KN-93, whereas steady-state MLC(20) phosphorylation responses were unaffected. In contrast, phorbol 12,13-dibutyrate (PDBu) did not activate CaM kinase II and PDBu-stimulated force development was unaffected by KN-93. Thus KN-93 appears to target a step(s) essential for force maintenance in response to physiological stimuli, suggesting a role for CaM kinase II in regulating tonic contractile responses in arterial smooth muscle. Pharmacological activation of protein kinase C bypasses the KN-93 sensitive step.