Kinase activation and smooth muscle contraction in the presence and absence of calcium

Purpose: The intracellular signalling mechanisms that modulate the sustained vascular smooth muscle contractions that occur with vasospasm are not well understood. The purpose of this investigation was to examine cell signalling mechanisms that account for sustained vascular smooth muscle contraction, independent of increases in intracellular Ca²⁺ concentrations ([Ca²⁺ ]_i). Methods: Fresh bovine carotid artery smooth muscles contractile responses were examined in a muscle bath. [Ca²⁺ ]_i was depleted by use of the extracellular Ca²⁺ chelator, ethylene glycol-bis(ß-aminoethylether) N, N, N′, N′-tetraacetic acid and the intracellular chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′,-tetraacetic acid. Results: In Ca²⁺ -free conditions, depolarizing the membrane with high extracellular KCI failed to elicit a contraction. In addition, in Ca²⁺-free conditions the ([Ca²⁺]_i) was less than 10 nmol/L as determined with the Ca²⁺-indicator, Fura 2. The protein kinase C (PKC) activator, phorbol 12, 13-dibutyrate (PDBu), induced slowly developing sustained contractions in bovine carotid artery smooth muscle, and the magnitude of the contractile response to PDBu (10 nmol/L to 10 μmol/L) was the same in the presence and absence of Ca²⁺ . PDBu induced contractions in Ca²⁺ -free conditions were not inhibited by the myosin light chain kinase inhibitor, ML-9 (50 μmol/L), but were inhibited by the PKC inhibitor, staurosporine (50 nmol/L). Conclusions: These data suggest that vascular smooth muscle contractions can occur under conditions where the [Ca²⁺ ]_i is low and fixed and that these contractions may be mediated by PKC. (J VASC SURG 1995;22:37-44.).