SMOOTH-MUSCLE PROTEIN-KINASE C-1

Protein kinase C (PKC) was first implicated in the regulation of smoot h muscle contraction with the observation that phorbol esters induce s lowly developing, sustained contractions. In some vascular smooth musc les, e.g., ferret aorta, phorbol ester induced contractions occur with out an increase in sarcoplasmic free-Ca2+ concentration ([Ca](i)) or m yosin light chain phosphorylation. This response appears to be mediate d by a Ca2+-independent isoenzyme of PKC (probably PKC epsilon), since saponin-permeabilized single ferret aortic smooth muscle cells, which retain receptor coupling, developed force in response to phenylephrin e at low free [Ca2+] (pCa 7.0-8.6) and the constitutively active prote olytic fragment of PKC (PKM) elicited a contraction at pCa 7 comparabl e with the phenylephrine-induced contraction. Both contractions were r eversed by a pseudosubstrate peptide inhibitor of PKC. These observati ons suggest a mechanism whereby cy-adrenergic agonists may elicit a co ntractile response without a Ca2+ signal: alpha-adrenergic stimulation of phosphatidylcholine-specific phospholipase C or D (the latter in c onjunction with phosphatidate phosphohydrolase) generates diacylglycer ol. In the absence of an increase in [Ca2+](i), diacylglycerol specifi cally activates so-called novel PKCs, of which epsilon is the only iso enzyme known to be expressed in vascular smooth muscle. Recent evidenc e suggests that PKC may trigger a cascade of phosphorylation reactions , resulting in activation of mitogen-activated protein kinase and phos phorylation of the thin filament associated protein caldesmon. Alterna tively, or additionally, PKC may directly phosphorylate calponin, anot her thin filament associated protein. These phosphorylations are predi cted to alleviate inhibition of the cross-bridge cycling rate by these thin-filament proteins. The slow development of force would then resu lt from a slow rate of cross-bridge cycling due to the low basal level of myosin phosphorylation.