CA2+-INDEPENDENT ISOFORMS OF PROTEIN-KINASE-C DIFFERENTIALLY TRANSLOCATE IN SMOOTH-MUSCLE

It is generally assumed that smooth muscle contraction is dependent on changes in intracellular Ca2+ concentration ([Ca2+]i); however, we have previously reported that a-agonist-induced contraction of aorta smooth muscle cells can occur in the absence of changes in [Ca2+]i [Collins, E. M., M. P. Walsh, and K. G. Morgan. Am. J. Physiol. 262 (Heart Circ. Physiol. 31): H754-H762, 1992]. The mechanism of this [Ca2+] i-independent contraction is controversial. We have now identified the Ca2+-independent protein kinase C (PKC) isoforms epsilon and zeta in ferret aorta and have used digital imaging microscopy to determine their subcellular distribution. At rest, epsilon-PKC is diffusely distributed in the cytosol, whereas zeta-PKC is concentrated in the perinuclear region; both isoforms are excluded from the nuclear space. Agonist stimulation causes a [Ca2+]i-independent translocation of epsilon-PKC to the surface membrane and of zeta-PKC to the intranuclear compartment. In comparison, ferret portal vein cells, which display a totally Ca2+-dependent agonist contraction, are lacking in epsilon-PKC but display perinuclear zeta-PKC, which translocates intranuclearly on activation. Thus the Ca2+-independent vascular contraction appears to be associated with plasmalemmal translocation of epsilon-PKC; in contrast, the intranuclear translocation of zeta-PKC may function in control of gene expression.