Agonist-induced redistribution of calponin in contractile vascular smooth muscle cells

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Abstract
Calponin is a thin filament-associated protein that has been implicated in playing an auxiliary regulatory role in smooth muscle contraction. We have used immunofluorescence and digital imaging microscopy to determine the cellular distribution of calponin in single cells freshly isolated from the ferret portal vein. In resting cells calponin is distributed throughout the cytosol, associated with filamentous structures, and is excluded from the nuclear area of the cell. The ratio of surface cortex-associated calponin to cytosol-associated calponin (R) was found to be 0.639 +/- 0.021. Upon depolarization of the cell with physiological saline solution containing 96 mM K+, the distribution of calponin did not change from that of a resting cell (R = 0.678 +/- 0.025, P = 0.369). Upon stimulation with an agonist (10 microM phenylephrine) that is known to activate protein kinase C (PKC) in these cells, the cellular distribution of calponin changed from primarily cytosolic to primarily surface cortex associated (R = 1.24 +/- 0.085, P < 0.001). This agonist-induced redistribution of calponin was partially inhibited by the PKC inhibitor calphostin, overlapped in time with PKC translocation, and preceded contraction of these cells. These results suggest that the physiological function of calponin may be to mediate agonist-activated contraction via a PKC-dependent pathway.