Caldesmon Inhibits Active Crossbridges in Unstimulated Vascular Smooth Muscle

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Abstract
—Caldesmon is a thin-filament–associated protein believed to be important in the regulation of smooth muscle contraction, although the precise mechanism is unknown. We used antisense oligodeoxynucleotides to produce intact swine carotid smooth muscle tissue deficient in h-caldesmon. Caldesmon content was decreased by 78% after 7 days in culture with antisense oligodeoxynucleotides but was unchanged in tissues in the presence of sense oligodeoxynucleotides or vehicle. Antisense oligodeoxynucleotides produced a significant decrease in the caldesmon/actin ratio, but no change was measured in the calponin/actin ratio, suggesting that the effect was specific to caldesmon and not other thin-filament–associated proteins. Basal and KCl-stimulated levels of myosin light chain phosphorylation were not different among tissues from all 3 groups. In contrast, h-caldesmon–deficient tissues produced 62% less KCl-induced force than controls. Unstimulated h-caldesmon–deficient smooth muscle tissues stretched and then released, redeveloped force, demonstrating active crossbridge cycling; strips containing normal h-caldesmon content did not redevelop force on release. We suggest that in resting vascular smooth muscle, active crossbridges are inhibited by caldesmon. Therefore, regulation of smooth muscle includes a thin-filament–based disinhibition component.