Inhibition of Mouth Skeletal Muscle Relaxation by Flavonoids of Cistus ladanifer L.: A Plant Defense Mechanism Against Herbivores

Abstract

Cistus ladanifer exudate is a potent inhibitor of the sarcoplasmic reticulum Ca²⁺-ATPase (Ca²⁺-pump) of rabbit skeletal muscle, a well- established model for active transport that plays a leading role in skeletal muscle relaxation. The low concentration of exudate needed to produce 50% of the maximum inhibition of the sarcoplasmic reticulum Ca²⁺-ATPase activity, 40–60 μg/ml, suggests that eating only a few milligrams of C. ladanifer leaves can impair the relaxation of the mouth skeletal muscle of herbivores, as the exudate reaches up to 140 mg/g of dry leaves in summer season. The flavonoid fraction of the exudate accounts fully for the functional impairment of the sarcoplasmic reticulum produced by the exudate (up to a dose of 250–300 μg/ml). The flavonoids present in this exudate impair the skeletal muscle sarcoplasmic reticulum function at two different levels: (i) by inhibition of the Ca²⁺-ATPase activity, and (ii) by decreasing the steady state ATP-dependent Ca²⁺-accumulation. Among the exudate flavonoids, apigenin and 3,7-di-O-methyl kaempferol are the most potent inhibitors of the skeletal muscle sarcoplasmic reticulum. We conclude that the flavonoids of this exudate can elicit an avoidance reaction of the herbivores eating C. ladanifer leaves through impairment of mouth skeletal muscle relaxation.