CALCIUM-ION ITSELF INHIBITS CA-FREE CONTRACTION OF VARIOUS SMOOTH MUSCLES IN RESPONSE TO ADRENERGIC AND CHOLINERGIC STIMULATIONS AND TO TPA IN THE ABSENCE OF CA CHANNEL BLOCKER AND EGTA

1. Smooth muscles such as thoracic aorta, stomach and vas deferens contract tonically in the absence of Ca ion (Ca) in response to various agonists such as norepinephrine, carbachol and TPA. 2. Addition of Ca alone to the Ca-free medium relaxes the muscle that has been contracting in response to the agonist in the Ca-free medium. 3. Ca itself has an inhibitory action on the contraction. 4. Acidification of the medium relaxes the muscle as Ca does, but weakly.