Calcium reversal, relaxation by calcium ion of various smooth muscles contracted by carbachol, norepinephrine or a phorbol ester in calcium-ion free medium.

1. Ca reversal was reported on the longitudinal smooth muscle of rat uterus and the fundic part of the circular smooth muscle of guinea pig stomach; that is, addition of Ca ion to the Ca-free bathing solution in which the muscle is contracting to oxytocin or carbachol, respectively, relaxes the muscle. 2. We tested whether this inhibitory action of Ca ion is seen in other smooth muscles including vascular, air way, gastric and genital smooth muscles. 3. We found that Ca reversal was observed in the smooth muscle of rat thoracic aorta contracted by norepinephrine or by a phorbol ester, TPA, guinea pig tracheal smooth muscle contracted to carbachol, fundic smooth muscle of rat stomach to carbachol, corporal and antral smooth muscle of guinea pig stomach to carbachol and rat vas deferens to norepinephrine. 4. Ca reversal was observed not only when the muscle contracted by an agonist of the surface receptor such as oxytocin, norepinephrine or carbachol but also by a phorbol ester that is believed to cause contraction in the cell by activating phosphorylation. 5. Thus, we conclude that Ca reversal is a universal phenomenon in smooth muscles.