Mechanical responses and calcium movements in rabbit iris smooth muscles

Ca movements in mechanical responses of the isolated sphincter and dilator muscles of the rabbit iris to some drugs were studied. D600, a Ca-blocker, was practically ineffective in doses which relax the vascular and intestinal smooth muscles, while other smooth muscle relaxants were effective on the iris muscles in the same doses as used for other smooth muscles. The contractile responses of both iris smooth muscles to K+ and acetylcholine were immediately abolished by Ca removal from the bath fluid, suggesting that these contractile responses were due to Ca influx. Furthermore, Ca-channels studied seem to be uninfluenced by D600. A residual response to norepinephrine was observed in the iris dilator muscle immersed in the Ca-free solution and the contractile response of the skinned fiber of this muscle to norepinephrine was also obtained at a low Ca solution. Accordingly, contraction of the dilator muscle by norepinephrine seems to be due mainly to a facilitation of the release of intracellularly sequestered Ca.