Ca²⁺ Movements in Smooth Muscle

¹ From the Department of Pharmacology, University of Miami School of Medicine, Miami

We describe the Ca²⁺ movements in smooth muscle cells at rest and during activation and relaxation as deduced from transplasmalemmal Ca²⁺ fluxes and contractile responses. The general picture which emerges is: the resting cell has a [Ca²⁺]_cyt below 10⁷ M and large gradients are poised across both the cell membrane and intracellular membranes. Excitation opens up Ca²⁺ channels which are linked to receptors and, if depolarization occurs, to other channels not linked to specific receptors but capable of sensing the membrane potential. Receptor activation also leads to release of Ca²⁺ from a limited intracellular Ca²⁺ pool which is superficially located because it has to be refilled from the outside. Relaxation is effected by Ca²⁺ accumulation by another intracellular Ca²⁺ pool, very likely sarcoplasmic reticulum, which does not release Ca²⁺ during activation. The sarcoplasmic reticulum Ca²⁺ pump can also decrease initial activation of the myofilaments. Elevation of cAMP levels may inhibit contraction by stimulating the sarcoplasmic reticulum Ca-ATPase. An enormous amount of research is still required to prove the above scheme and to localize and quantitate the various intracellular Ca²⁺ pools.