Relationship of ion channel activity to control of myometrial calcium

This article reviews the contribution of ion channels to membrane potential , the ion channels expressed in myometrium, and the effect of ion channel a ctivity on the control of myometrial intracellular free calcium. Plasma mem branes constitute barriers to permeability that establish concentration gra dients of ions inside versus outside the cell. Na+, Ca2+, and Cl- are norma lly in higher concentration gradients of ions inside whereas K+ is higher i nside. In myometrium, Ca2+ entry into cells mediated myometrial membrane po tential changes and serves as the internal signal for contraction. K+ efflu x is thought to promote repolarization after an action potential and to par ticipate in setting the resting membrane potential. Ions cross the cell mem brane through channels that have different regulated properties and selecti vities. Ion movement has been measured by a number of techniques, including radiolabeled ion flux, use of intracellular indicators,a nd patch-clamp me thodology. A number of myometrial Ca2+ channels have been described, includ ing voltage-regulated L-type channels and Ca2+ entry in response to intrace llular Ca2+ store depletion. Fast Na+ channels may contribute to cation ent ry late in pregnancy. K+ channels in myometrium include Ca2+-activated chan nels, a delayed rectifier, and an inward rectifier. A Ca2+-activated Cl- ch annel is also present in myometrium. In addition to being regulated by Ca2, the activity of a number of these channels can be regulated by uterine co ntractants and relaxants. regulation of ion channel activity can affect int racellular free Ca2+ concentrations in the myometrium. Therefore, control o f ion channel activity represents one of several approaches for controlling myometrial contractile activity. Copyright (C) 2000 by the Society for Gyn ecologic Investigation.