CHARACTERISTICS AND PHYSIOLOGICAL-ROLE OF THE CA2-ACTIVATED CL- CONDUCTANCE IN SMOOTH-MUSCLE()

In this review we discuss the properties of the Ca2+-activated Cl- cur rent [I-Cl(Ca)] recorded in isolated smooth muscle cells with electrop hysiological techniques and speculate on the possible physiological ro le(s) of I-Cl(Ca) in smooth muscle function. In particular, me concent rate on 1) the Ca2+ dependence of I-Cl(Ca), 2) the mechanisms that lin k pharmacological receptor stimulation on the cell surface membrane to activation of I-Cl(Ca), 3) the biophysical properties of I-Cl(Ca), an d 4) the pharmacology of I-Cl(Ca). It is evident that a diverse array of pharmacological agonists can evoke I-Cl(Ca) in many types of smooth muscle, and it seems that the well-established G protein-phosphoinosi tide metabolism (inositol 1,4,5-trisphosphate)-intracellular Ca2+ stor e pathway couples the receptor to the membrane channels. Also, the res ults indicate that the biophysical and pharmacological properties of I -Cl(Ca) are not only similar in the various smooth muscle types studie d so far but, possibly, are also similar to I-Cl(Ca) in non-smooth mus cle tissue. Evidence is presented that the Ca2+-activated Cl- channel exists in two states, open and closed, with a relatively long mean ope n time and that some of the agents that inhibit I-Cl(Ca) interact dire ctly with the open channel. It is suggested that the most likely role of I-Cl(Ca) in smooth muscle is to produce membrane depolarization and contraction to neurotransmitters and local mediators.