[Physiological functions and regulation of K+ and Cl- channels in single smooth muscle cells]

The major functional roles of K channels in smooth muscle cells are as follows: (i) Keeping resting membrane potential (RMP). (ii) Induction of hyperpolarization (HP) in response to bioactive substances. (iii) Facilitation of action potential repolarization (APR). (iv) Induction of action potential after-hyperpolarization (AH). (v) Facilitation of slow wave repolarization. (vi) Inhibition of depolarization and/or action potential. Major background K channels responsible for RMP have not been identified yet, whereas it is been postulated that several K channels including Ca-dependent K channels and ATP-dependent K channels may co-contribute in part to RMP. Further activation of these channels by bioactive substances results in hyperpolarization and muscle relaxation. APR and AH in several types of smooth muscles are due to activation of mainly large conductance Ca-dependent K channels (BK) and additionally delayed rectifier K channels. Moreover, another type of delayed rectifier K channels and early inactivating K channels may also contribute to some of the functional roles shown above. Although activation of Ca-dependent Cl channels in response to agonists depolarizes the cell membrane in several types of smooth muscle cells, a single channel current has not been identified. Ca-dependent K and Cl currents can be regulated by intracellular Ca store sites, which may be exhausted after a large release induced by inositol 1,4,5-trisphosphate or caffeine. The Ca-pump activity in the store sites may also regulate indirectly but strongly the activity of the channels and consequently membrane excitability.