Ionic channels in smooth muscle studied with patch-clamp methods

Application of the patch-clamp technique on smooth muscle has started only a few years ago, but many publications are already emerging. The Ca and K channels so far found in smooth muscles seem to be fundamentally similar to those in other tissues. However, it is likely that Ca channel in smooth muscle is not controlled by cyclic AMP, in contrast to the cardiac muscle. There are at least two different ("fast" and "slow") types of Ca channel, but their relative distribution in different smooth muscles and their physiological significance are not yet fully analyzed. Furthermore, analysis of the receptor-operated Ca channels is still very limited, although these channels are very important for the smooth muscle function. Further careful studies are necessary to examine regulatory mechanisms involved in different types of Ca channel and to clarify the receptor-operated channel. There are also different (voltage- and Ca-activated) types of K channel. The Ca-activated K channel seems predominant in many smooth muscles ad regulating their excitability. However, K channels responsible for the resting potential or for spontaneous activity should be defined and also contribution of K (and other ionic) channels to receptor-mediated responses remains to be investigated.