Some Smooth Muscles Possess Fast Sodium Channels

Most smooth muscle (SM) cells normally do not possess fast Na+ channels, and inward current for the action potential (AP) is carried primarily through slow (L- type) Ca2+ channels. However, it has been demonstrated that fast Na+ channels appear in myometrial SM cells during the latter half of pregnancy in the rat, presumably under steroid hormone control (4, 7). In addition, we have recently found functioning fast Na+ channels in rat and human intestinal (colonic) SM cells (23) and in rat portal vein cells (Xiong & Sperelakis, unpublished) under normal conditions. This review-type article provides a brief summary of (a) the changes in fast Na+ channels (and slow Ca2+ channels) in myometrial SM cells (from longitudinal layer) during pregnancy in the rat (4, 7); (b) the existence of functional fast Na+ channels in colonic SM cells (from circular layer) of the proximal (ascending) colon in both adult rat and human (13) and (c) the presence of functional fast Na+ channels in vascular SM cells from the rat protal vein (Xiong & Sperelakis unpublished).