Ionic Conductances in Dissociated Smooth Muscle Cells

Gastrointestinal and vascular smooth muscle cells appear to possess similar ion channels in their membranes but differ in how some of these channels are modulated. This, however, is not surprising since they react in a different manner physiologically to autonomic neurotransmitters such as acetylcholine and norepinephrine. Smooth muscle constrictors, norepinephrine in the case of vascular muscle, and acetylcholine in the case of gastrointestinal muscle, appear to share the common property of activating Ca2+ channels. However, whether these agents affect the Ca2+ channels directly or through second messenger systems remain to be determined. Vasodilators, on the other hand, appear to work via activation of K+ channels, specifically, the ATP-sensitive K+ channel. The ionic mechanism responsible for norepinephrine's decrease in the motility of gastrointestinal smooth muscle cells remains unknown at the present time.