The ionic mechanism of excitation in intestinal smooth muscle cells.

The ionic mechanism of excitation in intestinal smooth muscles has been reviewed. Mammalian smooth muscles are a very important group of excitable cells. There exists a considerable number of reports on the electrical and mechanical properties of these muscles. But little work has been done on the analysis of ionic mechanism of this excitable process. The recent successful application of the voltage-clamp technique using the double sucrose gap method has opened a new path for the study of the ionic mechanism in smooth muscles. In intestinal smooth muscle cells, the fast inward current responsible for spike generation is carried principally by the influx of Ca ions and not by Na ions. The outward currents which participate in the rate of repolarization are mainly carried by efflux of K ions. Sr ions and Ba ions can carry the current through the Ca channel. That is, Sr and Ba can substitute for Ca in generation of the fast inward current. The inactivation curve of the inward current of intestinal smooth muscle cells deviates from that of nerve cells and skeletal muscle cells in its hyperpolarizing range. Such an inactivation observed in the hyperpolarizing range is referred to hyperpolarizing inactivation. It seems likely that hyperpolarizing inactivation in smooth muscles is mainly due to the fast outward current. The electrophysiology of smooth muscles has made a remarkable advance lately due to the introduction of the double sucrose gap method. There exist, however, some serious limitations on the voltage control in this method and more quantitative, more accurate information can be expected upon further improvements of the voltage-clamp technique.