Chapter 17 - Ion Channels in Vascular Smooth Muscle

Abstract

This chapter surveys the various ion channels distributed in the membrane of vascular smooth muscle cells, which comprise the most important machinery for the regulation of [Ca2+]i. The physiological roles of some of them are uncertain, such as T-type Ca2+ and TTX-sensitive Na+ channels. Activation of these ion channels is speculated to contribute the initial membrane depolarization for generation of the action potentials in electrically active smooth muscle cells, and also may be in small arterial cells having the lower resting membrane potential. In these vascular cells, a part of the ion channels is in the resting state at the resting membrane potentials, and initial membrane depolarization by receptor activation opens these channels to initiate the action potentials. However, this concept is inapplicable to vascular cells having higher resting membrane potentials, such as aorta and pulmonary artery. Both T-type Ca2+ and TTX-sensitive Na+ currents close their inactivated gates rapidly by a sufficient depolarization, however, in a certain range of the membrane potential, channels are activated continuously with very low open probability. It is uncertain whether Ca2+ and Na+ influx through a window current is enough to elevate the [Ca2+]i and membrane depolarization, as re-evaluation of the channel properties under the physiological condition is necessary for elucidation of their physiological functions.