Sodium pump hyperpolarization-relaxation in rat caudal artery

Electrogenic ion transport contributes vitally to the Em in vascular muscle and thus is an important influence on contraction and relaxation. Agents that act on membrane ion transport will cause depolarization or hyperpolarization of sufficient magnitude to cause contraction or relaxation, respectively. In the caudal artery of the rat, the principal ion involved appears to be Na+. The transport process appears to be the Na+, K+-ATPase, which is ouabain sensitive, rather than other possible candidates such as the Na+-Ca2+ countertransport mechanism. The hyperpolarization and parallel relaxation found in caudal artery on return to K+ provide unequivocal evidence for an electrogenic Na+ pump. In contrast, the lack of a contraction on transition to O Na+ suggests that the caudal artery does not show an Na+-K+ countertransport system. Although other ion transport systems might be established later for caudal artery and other kinds of vascular muscle, it now appears that the electrogenic Na+ pump is the main ion transport system controlling contraction through a continuous contribution to Em.