A possible explanation for effects of Sr2+ on contraction-relaxation cycle in canine stomach.

Effects of Sr2+ on smooth muscle activity and Ca2+ movement by plasma membrane (PM) were examined in preparations isolated from canine stomach to determine how Sr2+ modulates the contraction-relaxation cycle in smooth muscle. Sr2+ caused contraction in Ca2+-free solution. In the presence of Sr2+, KCl caused a response similar to that in normal Ringer's solution, this contraction was inhibited by both nitroprusside and verapamil. Sr2+ inhibited Ca2+ uptake by PM in the presence and absence of ATP. Release of Ca2+ from loaded PM vesicles was accelerated by Sr2+. Results suggest that Sr2+ could replace Ca2+ in Ca2+ binding sites in the cell.