A new view of K+-induced contraction in rat aorta: the role of Ca2+ binding

Strong, K+-induced contractions of rat aorta in Ca-free, Mg-free media were not accompanied by increased intracellular calcium concentration, [Ca2+]i, whereas such contractions in the presence of the divalent cations were correlated with rising [Ca2+]i as assessed by fura-2. At the same time, calcium channel blockers, a modulator of Ca2+-binding proteins, and a modulator of actin polymerization, inhibited all types of K+-induced contractions. Increasing the K+ in isotonic medium evoked a rise of 45Ca2+ binding to the plasma membrane of freshly isolated aortic cells. Although Ca2+-dependent events underlie the mechanism of K+-induced vascular contractions in both the presence and absence of Ca2+, in contrast to the view that [Ca2+]i is a key regulator of excitation-contraction coupling in smooth muscle, we suggest that the modulation of Mg2+-dependent Ca2+ binding, probably within/at the L-type calcium channel by K+, is a trigger for aortic contraction. This Ca2+ binding may then activate actin-myosin interaction.