Induction of Ca²⁺ oscillations by vasopressin in the presence of tetraethylammonium chloride in cultured vascular smooth muscle cells

The change of cytosolic Ca²⁺ concentration ([ca²⁺]₁) caused by vasopressin was examined in indo-1-loaded A7r5 smooth muscle cells by use of the high-performance laser cytometer and ratiometric fluorescence method. Vasopressin (100 nM) caused an initial rapid rise and a delayed increase in [ca²⁺]₁) (n = 6). However, in the presence of tetraethylammonium chloride (10 mM), vasopressin consistently triggered sustained Ca²⁺ oscillations which were preceded by a large peak of [ca²⁺]₁). The latency for the development of this huge increase in [ca²⁺]₁) prior to the occurrence of sustained Ca²⁺ oscillations was always the same. The frequency and amplitude of this type of Ca²⁺ oscillation varied depending upon the extracellular Ca²⁺ concentration. Ca²⁺-free solution did not completely suppress the sustained Ca²⁺ oscillations, but caffeine (20 mM) effectively abolished them. The present findings indicate that in A7r5 smooth muscle cells, the sustained Ca²⁺ oscillations triggered by vasopressin in the presence of tetraethylammonium chloride were mainly due to Ca²⁺ release from 1P₃-sensitive Ca²⁺ stores and Ca²⁺ influx from extracellular space, and did not require the pacemaker activity derived from the surface membrane. Moreover, the vasopressin-induced change in [Ca²⁺]₁ appeared to be linked to pertussis toxin-insensitive GTP-binding protein(s).