Induction of Ca2+ Oscillations by Vasopressin in the Presence of Tetraethylammonium Chloride in Cultured Vascular Smooth Muscle Cells

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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 (10mM), 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 (20mM) 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 IP3-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).
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