Spontaneous oscillations in cytoplasmic calcium concentration in vascular smooth muscle

Fluorescence measurement of fura-2 and quin2 signals from confluent primary cultures of serum-deprived rat aortic smooth muscle cells have revealed spontaneous oscillations in intracellular calcium concentration ([Ca2+]i). The transients consist of a rapid increase in [Ca2+]i that averages 60 nM and lasts approximately 30 s. They are caused by intracellular calcium release and an influx of extracellular calcium. Exposure of cells to the calcium-channel antagonists verapamil and diltiazem or incubation in nominally calcium-free medium reduced both the duration and amplitude of the transients; in contrast, the calcium-channel agonist (-)BAY K 8644 increased their duration. The transients were abolished by caffeine and 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate, agents that interfere with calcium release from the sarcoplasmic reticulum. These findings demonstrate that the sarcoplasmic reticulum is a primary source for the spontaneous oscillations in cytoplasmic calcium and is closely associated with the influx of extracellular calcium. Although the function of these transients is unclear, they may be involved in the spontaneous contractions observed in some vessels and in the regulation of vascular resistance.