Spontaneous Ca2+ spiking in a vascular smooth muscle cell line is independent of the release of intracellular Ca2+ stores.

Monolayers of fura-2-loaded A7r5 cells, a cell line derived from rat embryonic aorta, generated spontaneous Ca2+ spikes that were synchronized within the cell population. These Ca2+ spikes were abolished by removal of extracellular Ca2+ or addition of nimodipine (50 nM), and their frequency was increased by depolarization with high K+ or by treatment with BAYK 8644 (1 microM), indicating that Ca2+ entry through L-type Ca2+ channels is required for Ca2+ spiking. Several lines of evidence indicate that mobilization of intracellular Ca2+ stores is not necessary for this Ca2+ spiking. 1) Ryanodine (0.1-50 microM) neither stimulated Ca2+ mobilization nor affected Ca2+ spiking; 2) the complex effects of caffeine were mimicked by theophylline, 8-bromo-cyclic adenosine 3':5'-monophosphate (8-bromo-cAMP), and forskolin, suggesting that the caffeine effects may be mediated by cAMP and not by ryanodine receptors; 3) prolonged incubation with thapsigargin (50 nM), which depletes intracellular Ca2+ stores, did not affect the frequency of Ca2+ spiking; 4) Ba2+ or Sr2+ could substitute for Ca2+ in the spike-generating mechanism even when intracellular stores were depleted of Ca2+. Under conditions where the sarcoplasmic reticulum (SR) contained Ca2+, Ba2+ spikes did not cause Ca2+ mobilization. The mechanisms involved in generating spontaneous Ca2+ spiking in A7r5 cells are therefore likely to reside in the sarcolemma and to operate independently of SR Ca2+ uptake and release.