Spontaneous oscillations and agonist-evoked changes in Ca2+ in cultured smooth muscle cells.

In confluent A10 aortic smooth muscle cells, at 37 degrees C and in the presence of 1 mM extracellular Ca2+ ([Ca2+]o), the resting intracellular free calcium [Ca2+]i) was found to be 111 +/- 6 nM (n = 17), and in single cells it was 82.4 +/- 7.6 nM. In the presence of 1 mM EGTA, in confluent cells, resting [Ca2+]i was 88.1 +/- 11 nM. The confluent cells showed spontaneous transients and elevations in [( Ca2+]i) as well as maintained oscillations. The oscillations had a periodicity of 6-9 s and were not present in single cells. One nanomolar and 2 microM vasopressin elevated [Ca2+]i to between 0.7 and 3 microM. In the presence of extracellular Ca2+, the initial transient was followed by a plateau phase that declined slowly; in EGTA, this plateau phase was not present but could be restored by adding back Ca2+. Although A10 cells show spontaneous action potentials, oscillations in [Ca2+]i have not been reported. The plateau phase of the vasopressin response appears to be mediated by Ca2+ influx, whereas the initial transient is consistent with the release of stored Ca2+.