Transients in intracellular free calcium in subconfluent and confluent cultures of a rat smooth muscle cell line.

The Ca2+ mobilizing mechanisms in the smooth muscle cell line A7r5 were found to undergo changes related to the degree of confluence of the cultures. In sparse cultures resting calcium was stable and exposure to arginine vasopressin (AVP) resulted in a single transient increase in intracellular free calcium (Ca2+i). In confluent cultures the cells could be divided into two general groups, those with a stable resting Ca2+i and those which demonstrated spontaneous brief elevations in Ca2+i of variable frequency. Application of AVP elevated Ca2+i, induced oscillations in quiescent confluent cells, increased the frequency of oscillatory activity in cells which were already active and, in cells which exhibited high frequency spontaneous fluctuations, inhibited this activity. Isotonic K+ depolarizing solution and normal solutions containing Co2+ inhibited Ca2+ spikes. These data suggest that the mechanism underlying the transients involves cyclical electrical phenomena at the cell membrane possibly utilizing calcium channels. There is no indication that the mechanism involves cytoplasmic oscillators.