Changes in cytosolic Ca2+ concentration ([Ca2+](i)) and in membrane po
tential were monitored in single A7r5 smooth-muscle cells during spont
aneous spiking and after arginine vasopressin stimulation. Spontaneous
Ca2+ oscillations, which were associated with the occurrence of actio
n potentials, occurred in about 90% of the confluent monolayers invest
igated. This spontaneous activity was synchronized amongst all the cel
ls of the monolayer, indicating that the cells were electrically coupl
ed. Arginine vasopressin stimulation produced a [Ca2+](i) rise that wa
s about 5 times higher than the amplitude of the spontaneous Ca2+ osci
llations and resulted in a subsequent cessation of spontaneous electri
cal activity and associated Ca2+ spiking, which persisted after [Ca2+]
(i) returned to baseline. Individual cells in the monolayer responded
to arginine vasopressin with a different latency. Agonist-induced Ca2 waves within one cell propagated much more slowly than spontaneous [C
a2+](i) rises. We conclude that agonist-induced [Ca2+](i) increases in
an electrically coupled cell monolayer can be asynchronous.