Cooperation of ATP and norepinephrine in inducing contraction in guinea pig vas deferens is not associated with change in intracellular Ca2+ level

Contractile responses and fura-2 fluorescence signals were simultaneously recorded in fura-2 loaded longitudinal muscle strips of guinea pig vas deferens to examine the relationship between cooperation of ATP and norepinephrine (NE) in the contractile response and intracellular Ca2+ level. ATP or NE induced a rapid or delayed contraction, respectively, with concomitant changes in fura-2 signal. Addition of both agonists potentiated the rapid, but not delayed contraction, while there was no potentiation in the fura-2 signal. In another series of experiments, NE (greater than or equal to 10 mu M) contracted the muscle without further concomitant increase in Ca2+ level, indicating ''Ca2+ sensitization'' of the contractile apparatus. These findings suggest that cooperation of ATP and NE in inducing a rapid contraction of guinea pig vas deferens is mainly due to the ''Ca2+ sensitization'' effect of NE.