Cooperation of ATP and Norepinephrine in Inducing Contractionin Guinea Pig Vas Deferens Is Not Associated with Change in Intracellular Ca²⁺ Level

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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 (≥10 μM) 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.