In vivo evidence that an adenylate cyclase-camp system dilates cerebral arterioles in mice

Pial arterioles of living mice anesthetized with urethane were monitored by television microscopy. I tested the existence of an adenylate cyclase-cyclic adenosine monophosphate (cAMP) system for dilating the arterioles by topically applying the following drugs: Camp (10⁻³ M), its more potent analogue dibutyryl cAMP (10⁻³ and 10⁻⁴ M), and forskolin (10⁻⁶ M). Forskolin activates endogenous adenylate cyclase, which leads to increases in endogenous cAMP. Each drug was applied for 30 seconds; all three produced dilation. I then applied either cAMP or forskolin in the presence or absence of 10⁻⁴ M isobutylmethylxanthine (IMX), an inhibitor of endogenous phosphodiesterase, which destroys cAMP. The presence of IMX significantly potentiated the dilation produced by exogenous cAMP and forskolin. These data indicate that cerebral surface arterioles of mice respond to cAMP with dilation and contain the enzymes for producing and inactivating this dilator. The existence of an adenylate cyclase-cAMP dilating mechanism in pial arterioles does not rule out the simultaneous existence of other dilating mechanisms.