The relaxation effects of forskolin and methylxanthines
on noradrenaline (NA)-induced contractions were
investigated by measuring isotonic contraction and
intracellular calcium concentration ([Ca2+]i) in the
epididymal side of guinea-pig vas deferens. NA (100
µM) and high K+ (55 mM) induced a biphasic
contraction; fast, transient (phasic) and slow, sustained (tonic) phases.
Both phases in either NA or high K+ stimulation were abolished in
Ca2+-free solution. Pretreatment with 10 µM nifedipine, an L-type Ca2+
channel blocker, reduced both phasic and tonic contractions induced by
high K+. In the case of NA-induced contraction, however, nifedipine
reduced the phasic contraction but not the tonic contraction. The
nifedipine-insensitive tonic contraction was relaxed by the application of
polyvalent cations (Mn2+, Co2+, Cd2+ and La3+). These findings
indicate that NA-induced biphasic contraction is mainly due to
nifedipine-insensitive Ca2+ influx, especially in the tonic phase. Cyclic
AMP-increasing agents such as forskolin (0.5-10 µM), IBMX (5-500
µM) and caffeine (1-20 mM) relaxed the NA-induced contraction
extensively in a concentration-dependent manner. However, these
agents only partially relaxed the high K+-induced contraction.
Forskolin (10 µM) and IBMX (100 µM) reduced the [Ca2+]i response to
NA, but had no effect on the [Ca2+]i response to high K+. These results
suggest that an increase in intracellular cAMP may relax the
NA-induced contraction by attenuating a nifedipine-insensitive Ca2+
influx and by a mechanism independent of a reduction in [Ca2+]i.