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In the present study we tried to determine the relative contributions of sarcoplasmic reticulum (SR) calcium release and gated calcium entry through voltage-dependent calcium-channels (VDC) to the contractions induced by norepinephrine (NE, 10 microM) and acetylcholine (ACh, 10 microM) in the vas deferens of the guinea pig. NE and ACh at 10 microM caused contractions composed of three phases: a prephasic, a phasic and a tonic component. Nifedipine strongly inhibited the contractions induced by potassium (K, 100 mM) and abolished phasic components of the contractions induced by NE or ACh. However, the tonic components of NE- or ACh-induced contractions were slightly attenuated by nifedipine. Pretreatment with ryanodine in combination with caffeine attenuated prephasic and tonic components of the contractions induced by NE and ACh in a dose-dependent manner. The inhibitory effects of ryanodine and caffeine on the phasic component of NE-induced contractions were divided into two types: 1) abolishment of the phasic contractions and 2) a delayed appearance of the phasic contractions without attenuation of magnitude. However, ryanodine with caffeine minimally affected phasic component by ACh- and K-induced contractions. Ryanodine pretreatment in combination with NE, ACh or K had no effect on the contractions induced by each agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

journal of pharmacobio-dynamics

Effects of ryanodine and nifedipine on contractions induced by norepinephrine, acetylcholine, and potassium in the isolated vas deferens of the guinea pig.