The effects of calcium channel inhibitors and other procedures affecting calcium translocation on drug‐induced rhythmic contractions in the rat vas deferens

Abstract

1 In the rat isolated vas deferens, methoxamine 8.1 gM produced an initial phasic response that declined towards baseline and was followed by rhythmic contractions that continued until wash-out.These responses were predominant in the epididymal half. BaCl2 1 mM produced a similar type of response which was not mediated by noradrenaline release or activation of a-adrenoceptors. The barium responses were similar in the epididymal and prostatic halves.2 Incubation in nominally Ca2'-free solution caused abolition or near abolition of rhythmic contractions produced by barium or methoxamine. The initial phasic response to methoxamine was abolished in Ca2+-free solution, whereas that produced by barium persisted.3 Rhythmic contractions produced by methoxamine or barium were inhibited by Mg2+ (2.4-20mM) and by La3+ (1-5mM). Mg2+ had selectivity for inhibition of the frequency of methoxamine-but not barium-induced rhythmic contractions.4 Despite their dependence on [Ca]2+0, barium-and methoxamine-induced rhythmic contractions were resistant to inhibition by calcium channel inhibitors. Verapamil, nifedipine and flunarazine inhibited the amplitude of rhythmic contractions more readily than the frequency (methoxamine IC50 for verapamil: amplitude=29.8±5.40AM, n =6, frequency=96.7±31.O0AM, n =5, for nifedipine-amplitude = 2.42 ± 0.34 pM, n =7, frequency = 3.24 ± 0.75JAM, n =7, and for flunarizine: amplitude = 15.9 ± 5.95 JAM, n =7, frequency = 153 ± 28.6 JAM, n = 7). There was no differentiation between inhibition of methoxamine and barium-induced responses. 5 Like Mg2+, methoxyverapamil selectively inhibited the frequency of methoxamine-induced contractions (IC50: amplitude = 16.8 ± 2.86 gM, n = 5, frequency -2.07 ± 0.81 JiM, n = 5) but not barium-induced contractions (IC50: amplitude = 13.9 ± 1.95 JAM, n =5, frequency = 48.5 ± 8.98 JAM, n =5).6 Diazoxide (43.3-2 167 JM) and nitroprusside (3.36-6712 MM) had only a small effect on barium contractions, but produced a dose-related reduction in the amplitude of methoxamine-induced responses. Diazoxide and nitroprusside caused methoxamine contractions to occur in groups, although they had no effect on their overall frequency. 7 It is concluded that barium-and methoxamine-induced rhythmic contractions in the rat vas deferens are mediated by the entry of [Ca2+]0 via membrane calcium channels that have a lower affinity (10-100 x) for calcium channel inhibitors than those mediating the KC1 response. Channels activated by methoxamine are concentrated in the epididymal half, whereas those opened by barium are evenly distributed. However, although responses to methoxamine and barium are similar in form, differences in the effects of some of the drugs tested, together with the results of previous studies, indicate that they produce contractions by different mechanisms.

Inhibitory Action of Propranolol on the Contractions Induced by Nerve Stimulations or Calcium in the Smooth Muscle of Rat Vas Deferens

“…It has been reported in the rat vas deferens that sensitivity of high K-induced contractions to inhibition by verapamil and nifedipine was higher than that of NE-induced contractions (12,13). Furthermore, nitroprusside which did not affect K-contractions inhibited rhythmic contractions induced by an a receptor agonist, methoxamine (14). These results suggest the presence of potential and receptor-operated Ca channels in the rat vas deferens.…”

Dissection of SNARE-driven membrane fusion and neuroexocytosis by wedging small hydrophobic molecules into the SNARE zipper

“…4B, red bars). The three most competent compounds, delphinidin, cyanidin, and myricetin, showed 75-80% inhibition at 10 μM, comparable to the calcium channel blocker verapamil (21,22) and only slightly lower than the Clostridium botulinum neurotoxin D T A A A A A A A A A A A A A A A A A T A A A A A A A A A A A A A A A A A T A A A A A A A A A A A A A A A A A W T P2 (BoNT/D). This high-K + pretreatment experiment appeared to mirror the in vitro preincubation experiment in which SHMs were added before 4°C preincubation ( Fig.…”

A botulinum neurotoxin-like function ofPotentilla chinensisextract that inhibits neuronal SNARE complex formation, membrane fusion, neuroexocytosis, and muscle contraction

“…Verapamil, an l-type calcium channel blocker, was used as a positive control instead of VpS because the long peptide chain of VpS is thought to not be internalized into the cell. At 20 μM, verapamil inhibited neurotransmitter release by approximately 70% in our experimental setting (Hay & Wadsworth, 1983;Richardson et al, 1996). The most potent inhibitors (Potentilla chinensis and Castanea crenata) blocked neurotransmitter release up to 60-85%, which is comparable to verapamil.…”