Local anaesthetic activity of organic calcium antagonists: relevance to their actions on smooth muscle

Local anaesthetic activity was measured on the rat phrenic nerve at 37 degrees C: Verapamil HCl had 5.1X and methoxyverapamil HCl had 3.0X the potency of lidocaine. Flunarizine was about equipotent with lidocaine, while nifedipine, diazoxide and sodium nitroprusside were without effect. It is concluded tht the smooth muscle inhibitory actions of the calcium antagonists, even when used in concentrations possessing local anaesthetic activity, are due to a reduction in Ca2+ permeability. In rat isolated vasa deferentia, lidocaine had both stimulant and inhibitory actions. On its own it induced rhythmic contractions (not blocked by phentolamine) and it also augmented the frequency of rhythmic contractions produced by methoxamine or barium. These actions may be due to block of K+ channels. Lidocaine also reduced the amplitude of methoxamine and barium induced contractions and both phases of the KCl contraction. The KCl response was restored by increasing "Ca2+]0, and it is therefore concluded that lidocaine was acting by blockade of Ca2+ channels.