Effect of calcium antagonists on the response to noradrenaline in the whole and bisected rat vas deferens C. Boselli, L. Bianchi, A. Barbieri & E. Grana Istituto di Farmacologia, Facolta Á di Farmacia, Universita Á di Pavia, Viale Taramelli, 14, 27100 Pavia, Italy Summary 1 The present study was carried out to look at the effect of different calcium antagonists on the response to noradrenaline in the whole and bisected rat vas deferens considering that the response consisted of three components (I) the phasic response (II) the tonic response and (III) the spikes (rhythmic contractions). 2 Nifedipine (3 ´ 10 ±9 ±1 ´ 10 ±7 m) inhibited all the components at the same concentration range, verapamil (1 ´ 10 ±7 ±1 ´ 10 ±5 m) inhibited the phasic and tonic response but not the rhythmic activity. This latter component, at a certain concentration range and especially in the prostatic portion was markedly potentiated. Diltiazem and flunarizine lay in an intermediate position. 3 Papaverine, a Ca 2+ antagonist that acts mainly intracellularly, inhibited preferentially the tonic component; ryanodine was practically inactive. 4 Cromakalim inhibited only partially the phasic and tonic components but totally inhibited the rhythmic contractions. 5 These results can be explained by postulating two types of calcium channels opened by a- adrenoceptor stimulation. The first one is verapamil- and nifedipine-sensitive and allows the entry of Ca 2+ directly available for the contraction and responsible for the phasic and partially responsible for the tonic component. The second channel is merely nifedipine-sensitive and allows the entry of Ca 2+ trigger which can release Ca 2+ from intracellular sites: the mobilized Ca 2+ is able to sustain the tonic component and is the main one responsible for the rhythmic activity. There is the possibility that this second channel is associated with ATP-sensitive K + channels. Introduction In our previous study (Boselli, Bianchi & Grana, 1997), three components of the response to exogenous noradrenaline of the rat vas deferens were identified, i.e. an early rapid phasic component followed by a second slower tonic component with spikes super- imposed. The time course of these components, as well as their ratio, were different depending on which half of the vas deferens was considered. The sources of calcium mobilized by noradrenaline upon activation of the a-adrenoceptors remain uncertain (for general review see Ruffolo, Nichols, Stadel & Hieble, 1991; Minneman & Eshenshade, 1994). Most of the work in this field has been carried out with functional experiments based on the employment of calcium antagonists. In the current literature some findings seem to be conflicting possibly because insufficient attention has been paid to the features of the response to noradrenaline that we have identified. Hay & Wadsworth (1983) studied the effects of verapamil and nifedipine on twitches and noradrenaline con- tractions of the rat bisected vas deferens and concluded that the calcium channel inhibitors can distinguish between three distinct types of calcium ion channel. Amobi & Smith (1986) demonstrated that in the epididymal portion nifedipine (10 ±5 m) abolished the noradrenaline-induced contractions and sponta- neous activity. In contrast, verapamil (10 ±5 m) abol- ished noradrenaline-induced contraction but not the spontaneous activity. Some findings suggested that noradrenaline-induced contractions may be related to the release of intracel- lular Ca 2+ in addition to the influx of extracellular Ca 2+ especially in the epididymal site (Nagata, Saito & Matsuki, 1991) or that, in prostatic portion, the phasic component is related to the release of intracellular Ca 2+ whereas the tonic component of the contraction involves calcium influx via membrane channels sensitive to calcium channel blockers (Ves- perinas, Feddersen, Lewin & Huidobro-Toro, 1989). Recently it has been proposed that the contraction of the rat (epididymal) vas deferens mediated by a 1A - adrenoceptors is dependent upon activation of protein kinase C by diacylglycerol, resulting in the influx of extracellular Ca 2+ through voltage-gated Ca 2+ chan- Correspondence: Enzo Grana ã 1998 Blackwell Science Ltd 297 Journal of Autonomic Pharmacology, 18, 297±306