Separation of putative α_1A‐ and α_1B‐ adrenoceptor mediated components in the tension response of the rat vas deferens to electrical field stimulation

Abstract

1. The effects of the putative alpha 1B-adrenoceptor antagonist, chloroethylchlonidine (CEC), on tension responses of the rat isolated whole vas deferens to single and multiple pulses of electrical field stimulation have been evaluated by use of a microcomputer system which enables the averaging of like-responses throughout their time course. 2. CEC (10(-7) to 3 x 10(-6) M) selectively and in a concentration-dependent manner blocked the noradrenergic component of the response to a single field stimulus in the absence or presence of nifedipine (10(-5) M, which blocked the purinergic but not the noradrenergic component of the response). The concentration-response curve of the vas to exogenously-applied noradrenaline (NA) was unaffected by CEC (10(-6) M) but was flattened by nifedipine (10(-5) M). 3. The tension response to 10 Hz trains of pulses was biphasic, with an early (less than 2 s) and a plateau (greater than 4 s) phase. We deduce from our pharmacological analysis that the early phase contains a putative alpha 1B-adrenoceptor component (susceptible to CEC or prazosin but not to nifedipine) and a P2-purinoceptor component (susceptible to suramin or nifedipine) whereas the plateau phase contains an alpha 1A-adrenoceptor component (susceptible to prazosin or nifedipine but not to CEC) and a P2-purinoceptor component (susceptible to suramin or nifedipine). 4. We suggest that the putative alpha 1B-adrenoceptors may be functionally confined to the synaptic region whereas the putative alpha 1A-adrenoceptors are excluded from this region. Trains of pulses would allow NA to accumulate and spill out beyond the synaptic region to reach and activate the putative alphalA-adrenoceptors.