Inhibition of field stimulation-induced contractions of rabbit vas deferens by muscarinic receptor agonists: selectivity of McN-A-343 for M-1 receptors

Inhibition of the field stimulation-induced twitch responses of the rabbit vas deferens by the muscarinic receptor agonist, McN-A-343, has been attrib uted to presynaptic muscarinic receptors of the M-1 subtype located on nora drenergic nerve terminals. Stimulation of these receptors causes inhibition of transmitter release and inhibition of the contractile response. However , the selectivity of McN-A-343 for M-1 receptors has been questioned and th is throws doubt on whether the prejunctional receptors of the rabbit vas de ferens are of the M-1 subtype. In this study we have undertaken a comprehen sive re-evaluation of the inhibition of prostatic a nd epididymal portions of the rabbit isolated field-stimulated vas deferens by several agonists, i ncluding McN-A-343, and quantified the antagonism by M-1-selective antagoni sts, pirenzepine and telenzepine. Prostatic and epididymal portions of vasa deferentia from New Zealand White rabbits were immersed in a low Ca2+ Krebs solution at 32 +/-0.5 degreesC g assed with 5% CO2 in oxygen. Yohimbine (1.0mM) was present throughout to bl ock prejunctional alpha (2)-adrenoceptors. Field stimulation was applied by repeated application of single pulses (30 V, 0.05 Hz, 0.5 ms) and isometri c contractions recorded. Carbachol and oxotremorine initially potentiated t he epididymal contractions but at higher concentrations there was inhibitio n. In the prostatic portion, oxotremorine only inhibited. McN-A-343 produce d inhibitory responses only in both epididymal and prostatic portions. Pire nzepine shifted the concentration-response curves for th e inhibitory respo nses to oxotremorine to the rig ht. However, the potentiation of the twitch es a Iso became more apparent with the lower concentrations of oxotremorine . Schild plots for the antagonism by pirenzepine yielded pA(2) values of 7. 96 +/-0.004 and 7.7 +/-0.02 for the epididymal and prostatic portions, resp ectively. The concentration-response curves for the inhibition of twitches by McN-A-343 were displaced to the right in a parallel manner by pirenzepin e in both prostatic and epididymal portions with no potentiation of the twi tches. The Schild plot for this antagonism generated pA(2) values of 7.68 /-0.01 and 8.07 +/-0.01, respectively. Telenzepine caused parallel shifts o f the McN-A-343 concentration-response curves to the right in prostatic por tions, the pA(2) value being 8.70 +/-0.13. Telenzepine (10(-7)M) abolished the inhibitory effect of carbachol to reveal only concentration-dependent p otentiation of the contractions. The Schild plot for antagonism of this con tractile effect yielded a pA(2) value (7.07 +/-0.09) that was significantly less by almost two orders of magnitude (1.70) than the value for the antag onism by telenzepine of the McN-A-343-induced inhibitory response. The pA(2) values of pirenzepine and telenzepine against the inhibitory resp onses of the rabbit vas deferens are consistent with the involvement of M-1 receptors, This reads to the conclusion that McN-A-343 causes inhibition t hrough this receptor type. The doubts concerning the selectivity of McN-A-3 43 for M-1 receptors are therefore unfounded. The fact that McN-A-343 does not display a selective binding profile suggests that its selectivity does not arise from affinity differences but probably resides in its intrinsic e fficacy.