Smooth-muscle effects of substance P.

The effects of substance P (SP) on smooth-muscle contractions induced by noradrenaline (NA) or by high- or low-frequency electrical stimulation (ES) were studied on isolated rat vas deferens and musculus anococcygeus. Neither the contractile effects of NA nor those of high-frequency ES were significantly changed by SP. The peptide in concentrations of 1 x 10(-7) M and 1 x 10(-6) M strongly potentiated (up to 70%) vas deferens contractions in response to low-frequency ES but slightly reduced those of musculus anococcygeus. Inhibition of the opiate receptors by 1 x 10(-6) M naloxone did not change the potentiating SP-effect on vas deferens contractions. Blockade of the prostaglandin (PG) receptors by polyphloretin phosphate (PPP), 5 micrograms/ml, or of the PGs synthesis by indomethacin, 5 x 10(-6) M, resulted in a slight increase of the SP-potentiating effect. The present results suggest that specific presynaptic SP-receptors (excitatory or inhibitory) play a major role in the mechanism of the potentiating action of substance P.