Substance P and somatostatin and excitatory neurotransmission in rabbit urinary bladder

The influence of substance P (SP) and somatostatin (SS) on contractions induced by electrical field stimulation, ATP and carbachol were investigated in isolated rabbit urinary bladder. Some experiments were also carried out in rat and guinea-pig detrusor. When added cumulatively to rabbit and rat preparations, SP and SS caused a gradual increase in tone of the preparations. There were no effects on the electrically induced contractions. In rabbit and rat detrusor pretreated with guanethidine, atropine and indomethacin, the contractile response to nerve stimulation was markedly reduced. In this situation SP and SS caused 2-4 fold increase in the electrically induced contractions. Indomethacin abolished the tonic response to ATP in the rabbit detrusor, and reduced the initial phasic contraction by about 30%. Both SP and SS were able to restore the phasic contraction to the level obtained before indomethacin addition. The contractions induced by carbachol in rabbit and rat detrusor were not affected by SP and SS, not even in indomethacin pretreated preparations. The SP-antagonist (D-Pro2, D-Phe, D-Trp9)-SP was tested in isolated rabbit, rat, and guinea-pig detrusor. Concentrations of the antagonist which effectively inhibited contractions induced by a submaximum concentration of SP had no effect on the contractile response to electrical field stimulation in rabbit and guinea-pig preparations, and rather tended to increase the contractions in the rat detrusor. The results suggest that neither SS nor SP is the mediator of excitatory non-cholinergic, non-adrenergic activation of rabbit urinary bladder. A role as local modulators of neuromuscular transmission cannot be excluded.