It has been known that the excitatory response of the detrusor to pelvic nerve stimulation is largely atropine resistant and mediated by non-cholinergic, non-adrenergic excitatory nerves.

Recently, 5-hydroxytryptamine(5-HT) and vasoactive intestinal polypeptide (VIP) have been suggested as possible non-cholinergic, non-adrenergic inhibitory transmitter in the urinary bladder of the pig and of man.

VIP, a 28-amino acid peptide isolated from porcine duodenum, has been detected by immunocytochemistry and radioimmunoassay in many parts of the peripheral as well as central nervous systems.

The effect of VIP on urinary tract smooth muscle has been well documented, but the results are still conflicting. It may produced relaxation or contraction, or be inactive.

In this study, the effect of VIP was investigated, using the isolated rabbit urinary bladder strip with special reference to mobilization of Ca²⁺ and K⁺ ions.

Bladder strips of 5-10 mm long were carefully prepared from dome and trigone of the rabbit and were suspended in an organ bath containing 1 ml of Tyrode solution maintatined at 37^oC. Tyrode solution was aerated with 95% oxygen and 5% carbon dioxide. One end of the bladder strip was attached at the bottom of the bath and the other end to the force displacement transducer (FT. 03, Grass, USA).

When a stable tension level of the strip had been attained. drugs were added to the organ bath and the change of motility of the strip was recorded on a Polygraph (Grass Model 7, USA).

The results obtained were as follows;

1) VIP inhibited the spontaneous contraction of detrusor and trigone strips of rabbit bladder.

2) VIP exerted relaxation of detrusor strip contraction induced by cholinergic drugs.

3) The contraction of trigone strip induced by adrenergic drugs was relaxed by VIP.

4) In calcium-free high-potassium depolarizing solution, VIP exerted a relaxing effect on detrusor and trigone strip contraction induced by CaCl₂.

5) VIP exerted a relaxing effect on detrusor and trigone strip contraction induced by KCI.

From the above results, it was concluded that VIP inhibits the spontaneous motility as well as cholinergic- or adrenergic- induced contraction, and that these effects were associated with the inhibition of Ca⁺² influx and K⁺-channel opening.