We investigated differences in the density of muscarinic receptors between emptied and distended canine bladders after urinary diversion. Adult female mongrel dogs were used. The experimental groups were as follows: 1) control bladders (n= 8), 2) disused empty bladders (n= 9), 3) disused bladders, distended with 100ml saline (n= 5). Disuse was surgically achieved by bilateral ureterocutaneostomy. One week postoperatively, the bladder was removed and separated into body and base at the level of

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... he ureteral orifices. A homogenate of each specimen was incubated with 10 different concentrations of 3H-quinuclidinyl benzilate. The maximal number of binding sites (Bmax), i.e. the receptor density, was derived by Scatchard analysis. Compared to the controls, the density of muscarinic receptors in disused bladder body decreased significantly in the bladders which had been kept empty, but increased in those distended with 100ml saline (p <0.05). In bladder base, there was no significant difference in the receptor density between the controls and the emptied bladders, though the density increased significantly in distended bladders (p <0.05). The bladder body contained a significantly greater number of muscarinic receptors than the bladder base in all experimental groups. We conclude that when filling and voiding cycles cease, tension on the smooth muscle governs the density of muscarinic receptors: loss of tension decreases the receptor density and maintenance of tension increases it.