Studies on binding sites, contents, and effects of AVP in isolated bladder and urethra from rabbits and humans.

The binding sites, contents, and effects of arginine vasopressin (AVP) were studied in isolated bladder and urethral preparations from rabbits and humans. In all tissues, higher levels of AVP-like immunoreactivity (AVP-LI) were detected than those normally found in plasma. Radioligand membrane binding studies using [3H]AVP as the ligand revealed the existence of a single population of binding sites in the rabbit bladder, and displacement experiments indicated that the receptor was of the V1 subtype. By autoradiography, [3H]AVP binding sites in the rabbit bladder were shown to be located on both circularly and longitudinally oriented smooth muscle cells, as well as in the submucosa at the part adjacent to the urothelium. In the rabbit urethra, the binding sites were confined mainly to the circular smooth muscle layer. Neither radioligand membrane binding studies nor autoradiography revealed any specific [3H]AVP binding sites in the human bladder. AVP contracted rabbit bladder and urethral preparations concentration dependently. The contractions were inhibited by the V1-receptor selective antagonist A16 in a noncompetitive manner. However, A16 had no effects on contractions elicited by electrical-field stimulation. In preparations of the human bladder and urethra, AVP in concentrations up to 10(-5) M did not have any contractile effects. These results suggest that in the rabbit and human lower urinary tract, AVP-LI is synthesized locally and/or extracted from the circulation. It is unlikely that AVP is directly involved in the neurotransmission in these tissues, although in the rabbit bladder and urethra a modulatory role cannot be excluded.