The mechanism(s) involved in determining the voiding cycle of the rat urinary bladder have been investigated in urethan-anesthetized animals. Fluid emission is almost confined to that phase of the voiding cycle which is characterized by the presence of a series of high-frequency oscillations in intraluminal pressure (IPHFO). During this phase the mean urethral flow rate reached a maximum and fluid was expelled in a stream-like fashion. The index obtained by multiplying the amplitude of IPHFO by their duration was significantly related to the maximal value of urethral flow rate. The IPHFO were selectively abolished by administration of d-tubocurarine at a dose that barely affects detrusor contractility. Moreover, d-tubocurarine reduced mean urethral flow rate and increased residual volume. The reflex (hexamethonium sensitive) mechanism(s) responsible for the generation of IPHFO is more developed in male than female rats. This mechanism, which involves activation of skeletal muscle, plays a significant role in determining bladder voiding in this species.

In the rat isolated urinary bladder, NaHS (30 mM-3 mM) and capsaicin (10 nM-3 mM) produced concentration-dependent contractile responses (pEC 50 ¼ 3.570.02 and 7.170.02, respectively) undergoing dramatic tachyphylaxis. In preparations in which sensory nerves were rendered desensitized (defunctionalized) by high-capsaicin (10 mM for 15 min) pretreatment, neither capsaicin itself nor NaHS produced any motor effect. NaHS-induced contractile effects were totally prevented by the simultaneous incubation with tachykinin NK 1 (GR 82334; 10 mM) and NK 2 (nepadutant; 0.3 mM) receptor-selective antagonists. Tetrodotoxin (1 mM) only partially reduced the response to NaHS. These results provide pharmacological evidence that H 2 S stimulates capsaicin-sensitive primary afferent nerve terminals, from which tachykinins are released to produce the observed contraction by activating NK 1 and NK 2 receptors. While the molecular site of action of H 2 S remains to be investigated, our discovery may have important physiological significance since H 2 S concentrations capable of stimulating sensory nerves overlap those occurring in mammalian tissues under normal conditions.

In this study we describe the ability of two human urotensin-II (hU-II) derivatives [Pen 5 , .04, n ¼ 4, respectively). To our knowledge, urantide is the most potent UT receptor antagonist so far described, and might represent a useful tool for exploring the (patho)physiological role of hU-II in the mammalian cardiovascular system.