Rabbit vas deferens

FIGURE 6. Affinities (Jvl2 values) of the (/ ) and (.V) enantiomers of 11 and 13 for muscarinic Ml (rabbit vas deferens), M2 (guinea-pig atria) and M3 receptors (guinea-pig ileum)... 

The involvement of releasable enzymes in the metabolism of extracellular nucleotides has been suggested to explain an increase in the capacity to metabolize ATP observed in some tissues following nerve stimulation (Todorov et al. 1996 Mihaylova-Todorova et al. 2002). Released enzymes resembling the E-NTPDases mentioned above seem to be able to convert released ATP to AMP. In some tissues (guinea pig but not rabbit vas deferens), the released enzymes also hydrolyse AMP into adenosine (see Westfall et al. 2002). 

From these data it becomes evident that the effects of McN-A-343 and of most of its analogues differ clearly in the three functional assays. McN-A-343 and its 4-chloro derivative had comparable potency in rabbit vas deferens, whereas in both atria and ileum they behaved as partial or full agonists, being more potent and more efficacious in the M3 assay. The incorporation of chirality into this series of acetylenic McN-A-343 derivatives had a beneficial effect upon the potency (affinity) and muscarinic receptor subtype-selectivity, the (S)-enantiomers consistently showing higher activity than the corresponding (R)-isomers. 

In ntro fiinaional activity of McN-A-343 analogues at Mj receptors in rabbit vas deferens (RVD), M2 receptors in guinea-pig atria (GPA) and M3 receptors in guinea-pig ileum (GPI). Abbreviations for the muscarinic agents are explained in the legend of Figure 1... 

Figure 2. Inhibitory potency of suramin and of three of its analogues (for chemical structure, see Figure 1) at P2X P2Y"Purinoceptors. — Inhibition of neurogenic contractions (0.05 Hz plCfo values) in rabbit vas deferens (P2X-receptors). -- Inhibition of ADP(3 binding (pKj values) to turkey erythrocyte membranes (P2Y- "sceptors) [35].

Rabbit vas deferens Rat mesentery Guinea-pig taenia coli Rat bladder... 

These results show that the phosphate and aldehyde moieties of pyridoxal-5-phosphate and its analogues are required for antagonist activity at P2X Punnoceptors in rabbit vas deferens. Among this series of compounds, PPADS was found to be the most interesting P2-antagonist, and it was therefore the subject of a more detailed pharmacological investigation. 

The time course of action of PPADS at 5 pM, a concentration which inhibited the effects of endogenous ATP by about 70%, was examined in the field-stimulated rabbit vas deferens. In these experiments, PPADS acted as a slowly-equilibrating and a slowly-reversible P2X purinoceptor antagonist. It was found to reach apparent equilibrium in about 120 min, and the tissue took about 90 min to regain its control twitch height by continual washout of PPADS. Importantly, these findings indicate that PPADS is not an irreversible antagonist at P2X Purinoceptors in rabbit vas deferens. 

Relaxant responses to 2-methylthio ATP in the carbachol-contracted rat duodenum were antagonized by PPADS (10 - 100 pM) in a concentration-related manner. The resulting Schild plot was linear and its slope (1.02) was not significantly difiTerent fi-om unity, indicating competitive antagonism. The affinity estimate for PPADS in rat duodenum (pA2 = 5.09 Table 3) was very similar to that obtained for pyridoxal-5-phosphate in guinea-pig aorta (5.39), but clearly lower compared to the apparent pKg values obtained for the P2X subtype in rabbit vas deferens (6.34) and rat mesenteric artery (6.38 Table 3). In rat mesenteric arterial bed, the tone of which was raised by methoxamine, vasodilator responses to 2-methylthio ATP were slightly inhibited by 10 pM PPADS (pA2 = 5.46), a concentration which virtually abolished P2X punnoceptor-mediated vasoconstriction. It is noteworthy that PPADS (30 pM) was found to be ineffective at vascular P2Y-punnoceptors in rabbit mesenteric artery and aorta. These results clearly demonstrate that PPADS is less effective at duodenal and vascular P2Y purinoceptors than at P2X-receptors (Table 3). 

The characterization of the in vitro pharmacological profile of SR 120819A is illustrated here in two relevant well-known Y1-receptor preparations the human neuroblastoma SK-N-MC cell line (Fuhlendorff el al., 1990 Feth et al., 1991) and the rabbit vas deferens (Doods and Krause, 1991). 

Figure 5 Inhibitory effect of SR 120819A on the electrically stimulated rabbit vas deferens. Cumulative concentration-response curves for [Leu31,Pro34]NPY on the amplitude of twitch contractions elicited by electrical field stimulation were established in the absence ( ) or in the presence of 0.1 (A), 0.3 ( ) and 1 (T) p.M of SR 120819A. Results are expressed as a percentage of the twitch control responses measured after 45 min incubation with or without SR 120819A, and are the mean SEM of 5-6 experiments.

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