Parathyroid hormone related peptide (PTHrP) - a potent relaxatory agent in rat, guinea-pig and human tissues

Sonia Roberts1, Mark Shaxted1, Wei Xun1, Carol Austin1, John Rennie2 Robert Hull1. 1James Black Foundation, Dulwich, London. U.K., 2Kings College School of Medicine and Dentistry, Denmark Hill, London, U.K.

PTH and PTHrP are reported to relax smooth muscle preparations obtained from rat (Mok et al., 1987), guinea-pig (Botella et al., 1994) and man (Chadwick et al., 2000). These studies aim to identify suitable isolated tissue assays which would enable quantitative estimates of the activity of novel PTH1-R ligands to be determined.

PTH1-34 and PTHrP1-34 (10pM-0.1 μM)-mediated effects were examined in organ bath studies with isolated tissue preparations from rat (Wistar, male, 250-400g) and guinea-pig (gp) (Dunkin Hartley, male, 350-500g). The effect of PTHrP (0.1 μM) was also examined in several specimens of human smooth muscle tissue which were obtained from routine gastric surgery following ethical approval and informed patient consent. The tissues selected for assay development were determined on the basis of species, agonist potency and robustness of agonist response.

Preliminary studies identified that PTHrP (0.1 μM) produced measurable relaxation of some pre-contracted (5-methyl furmethide), smooth muscle preparations obtained from rat, gp and man; whilst in gp atria a positive chrontropic response was produced. More extensive studies with tissues selected from both the rat (anococcygeus, trachea, fundus) and gp (ileum and atria) indicated that cumulative doses of PTHrP (0.01nM-0.1 μM) produced concentration-dependent responses in these tissues (Table 1). In the gp ileum and atria and rat anococcygeus the PTH1-receptor selective antagonist, Nle30-TIP7-39 (1μM), inhibited PTHrP-mediated responses (Table 1). The PTH2-receptor agonist, TIP1-39 ( 1 n M-1μM) , was inactive in these assays.

Table 1. Potency of PTHrP concentration-response curves and affinity estimates for Nle30-TIP7-39 in isolated tissues from the guinea-pig and rat

In the rat and gp, PTHrP produced potent relaxation of pre-contracted smooth muscle. The variable responsiveness of human tissue makes it less suitable for screening purposes but clinical relevance supports its use for examining key compounds. The antagonism of PTHrP-mediated effects by Nle30-TIP7-39 (1μM) (pA2~6) confirms the presence of PTH1-receptors in these assays. The lack of agonist response with TIP1-39 indicates that, if present, PTH 2-receptors are not coupled to either smooth muscle relaxation or heart rate in these tissues.

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Chadwick, D.R. et al . (2000). Br. J. Surgery, 87: 1529-1533.