Differences between the abilities of tegaserod and motilin receptor agonists to stimulate gastric motility in vitro

Abstract

Background and purpose: Motilin or 5-HT4 receptor agonists stimulate gastrointestinal motility. Differences in activity are suggested but direct comparisons are few. A method was devised to directly compare the gastric prokinetic activities of motilin, the motilin receptor agonist, erythromycin, and the 5-HT4 receptor agonist, tegaserod.

Experimental approach: Gastric prokinetic-like activity was assessed by measuring the ability to facilitate cholinergically-mediated contractions evoked by electrical field stimulation (EFS) in rabbit isolated stomach. Comparisons were made between potency, maximal activity and duration of responses.

Key results: Rabbit motilin (r.motilin) 0.003-0.3 microM, [Nle13]motilin 0.003-0.3 microM, erythromycin 0.3-10 microM and tegaserod 0.1-10 microM caused concentration - dependent potentiation of EFS-evoked contractions. The potency ranking was r.motilin = [Nle13]motilin > tegaserod > erythromycin. The Emax ranking was r.motilin = [Nle13]motilin = erythromycin > tegaserod. Responses to r.motilin and [Nle13]motilin faded rapidly (t1/2 9 and 11 min, respectively) whereas those to erythromycin and tegaserod were maintained longer (t1/2 24 and 28 min). The difference did not appear to be due to peptide degradation. A second application of [Nle13]motilin was excitatory after 60 min contact and fade of the initial response (responses to 0.03 and 0.1 microM [Nle13]motilin were not different from those caused by the first application).

Conclusions and implications: Prokinetic-like activities of the 5-HT4 agonist tegaserod and the motilin receptor agonists were compared by measuring changes in cholinergically-mediated contractions. This novel approach highlighted important differences between classes (greater Emax of motilin, compared with tegaserod) and for the first time, within each class (short t1/2 for motilin, compared with erythromycin).

Figure 1

Representative traces showing the effects of r.motilin, [Nle¹³]motilin, erythromycin and tegaserod on EFS-evoked, nerve-mediated contractions in rabbit isolated gastric antrum circular muscle preparations. EFS (0.5 ms width, 50 V for 30 s, every 1 min) was applied using a fixed frequency (1–14 Hz) that varied between tissues so that the muscle tension generated was greater than 0.1 g but less than 1 g.

Figure 2

The abilities of r.motilin, [Nle¹³]motilin, erythromycin and tegaserod to facilitate cholinergically-mediated contractions in rabbit isolated gastric antrum circular muscle preparations, measured over a 60 min period. Following addition of the compound the averages of three consecutive responses to EFS were measured over the 60 min period to give 20 separate data points. Changes in the responses to EFS were then expressed as a percentage of the mean of at least five pre-dose responses (basal activity). Data are expressed as means±s.e.m.; n=4–6. Responses to vehicles and to the maximal and a submaximally-effective concentration are shown for (a) r.motilin 100 and 300 nM; (b) [Nle¹³]motilin 100 and 300 nM; (c) erythromycin 3 μM and 10 μM and (d) tegaserod 3 and 10 μM.

Figure 3

Concentration-dependent facilitation of cholinergically mediated contractions in rabbit isolated gastric antrum circular muscle preparations by r.motilin, [Nle¹³]motilin, erythromycin and tegaserod. Data shown are the maximum values of potentiation at each concentration and are expressed as means±s.e.m.; n=3–5.

Figure 4

Representative trace showing the ability of a second application of 30 nM [Nle¹³]motilin to facilitate cholinergically-mediated contractions in rabbit isolated gastric antrum circular muscle preparations. Tissues were initially exposed to 30 nM [Nle¹³]motilin and left in contact with the peptide for 60 min before re-application of the same concentration of [Nle¹³]motilin. Using this protocol, the amplitude of the EFS-evoked contractions tends to fade with time.