The role of intracellular calcium stores in motilin induced contractions of the longitudinal muscle of the rabbit duodenum

Summary The contraction of longitudinal muscle strips of the rabbit duodenum in response to motilin and acetylcholine was investigated in normal and high K⁺-solutions in the presence and absence of external calcium, in order to demonstrate the existence of pharmaco-mechanical coupling for motilin and to examine whether the peptide mobilizes calcium from an intracellular store. In depolarized smooth muscle (140 mM K⁺), motilin (3.2×10⁹ –1×10^–7 M) and acetylcholine (1×10^–5 M) were still capable of causing a considerable, transient, concentration-dependent contraction in the presence of Ca²⁺. The extra-contraction to motilin was not blocked by tetrodotoxin (1 g/ml) nor by atropine (10^–7 M), but acetylcholine (10^–5 M) was blocked by atropine. Verapamil (10^–7 M) could selectively block the K⁺ contraction without affecting the extra agonist contraction. Nitroprusside was ineffective up to 10^–4 M in high K⁺-solutions, but in normal Hepes-buffer it caused a concentration-dependent rightward shift of the concentration-response curve of motilin and acetylcholine contractions. In a calcium-depleted medium, high K⁺-depolarized muscle strips were still responsive to motilin and acetylcholine, but higher concentrations (10^–6 M) were needed than in the presence of calcium and the contractions reached only 57 +- 11% and 74 +- 9% respectively of the maximal contraction in 1.2 mM Ca²⁺ containing solutions. The response to motilin (10^–6 M) was not only smaller than that to acetylcholine (10^–5 M), it also faded more rapidly with time. The response to one agonist could not be repeated except by using a higher concentration of the same or the other agonist, and the magnitude of this second response depended upon the dose used in the first one. We conclude that pharmaco-mechanical coupling exists for motilin and that this peptide is able to elicit contractions by mobilization of calcium from an intracellular store. This store overlaps with the one used by acetylcholine. Our experiments also reinforce the hypothesis that in the rabbit motilin exerts a direct action upon smooth muscle cells.