Abstract
The spontaneous activity of the rat isolated colon is suppressed by prostacyclin analogues such as cicaprost (IC(50)=4.0 nM). Activation of prostanoid IP(1)-receptors located on NANC inhibitory neurones is involved. However, several non-prostanoids, which show medium to high IP(1) agonist potency on platelet and vascular preparations, exhibit very weak inhibitory activity on the colon. The aim of the study was to investigate this discrepancy. Firstly, we have demonstrated the very high depolarizing potency of cicaprost on the rat isolated vagus nerve (EC(50)=0.23 nM). Iloprost, taprostene and carbacyclin were 7.9, 66, and 81 fold less potent than cicaprost, indicating the presence of IP(1) as opposed to IP(2)-receptors. Three non-prostanoid prostacyclin mimetics, BMY 45778, BMY 42393 and ONO-1301, although much less potent than cicaprost (195, 990 and 1660 fold respectively), behaved as full agonists on the vagus nerve. On re-investigating the rat colon, we found that BMY 45778 (0.1 - 3 microM), BMY 42393 (3 microM) and ONO-1301 (3 microM) behaved as specific IP(1) partial agonists, but their actions required 30 - 60 min to reach steady-state and only slowly reversed on washing. This profile contrasted sharply with the rapid and readily reversible contractions elicited by a related non-prostanoid ONO-AP-324, which is an EP(3)-receptor agonist. The full versus partial agonism of the non-prostanoid prostacyclin mimetics may be explained by the markedly different IP(1) agonist sensitivities of the two rat neuronal preparations. However, the slow kinetics of the non-prostanoids on the NANC system of the colon remain unexplained, and must be taken into account when characterizing neuronal IP-receptors.