Actions of endothelin and corticotropin releasing factor in the guinea-pig ileum: no evidence for an interaction with capsaicin-sensitive neurons

Abstract

Both endothelins and corticotropin releasing factor (CRF) appear in capsaicin-sensitive neurons. We have investigated the effects of human endothelin-1 (ET-1) and CRF in the guinea-pig ileum longitudinal and circular preparations and sought for ways of specific antagonism. With the aid of tachyphylaxis to capsaicin (i.e., rendering capsaicin-sensitive neurons functionally impaired) it was tested if these neurons played a mediating role in the effects of ET-1 or CRF. We also tried to find out whether endogenous endothelin or CRF plays a role in the excitatory and inhibitory effects of capsaicin in the ileum. In preparations at basal tone, both exogenous ET-1 (1–100 nM) and CRF (3–100 nM) caused contraction. These responses were not influenced by capsaicin tachyphylaxis. The contractile effect of ET-1 was not affected by tetrodotoxin (1 μM), atropine (1 μM), methysergide (100 nM), chloropyramine (100 nM) or SR140333 (100 nM) but was significantly inhibited or even abolished by the receptor antagonist BQ123 (3 μM) or BQ788 (3 μM). CRF caused contraction that was fully sensitive to tetrodotoxin (1 μM), tachyphylaxis to CRF or to atropine (1 μM) plus the tachykinin NK 1 receptor antagonist SR140333 (200 nM). Atropine alone had a weak inhibitory effect on the contractile action of CRF. Neither the antagonist BQ123 (3 μM) nor CRF tachyphylaxis inhibited the contractile action of capsaicin (2 μM), even in the presence of a mixture of GR82334 (3 μM) and SR142801 (100 nM), for blocking tachykinin NK 1 and NK 3 receptors, respectively – a treatment that by itself significantly reduced the effect of capsaicin. Exogenous ET-1 (0.3–5 nM), but not CRF (30–100 nM), caused relaxation of the atropine-treated, histamine-precontracted ileum. This effect of ET-1 was significantly inhibited or abolished by BQ123 (10 μM), or BQ788 (3 μM), but was not influenced by capsaicin tachyphylaxis. Likewise, relaxation of the atropine-treated, histamine-precontracted ileum in response to capsaicin was not influenced by the endothelin receptor antagonist BQ788 (3 μM) or BQ788 (3 μM) plus BQ123 (3 μM). Apamin (300 nM) was also without effect on the capsaicin-induced relaxation. In circular muscle strips ET-1 inhibited the indomethacin-induced spontaneous activity. This effect was abolished by BQ123 (3 μM) or BQ788 (3 μM). CRF caused a stimulation of the circular muscle. This stimulatory effect was not influenced by atropine (1 μM) alone, but was inhibited by atropine plus tachykinin NK 1 and NK 2 receptor antagonists (SR140333 (200 nM) and SR48968 (200 nM)) and also by tetrodotoxin (1 μM). It is concluded that capsaicin-sensitive neurons do not play a role in the effects of exogenous ET-1 or CRF in the guinea-pig ileum. ET-1 can both contract and relax the ileal longitudinal smooth muscle directly, probably via both ET A and ET B receptors. CRF acts by specifically stimulating excitatory (but not inhibitory) neurons of the myenteric plexus. Neither endogenous ET-1 nor CRF seems to play a role in the excitatory or inhibitory effects of capsaicin.