α₂‐Adrenoceptor‐mediated contractile response to catecholamines in smooth muscle strips isolated from rainbow trout stomach (Salmo gairdneri)

Abstract

1The type of adrenoceptor involved in the contractile response to catecholamines in smooth muscle strips isolated from rainbow trout stomach was determined.2 Noradrenaline (10 nM-10 ptM) and adrenaline (10 nM-3 pM) caused non-sustained contractions which were markedly decreased by phentolamine (5.4 pM) but not by carteolol (5 PM). Phenylephrine(1 jiM -1 mM) was less effective in causing muscle contraction and methoxamine produced no contraction. Clonidine (100 nM-300JM) caused no mechanical response but inhibited the contraction to noradrenaline or adrenaline but not acetylcholine or 5-hydroxytryptamine. 3 Yohimbine (10nM-1 pM) decreased the contraction induced by noradrenaline or adrenaline but prazosin (1 piM) did not. 4 Tetrodotoxin (780 nM) partially reduced the contraction induced by noradrenaline or adrenaline but atropine (500 nM) did not. 5 In the presence of atropine (1 pM), electrical transmural stimulation caused frequency-dependent, tetrodotoxin-sensitive contractions. 6 These results suggest that the contractile response induced by noradrenaline or adrenaline is mediated by M2-adrenoceptors. It is also suggested that noradrenaline and adrenaline contract the smooth muscle by direct action and by indirect action through the non-cholinergic excitatory nerve.

5‐Hydroxytryptamine is a possible neurotransmitter of the non‐cholinergic excitatory nerves in the longitudinal muscle of rainbow trout stomach (Salmo gairdneri)

“…The results of the present experiments confirm the previous observations that TMS causes tetrodotoxin-sensitive frequency-dependent contractions of the rainbow trout stomach strips (Holmgren & Nilsson, 1981;Kitazawa et al, 1986). Atropine significantly decreased the response to low frequency stimulation, but did not decrease that to highfrequency stimulation, and, therefore, it was suggested that low frequency stimulation mainly excited cholinergic excitatory nerves and high frequency stimulation excited non-cholinergic excitatory nerves.…”

“…On the other hand, histamine did not cause any mechanical response at a fairly high concentration (100pM) and pyrilamine had no inhibitory effect on the response to TMS (Table 1) (n = 5). This response to noradrenaline was selectively decreased by phentolamine (5.4pM) and yohimbine (1 M), but not by prazosin (1 pM), indicating that the excitatory response to noradrenaline was mediated by a-adrenoceptors, as previously observed (Kitazawa et al, 1986). The a-adrenoceptor antagonist, phentolamine (5.4pM), had no inhibitory effect on the atropine-resistant contraction caused by TMS (Table 1).…”

“…While the mechanical response to transmural stimulation (TMS) of rainbow trout stomach strips was being investigated in vitro, an atropine-resistant contractile response induced by TMS was observed (Kitazawa et al, 1986). However, the substance mediating the non-cholinergic response has not, as yet, been identified.…”

“…However, the substance mediating the non-cholinergic response has not, as yet, been identified. Physiological experiments showed that catecholamines, 5-HT, ATP and several neuropeptides, including substance P, affect the mechanical activity of the smooth muscle layer of the rainbow trout stomach (Holmgren, 1983;Kitazawa et al, 1986).…”

Effect of acetylcholine, pilocarpine, serotonin and adrenaline on the in vitro gastric motility of Hoplias malabaricus (Teleostei)

“…The contractile response to noradrenaline and adrenaline in this species involves both a direct action on smooth muscle and an indirect action through a non-cholinergic excitatory nerve (Kitazawa, 1986). Enteric neurons may release acetylcholine, 5-hydroxytryptamine and neuropeptides in various combinations (Holmgren and Nilsson, 1991;Jensen and Holmgren, 1993 (Holmgren and Nilsson, 1976;Winberg et ai., 1981).…”