Regulation of atrial contraction in the seawater-adapted eel, Anguilla japonica

Post- and pre-synaptic action of isotocin in the upper esophageal sphincter muscle of the eel: its role in water drinking

Isotocin is a fish analogue of the mammalian hormone oxytocin. To elucidate sites of action of isotocin (IT) in the upper esophageal sphincter (UES) muscle, a key muscle in swallowing, IT was applied after treatment with tetrodotoxin (TTX). Even after blocking nerve activity with TTX, IT relaxes the UES muscle in a concentration-dependent manner, suggesting that IT receptor(s) is present on the muscle cells. Similar relaxation was also obtained by application of 3-isobutyl-1-methylxanthine (IBMX), forskolin (FSK) and 8-bromo-adenosine, 3',5'-cyclic monophosphate (8BrcAMP) after pretreatment with TTX, suggesting that the relaxing effect (postsynaptic action) of IT may be mediated by cAMP. In contrast to such relaxing effect, IT enhanced the UES contraction induced by repetitive electrical field stimulation (EFS). Such enhancement was blocked by an IT receptor antagonist, suggesting that this effect is also mediated by IT receptor(s). Similar enhancement was also induced by IBMX, FSK and 8BrcAMP, suggesting the enhancing effect is also mediated by cAMP. However, no enhancing effect of IT was observed when the muscle was stimulated by carbachol, or after treatment with curare or TTX, denying the postsynaptic modulatory action of IT and suggesting presynaptic action for IT, i.e., accelerating acetylcholine release. Summarizing these results, role of IT in precisely regulating the drinking rate in the seawater eel is discussed.

Glucagon-like peptide isolated from the eel intestine: effects on atrial beating

A new glucagon-like peptide was isolated from the intestine of the eel Anguilla japonica. The primary structure was determined by sequence analysis after cleavage with lysyl endopeptidase, quantitative amino acid analysis and fast atom bombardment mass spectrometry as HSQGTFTNDY10SKYLETRRAQ20DFVQWLMNSK30RSGGPT. Since its structure is similar to that of oxyntomodulins (OXMs) reported in various vertebrates, we named this peptide eel oxyntomodulin (eOXM). We found that eOXM enhanced the contractile force and the beating rate of the eel atrium in a dose-dependent manner. These effects of eOXM were not inhibited by betaxolol, a β1-adrenoceptor antagonist, indicating that the actions of eOXM were independent of those of adrenaline. eOXM enhanced the intracellular Ca2+ concentration of the myocardium. The contractility of the eel atrium was greatly reduced after omitting Ca2+ from the bathing medium or after treatment with verapamil, a Ca2+ channel blocker. After inhibiting Ca2+ entry under these conditions, the inotropic effect of eOXM was markedly reduced, but the chronotropic effect was not altered significantly. These results indicate that the inotropic effect of eOXM is via a stimulation of Ca2+ influx but that the chronotropic effect may be independent of extracellular Ca2+.

The role of circulating catecholamines in the regulation of fish metabolism: an overview

The physiological role of the catecholamines (CA), adrenaline and noradrenaline in fish has been frequently reviewed, but the metabolic consequences of these hormones have received less attention. The purpose of this review is to examine the recent literature dealing with CA actions on whole fish and tissue metabolism. The CA increase glucose production both in vivo and in vitro, at least in isolated hepatocytes. Although the data are less clear, lipid mobilization is also a consequence of elevated circulating CA. The difficulty with using the whole fish for such studies is that CA may alter other circulating hormone levels, CA turnover in the circulation quickly, and it is difficult to define precisely the tissue being affected. Much of our understanding is derived, therefore, from the study of isolated tissues, and especially the hepatocyte. Catecholamines stimulate both glycogenolysis and gluconeogenesis in hepatocytes isolated from a large number of fish species. This review examines the steps involved in the signal transduction system, from the binding of CA to alpha- and beta-adrenoceptors to the ultimate effects of specific enzyme phosphorylation. Recent literature demonstrates that the complexity of the adrenoceptor system noted for mammals, also is expressed in fish. Adrenoceptor subtypes are specific to species, to tissues and to function of the tissues, and these issues are discussed especially as they are related to external and to internal stressors. Future research will pursue better definitions of the adrenoceptor systems, molecular biology of the components of these receptor systems and development of alternative cell models. There still remains a poor explanation of the reason for the diversity of adrenoceptor systems, and there are a number of fish systems that may provide unique opportunities to understand this question.