Post-prandial metabolic alkalosis in the seawater-acclimated trout: the alkaline tide comes in

Abstract

SUMMARYThe consequences of feeding and digestion on acid-base balance and regulation in a marine teleost (seawater-acclimated steelhead trout; Oncorhynchus mykiss) were investigated by tracking changes in blood pH and [HCO 3 -], as well as alterations in net acid or base excretion to the water following feeding. Additionally the role of the intestine in the regulation of acid-base balance during feeding was investigated with an in vitro gut sac technique. Feeding did not affect plasma glucose or urea concentrations, however, total plasma ammonia rose during feeding, peaking between 3 and 24 h following the ingestion of a meal, three-fold above resting control values (~300 μmol ml -1). This increase in plasma ammonia was accompanied by an increase in net ammonia flux to the water (~twofold higher in fed fish versus unfed fish). The arterial blood also became alkaline with increases in pH and plasma [HCO 3 -] between 3 and 12 h following feeding, representing the first measurement of an alkaline tide in a marine teleost. There was no evidence of respiratory compensation for the measured metabolic alkalosis, as Pa CO2 remained unchanged throughout the post-feeding period. However, in contrast to an earlier study on freshwater-acclimated trout, fed fish did not exhibit a compensating increase in net base excretion, but rather took in additional base from the external seawater, amounting to ~8490 μmol kg -1 over 48 h. In vitro experiments suggest that at least a portion of the alkaline tide was eliminated through increased HCO 3 -secretion coupled to Cl -absorption in the intestinal tract. This did not occur in the intestine of freshwater-acclimated trout. The marked effects of the external salinity (seawater versus freshwater) on different post-feeding patterns of acid-base balance are discussed.

Revisiting the effects of crowding and feeding in the gulf toadfish, Opsanus beta: the role of Rhesus glycoproteins in nitrogen metabolism and excretion

“…Under these circumstances, ammonia liberated from post-prandial catabolic processes would begin to accumulate in the plasma and eventually translate into elevated rates of ammonia excretion, as was the case in the present study. This pattern is also observed in other ammoniotelic teleosts following feeding (Kaushik and Teles, 1985;Kaushik and Gomes, 1988;Alsop and Wood, 1997;Wicks and Randall, 2002;Bucking and Wood, 2008;Bucking et al, 2009;Zimmer et al, 2010). In addition, this explanation also clarifies why the increases in plasma ammonia levels (Fig.1B) and ammonia excretion rates (Fig.1A) observed in crowded and unconfined fish during the post-prandial period were of similar magnitude.…”

“…Higher hepatic Rh mRNA expression was accompanied by an upregulation of both Rhcg1 and Rhcg2 at the gill (Fig.5A). This scenario is similar to that of most teleosts that simply excrete the majority of excess ammonia across the gills (Kaushik and Teles, 1985;Wicks and Randall, 2002;Bucking and Wood, 2008;Bucking et al, 2009). Indeed, physiological data from the present study confirmed that unconfined toadfish excreted the majority of their wastes as ammonia (Table2), and also had higher rates of ammonia excretion than their crowded counterparts.…”

Measuring intestinal fluid transport in vitro: Gravimetric method versus non-absorbable marker

“…However, following direct evaluation of these two methods, we found [ 14 C] PEG to be inconsistent and unreliable for most species. The average rates of net fluid absorption by gut sacs measured directly using the gravimetric method from all five species compared favourably with previous studies under similar conditions (Mainoya and Bern, 1982;Marshall et al 2002;Grosell et al 2005;Bucking et al 2009). Net fluid absorption measured simultaneously by [ 14 C] PEG was on average 2-7 times higher in the toadfish intestine (Fig.…”

Feeding ratio and frequency affects cadmium bioaccumulation in black sea bream Acanthopagrus schlegeli

“…Here, the most obvious interpretation for the observed protection by feeding against metal accumulation is that the ingestion, digestion and assimilation of food prevents the uptake of waterborne metals in the gastro intestinal tract by decreasing the drinking of water (Wood et al 2010) and/or changing the physico−chemical environment of the intestines (e.g. pH and acid−base disturbances [Taylor et al 2007, Bucking et al 2009], gut fluid volume [Wood et al 2010], bulk ion exchange and ion concentration [Bucking et al 2011]). Alternatively, it is noteworthy that the fish in all these studies (including the present study) were fasted for >10 d, which usually resulted in an insufficient intake of essential nutrients, thus resulting in po tential disturbances in normal phy siological processes/functions, such as energy metabolism and partitioning (Sun et al 2003), osmoregulation (e.g.…”

“…Since most marine fish drink seawater continuously to maintain osmotic homeostasis (Grosell 2006) and intestines are the major sites for the uptake of dissolved metals (Zhang & Wang 2007a), feeding and digestion can alter the waterdrinking rate (Wood et al 2010) and physico-chemical environment of fish intestines (e.g. pH and acid−base levels [Taylor et al 2007, Bucking et al 2009] and major ion concentrations [Bucking et al 2011]). For instance, Wood et al (2010) found that gulf toadfish Opsanus beta under daily satiation feeding accumulated much less waterborne Ag in whole body, liver and white muscle samples than those without feeding.…”