Metabolic organization and effects of feeding on enzyme activities of the dogfish shark (Squalus acanthias) rectal gland

Abstract

SUMMARY In order to investigate the metabolic poise of the elasmobranch rectal gland, we conducted two lines of experimentation. First, we examined the effects of feeding on plasma metabolites and enzyme activities from several metabolic pathways in several tissues of the dogfish shark, Squalus acanthias, after starvation and at 6, 20, 30 and 48 h post-feeding. We found a rapid and sustained ten-fold decrease in plasma β-hydroxybutyrate at 6 h and beyond compared with starved dogfish, suggesting an upregulation in the use of this substrate, a decrease in production, or both. Plasma acetoacetate levels remain unchanged, whereas there was a slight and transient decrease in plasma glucose levels at 6 h. Several enzymes showed a large increase in activity post-feeding, including β-hydroxybutyrate dehydrogenase in rectal gland and liver, and in rectal gland, isocitrate dehydrogenase, citrate synthase, lactate dehydrogenase, aspartate amino transferase, alanine amino transferase, glutamine synthetase and Na+/K+ ATPase. Also notable in these enzyme measurements was the overall high level of activity in the rectal gland in general. For example, activity of the Krebs' TCA cycle enzyme citrate synthase (over 30 U g-1) was similar to activities in muscle from other species of highly active fish. Surprisingly, lactate dehydrogenase activity in the gland was also high (over 150 U g-1), suggesting either an ability to produce lactate anaerobically or use lactate as an aerobic fuel. Given these interesting observations, in the second aspect of the study we examined the ability of several metabolic substrates (alone and in combination) to support chloride secretion by the rectal gland. Among the substrates tested at physiological concentrations (glucose, β-hydroxybutyrate, lactate,alanine, acetoacetate, and glutamate), only glucose could consistently maintain a viable preparation. Whereas β-hydroxybutyrate could enhance gland activity when presented in combination with glucose, surprisingly it could not sustain chloride secretion when used as a lone substrate. Our results are discussed in the context of the in vivo role of the gland and mechanisms of possible upregulation of enzyme activities.

Established and potential physiological roles of bicarbonate-sensing soluble adenylyl cyclase (sAC) in aquatic animals

“…Furthermore, as sAC is also present in multiple other shark tissues (Fig. 4) (see also Roa et al, 2012), it is a good candidate to mediate some of the profound metabolic changes that take place throughout the shark's body during the alkalotic post-feeding period (Walsh et al, 2006;Wood et al, 2008), including the stimulation of NaCl secretion across the rectal gland (Shuttleworth et al, 2006;Wood et al, 2007b).…”

Osmoregulation, ionoregulation and acid–base regulation by the gastrointestinal tract after feeding in the elasmobranch (Squalus acanthias)

“…recently, we have begun to address this deficit with studies on Squalus acanthias Kajimura et al, 2006;Walsh et al, 2006). These investigations have revealed pronounced systemic disturbances including a marked alkaline tide in the bloodstream peaking 6·h after feeding, presumably reflective of HCl secretion into the stomach , and a slower activation of the ornithine urea cycle in both liver and skeletal muscle associated with a rise in plasma urea and osmolality levels .…”

A critical analysis of carbonic anhydrase function, respiratory gas exchange, and the acid-base control of secretion in the rectal gland of Squalus acanthias

“…After 0.75·h, samples of the rectal gland secretion, venous perfusate and arterial perfusate were collected for gas and acid-base analyses, and the secretion was frozen (-20°C) for the later analysis of ions. These sampling procedures took 15·min, and then at 1·h, 5310 -6 ·mol·l -1 forskolin (Sigma-Aldrich) was added to the perfusion reservoir to stimulate secretion (Moran and Valentich, 1991;Forrest, Jr et al, 1997;Walsh et al, 2006). After a further 0.75-1·h, the sampling and analyses were repeated, and then the perfusion pressure was lowered to ~12·mmHg, a procedure chosen to decrease the perfusion flow through the gland without altering secretion flow so as to increase the precision of measurement of arterial-venous differences (see Results).…”

“…Overall, there was good agreement with the in vivo data reinforcing the present in vitro data, such that the relationship between pHe and relative secretion rate did not change, but became more significant due to the greater number of treatment means (r=0.94, P<0.0001; exponential model). Again there was no significant relationship with [HCO 3 -]e. These responses of rectal gland secretion to pHe are probably adaptive in the context of the intact animal because feeding will cause both a metabolic alkalosis ('alkaline tide'; Wood et al, 2005) and volume loading due to the ingestion of seawater and/or salty food (Mackenzie et al, 2002), necessitating activation of rectal gland secretion (Walsh et al, 2006). The highest pHe of about 8.6 from the in vitro experiments is probably outside the physiological range and may explain the non-linearity of response in this range (Fig.·6).…”

“…The acid-base status of the secreted fluid varied with that of the perfusate, secretion pH remaining about 0.3-0.5 units lower, and changing in concert with pHe rather than pHi; secretion HCO 3 which secretion can be directly stimulated using forskolin (Moran and Valentich, 1991) to activate the adenyl cyclase pathway (e.g. Forrest, Jr et al, 1997;Walsh et al, 2006).…”