How do you feel after a big steak? Stuffed and sleepy? Or perhaps more alkaline? After guzzling a protein-rich meal, many mammals experience a phenomenon known as the alkaline tide; when stomach cells secrete acid to aid digestion, they pump their surplus bicarbonate ions into the bloodstream,leading to a rise in blood alkalinity. But how does feeding affect blood pH in other types of animal? Chris Wood, a keen fisherman and physiologist at McMaster University, has now discovered alkaline tides in an elasmobranch fish for the first time (p. 2693).
To find out whether fish have alkaline tides, Wood and his colleagues Makiko Kajimura, Tom Mommsen, and Pat Walsh at Bamfield Marine Station needed to study a protein-munching species. They chose the Pacific spiny dogfish, a shark that feasts on bony fishes and invertebrates. But working with sharks wasn't easy, as Wood explains: `they're pretty vicious and I've been bitten once, but what do you expect from a top predator? They are also uncooperative animals and don't feed well in captivity.' Wood soon found a solution to that;to ensure that the dogfish ate their dinners, he fitted the sharks with feeding tubes. He pumped a flatfish paste into their stomachs and took blood samples from the fish to search for evidence of an alkaline tide. He saw a marked increase in blood pH after three hours, peaking around 6 hours and returning to pre-feed levels by 17 hours. Levels of bicarbonate ions in the blood rose and ebbed in a similar manner – the hallmark of a classic alkaline tide.
We know very little about nitrogen metabolism in elasmobranchs, so Wood also examined nitrogen excretion after he fed the dogfish. He explains that elasmobranchs exhibit a peculiar form of osmoregulation. `Unlike bony fish that constantly need to excrete salt in order to maintain their bodies at a lower salt concentration than the sea, dogfish keep their blood and tissues at an osmotic concentration close to seawater,' he says. One way to achieve this is to hang on to organic molecules like urea, the nitrogenous waste from a protein-rich diet. But dogfish are sporadic feeders, gorging themselves about twice a week, so their nitrogen reserves are scarce. Wood was curious to know if dogfish conserve nitrogen from their diet to make urea or excrete it in ammonia, which is much cheaper to produce. By measuring the ammonia and urea excretion of the tube-fed dogfish, Wood found that the sharks retained virtually all the nitrogen from their food. This nitrogen conservation suggests that dogfish can produce enough urea to maintain the correct osmotic concentration, despite their irregular feeding habits.
Dogfish may also benefit from their alkaline tide. Mammals counteract the alkaline tide by hoarding acid-generating carbon dioxide in their blood. But Wood didn't see carbon dioxide levels escalate in fed dogfish, which suggests that their blood pH rises unchecked. He suspects that the alkaline tide has knock-on effects for the sharks' metabolism, and may even kick-start urea production. Wood now has plenty to get his teeth into, as he tries to piece together what happens when sharks feed.