THE RECTAL GLAND OF SQUALUS-ACANTHIAS - A MODEL FOR THE TRANSPORT OF CHLORIDE

The rectal gland of the spiny dogfish shark, Squalus acanthias, secret es chloride by a furosemide sensitive process that has been termed ''s econdary active.'' Chloride enters the cell across the basolateral cel l membrane via the sodium:potassium:2 chloride cotransporter. The ener gy for this electroneutral uptake step is provided by the electrochemi cal gradient for sodium directed into the cell. This is maintained by Na-K-ATPase present in the basolateral cell membrane. Present as wall in the basolateral cell membrane is a potassium conductance that permi ts potassium to exit passively. Chloride leaves the cell across the lu minal membrane via a chloride conductance closely similar to CFTR. The rectal gland is thus a model for the mechanism of secondary active ch loride transport utilized by various epithelial organs throughout the vertebrate kingdom. This report reviews the humoral agents that regula te the secretion of chloride by the rectal gland and the intracellular mechanisms that mediate it. CNP, released from the heart in response to a volume stimulus, causes the release of VIP from nerves within the gland and together with VIP directly activates the rectal gland cell.