Urea transport and its regulation across the gall bladder ofTorpedo marmorata

Abstract

The gall bladder of Torpedo marmorata possesses two mechanisms in order to avoid urea loss: 1) low urea permeability, and 2) net urea absorption. In fact, the urea serosamucosa flux was 26 times lower than that of Bufo bufo and 2.4 times lower than that of Necturus maculosus. The low urea permeability seems to be regulated by an unidentified hormone utilizing cAMP as a second messenger. This hypothesis is supported by the observation that an activator of adenyl cyclase, i.e., forskolin, decreased by 42% the urea flux across the gall bladder of Torpedo. Noradrenaline, with its β effect, may be the hormone in question, since isoprenaline inhibits the urea flux by 27%. Another mechanism useful for minimizing urea loss, i.e., net absorption, seems to be present only in more permeable gall bladders. Net absorption may be due to active transport or solvent drag, but the second mechanism is supported by the observation that net absorption also occurs in a molecule quite different from urea, i.e., ethylene glycol.