The cellular mechanisms involved in the control of sodium transport across the gills of a seawater stenohaline fish Gadus morhua were investigated with particular emphasis on the role of diacylglyc√©rol (DAG) as a second messenger. A whole-body perfusion system identified several agents that induced changes in sodium efflux across the gills. The effects of these agents on Na+-K+-ATPase activity and phospholipid metabolism in isolated chloride cells were then investigated in an attempt to identify the second messengers involved in receptor-mediated stimulation of salt secretion in chloride cells.
The hormone atrial natriuretic factor (ANF) was found to reduce the sodium efflux in perfused gills and reduce Na^-K^-ATPase activity in homogenates of chloride cells. Thus its action appears to be at least partly due to an effect on active sodium transport by Na^-K^-ATPase. The mechanism by which ANF brings about a change in Na’*‚Äô-K^-ATPase activity was not via a receptor-mediated phospholipa.se C hydrolysis of phosphatidylinositides or indeed phospholipase C mediated hydrolysis of any other phospholipid. However receptor-mediated phosphatidylcholine (PtdCho) metabolism was probably involved and this may be accomplished by changes in cytidyltransferase activity.
The phosphatidylinositol cycle in unstimulated chloride cells and in cells treated with ANF was relatively inactive. Radioactive tracer studies revealed a high proportion of radio-incorporation into the phospholipid phosphatidyl- ethanolamine and relatively low amounts into the phosphoinositides unlike the situation in other salt transporting epithelia or in cod brain. The significance of this in terms of signal transduction in the chloride cells of cod remains to be determined.
Diacylglyc√©rol appears to have a role as a second messenger in the control of sodium efflux and Na^-K^-ATPase activity in the gills of cod. Both sodium efflux and Na^-K^-ATPase activity were reduced on administration of phorbol ester or a synthetic DAG analogue. The exact mechanism by which this inactivation was mediated remains unclear though it seems probable that it is mediated via activation of protein kinase C. A physiological effector for this pathway remains to be identified. Attempts to link the action of the hormone ANF with production of the second messenger DAG and the subsequent activation of PKC proved unsuccessful.