Na-Ca exchange in vascular smooth muscle.

The above section makes it clear that even if an Na-Ca exchange exists in vascular smooth muscles, the clear involvement of Na ions in the generation of tension can in many cases be explained by the operation of mechanisms other than Na-Ca exchange. Readers who have got this far may still be unclear whether we believe that smooth muscle cells do possess the exchange mechanism or not. In the next section we will briefly describe two more recent approaches which have convinced us that at least some smooth muscles do possess an Na-Ca exchange mechanism, one involving isolated smooth muscle membranes and the mechanisms they possess for transporting Ca, and one using ion-sensitive microelectrodes to follow in more detail changes in intracellular Na activity and membrane potential that occur when the extracellular ionic environment is manipulated.