The interior of the cells is electronegative respect to the extracellular media. However, against the natural tendency of the positively charged Na and Ca ions to accumulate into the cells, they are much more concentrated in the external side. To maintain that situation a net steady Na and Ca extrusion must take place while energy is consumed. For Ca, two proteins located in the plasma membrane cooperate to that end: (i) the Ca pump, which uses energy from ATP hydrolysis, and (ii) the Na/Ca exchange which, though it takes that energy from the inward Na gradient, is also stimulated by ATP. The studies will characterize the structure-function relationships of the Na/Ca exchanger employing the squid giant axon. Under dialysis, this preparation is unique for it allows almost complete biochemical control of intra and extracellular environments and also of the transmembrane electrical potential. Answers to the following specific questions can be expected to provide an increase in our understanding of how extraordinary electrochemical barriers to their movements. (i) Is ATP stimulation is exerted directly on the exchanger or are other structures, perhaps kinases, involved? (ii) What amino acids (structural units of proteins) are relevant to the functions of the exchanger? In this case compounds known to selectively block different amino acids or chemical groups will be used. The physiological relevance of the Ca extruding systems is evidenced by the fact that intracellular ?Ca! regulated blood flow and pressure and neural and endocrine functions.
