Ca(2+)(out) + Na(+)(in) => Ca(2+)(in) + Na(+)(out)
ca(2+)(out) + na(+)(in) => ca(2+)(in) + na(+)(out)
Ca(2+)(out) + Na(+)(in) => Ca(2+)(in) + Na(+)(out)
ca(2+)(out) + na(+)(in) => ca(2+)(in) + na(+)(out)