histamine(out) + 2 H(+)(in) => histamine(in) + 2 H(+)(out)
histamine(out) + 2 h(+)(in) => histamine(in) + 2 h(+)(out)
histamine(out) + 2 H(+)(in) => histamine(in) + 2 H(+)(out)
histamine(out) + 2 h(+)(in) => histamine(in) + 2 h(+)(out)