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