serotonin(in) + 2 H(+)(out) <=> serotonin(out) + 2 H(+)(in)
serotonin(in) + 2 h(+)(out) <=> serotonin(out) + 2 h(+)(in)
serotonin(in) + 2 H(+)(out) <=> serotonin(out) + 2 H(+)(in)
serotonin(in) + 2 h(+)(out) <=> serotonin(out) + 2 h(+)(in)