  It is shown that the kinematical difference in the decays B^- -> D^0 \ell
\tilde \nu and B^0 -> D^+ \ell \tilde \nu due to the isotopic mass splittings
of the B and D mesons is compensated in the total decay rate by an appropriate
difference in the lepton spectra.
  Thus there is no effect on the total decay rates in the linear order in the
isotopic mass splittings, as required by the general consideration based on the
operator product expansion. Although phenomenologically the isotopic difference
in the spectra amounts to at most about 1%, in the theoretical aspect this
effect can be viewed as an additional illustration of how the general OPE
results emerge from the properties of exclusive channels.
