  Meson transition amplitudes of the vector, axial-vector, and tensor quark
currents are analyzed within dispersion formulation of the constituent quark
model. The form factors in the decay region are given by relativistic double
spectral representations through the wave functions of the inital and final
mesons. We perform heavy quark expansion of the quark-model mesonic matrix
elements with a next-to-leading order accuracy and demonstrate that matching
this expansion to the heavy-quark expansion in QCD requires subtractions in the
double spectral representations for the form factors and allows fixing the
subtraction terms. The Isgur-Wise function and next-to-leading order universal
form factors are calculated.
