[en] Results of the low-temperature specific heat measurements (2-80 K) for one austenitic and three martensitic Ni-Mn-Ga ferromagnetic alloys are presented. The alloy compositions are chosen to comprise a wide span of valence electron concentrations e/a=7.3-7.78. Debye temperature (261-345 K) is found to be an increasing function of e/a while the experimental values of the Sommerfeld coefficient (2.9-3.4 mJ/mol K²) appear to be increasing in the martensitic region only. Observation of those trends rekindles the discussion about the role of vibrational and electronic contributions to the lattice instability and transformation mechanism of studied alloys