For a series of cis-polyisoprene (PI) chains having nearly the same molecular weight (M congruent to 50000) but differently once-inverted dipoles parallel along the chain backbone, dielectric behavior was examined in homogeneous blends in an entangling polybutadiene (PB) matrix with M = 263 000. In the blends, the PI chains (5 vol %) were entangled only with the matrix chains. The constraint release (CR) mechanism made a negligible contribution to the global relaxation of those PI chains. The series of PI chains exhibited quite different dielectric losses epsilon ". These differences, reflecting the different location of the dipole inversion point in the chain backbone, were utilized to experimentally determine the eigenfunctions f(p)(n) and relaxation times tau(p) defined for a local correlation function, C(n,t;m) = (1/a(2))-[u(n,t).u(m,0)] = (2/N)Sigma(p)f(p)(n) f(p)(m) exp(-t/tau(p)) with u(n,t) being the bond vector for the nth segment at time t. The f(p)(n) and tau(p)/tau(1) ratio (p = 1-3) evaluated for the PI chains in the high-M PB matrix were found to be close to those determined in a low-M entangling matrix (M = 9240) as well as in the monodisperse bulk state of the same PI chains. The PI relaxation was dominated by the CR mechanism in this low-M matrix, while this mechanism made a moderate contribution to the PI relaxation in the monodisperse bulk state. Thus the close coincidence of f(p)(n) and the tau(p)/tau(1) ratio (p = 1-3) in the three environments indicated that the distribution of the slowest three eigenmodes was hardly affected by the CR mechanism. In contrast, for higher eigenmodes, modest CR effects were observed through differences in the shape of the epsilon " curves at intermediate frequencies for respective dipole-inverted PI chains in the three environments.