An antisense compound for use in treating myotonic dystrophy DM1 or DM2, a method of enhancing antisense targeting to heart and quadricep muscles, and a method for treating DM1 or DM2 in a mammalian subject are disclosed. The oligonucleotide has 8-30 bases, with at least 8 contiguous bases being complementary to the polyCUG or polyCCUG repeats in the 3′UTR region of dystrophia myotonica protein kinase (DMPK) mRNA in DM1 or DM2, respectively. Conjugated to the oligonucleotide is a cell-penetrating peptide having the sequence (RXRR(B/X)R)2XB, where R is arginine; B is β-alanine; and each X is —C(O)—(CH2)n—NH—, where n is 4-6. The antisense compound is effective to selectively block the sequestration of muscleblind-like 1 protein (MBNL1) and/or CUGBP, in heart and quadricep muscle in a myotonic dystrophy animal model.