The coupling in z-direction perpendicular to the chains happens via hybridization with the unoccupied cation d-orbitals, mainly with the Cu d orbitals, and is found in the LDA calculations two times stronger in the Ca case than in the Sr case. In y direction there is nearly no electronic coupling. (There is, however, dipolar spin-spin interaction.)
[K. M. Kojima et al. PRL 78, 1787 (1997)]