We report the results of a ^63Cu and ^17O NMR study of the nuclear quadrupole interaction tensor, ^(17,63)nu_{Q,alpha}, in the hole doped spin ladder system Sr_(14-x)Ca_xCu_24O_41 (x = 0 and 12) performed under ambient and high pressures. NMR data show that the hole density in the Cu_2O_3 ladder layer grows with temperature, Ca content and an applied pressure. We have derived the hole occupation of Cu 3d and O 2p orbitals at the different ion sites in the Cu_2O_3 ladder as a function of the temperature, Ca substitution and pressure. We also suggest that the most important role of high pressure for the stabilization of a superconducting ground state in Ca-rich two-leg ladders is an increase of the hole concentration in the conducting Cu_2O_3 planes. We have obtained an estimate of 0.10 hole per Cu1 for the hole concentration at low temperature in Ca12 under 32 kbar when this compound undergoes a superconducting transition at 5K. Such a value fits fairly well with the doping phase diagram of cuprate superconductors.
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