  We present spectra of 14 A-type supergiants in the metal-rich spiral galaxy
M83. We derive stellar parameters and metallicities, and measure a
spectroscopic distance modulus m-M = 28.47 +\- 0.10 (4.9 +\- 0.2 Mpc), in
agreement with other methods. We use the stellar characteristic metallicity of
M83 and other systems to discuss a version of the galaxy mass-metallicity
relation that is independent of the analysis of nebular emission lines and the
associated systematic uncertainties. We reproduce the radial metallicity
gradient of M83, which flattens at large radii, with a chemical evolution
model, constraining gas inflow and outflow processes. We carry out a
comparative analysis of the metallicities we derive from the stellar spectra
and published HII region line fluxes, utilizing both the direct, Te-based
method and different strong-line abundance diagnostics. The direct abundances
are in relatively good agreement with the stellar metallicities, once we apply
a modest correction to the nebular oxygen abundance due to depletion onto dust.
Popular empirically calibrated strong-line diagnostics tend to provide nebular
abundances that underestimate the stellar metallicities above the solar value
by ~0.2 dex. This result could be related to difficulties in selecting
calibration samples at high metallicity. The O3N2 method calibrated by Pettini
and Pagel gives the best agreement with our stellar metallicities. We confirm
that metal recombination lines yield nebular abundances that agree with the
stellar abundances for high metallicity systems, but find evidence that in more
metal-poor environments they tend to underestimate the stellar metallicities by
a significant amount, opposite to the behavior of the direct method.
