We have used the TAURUS Fabry-Perot mapping spectrometer on the WHT to produce a complete kinematic map of the disc of M100 in H-alpha. Here we show how the internal velocity dispersion of the principal emission components of the brightest regions varies with their H-alpha luminosity. The plot shows ample scatter, but an upper envelope in the velocity dispersion is clearly linear (in the log-log plane) with a slope of 2.6, a result which agrees precisely with an earlier graph by Arsenault et al., who selected instead the regions of highest surface brightness. We show that this result, which differs from the conventional prediction from the virial theorem, is consistent with virialisation if the HII regions are density bounded and thus offers evidence in support of the density bounding hypothesis for the most luminous regions in disc galaxies.