  For disc galaxies, a close relation exists between the distribution of light
and the shape of the rotation curve. We quantify this relation by measuring the
inner circular-velocity gradient d_{R}V(0) for spiral and irregular galaxies
with high-quality rotation curves. We find that d_{R}V(0) correlates with the
central surface brightness mu_{0} over more than two orders of magnitude in
d_{R}V(0) and four orders of magnitudes in mu_{0}. This is a scaling-relation
for disc galaxies. It shows that the central stellar density of a galaxy
closely relates to the inner shape of the potential well, also for
low-luminosity and low-surface-brightness galaxies that are expected to be
dominated by dark matter.
