  We investigate the relation between circular velocity vc and bulge velocity
dispersion sigma in spiral galaxies, based on literature data and new
spectroscopic observations. We find a strong, nearly linear vc-sigma
correlation with a negligible intrinsic scatter, and a striking agreement with
the corresponding relation for elliptical galaxies. The least massive galaxies
(sigma < 80 km/s) significantly deviate from this relation. We combine this
vc-sigma correlation with the well-known MBH-sigma relation to obtain a tight
correlation between circular velocity and supermassive black hole mass, and
interpret this as observational evidence for a close link between supermassive
black holes and the dark matter haloes in which they presumably formed. Apart
from being an important ingredient for theoretical models of galaxy formation
and evolution, the relation between MBH and circular velocity has the potential
to become an important practical tool in estimating supermassive black hole
masses in spiral galaxies.
