  In this paper we study static spherically symmetric wormhole solutions
sustained by matter sources with isotropic pressure. We show that such
spherical wormholes do not exist in the framework of zero-tidal-force
wormholes. On the other hand, it is shown that for the often used power-law
shape function there is no spherically symmetric traversable wormholes
sustained by sources with a linear equation of state $p=\omega \rho$ for the
isotropic pressure, independently of the form of the redshift function
$\phi(r)$. We consider a solution obtained by Tolman at 1939 for describing
static spheres of isotropic fluids, and show that it also may describe wormhole
spacetimes with a power-law redshift function, which leads to a polynomial
shape function, generalizing a power-law shape function, and inducing a solid
angle deficit.
