  In this paper we first prove some linear isoperimetric inequalities for
submanifolds in the de Sitter-Schwarzschild and Reissner-Nordstrom manifolds.
Moreover, the equality is attained. Next, we prove some monotonicity formulas
for submanifolds with bounded mean curvature vector in warped product manifolds
and, as consequences, we give lower bound estimates for the volume of these
submanifolds in terms of the warping function. We conclude the paper with an
isoperimetric inequality for minimal surfaces.
