  Let $({\mathcal X}, d, \mu)$ be a metric measure space and satisfy the
so-called upper doubling condition and the geometrically doubling condition. In
this paper, the authors establish an interpolation result that a sublinear
operator which is bounded from the Hardy space $H^1(\mu)$ to
$L^{1,\,\infty}(\mu)$ and from $L^\infty(\mu)$ to the BMO-type space
${\mathop\mathrm{RBMO}}(\mu)$ is also bounded on $L^p(\mu)$ for all
$p\in(1,\,\infty)$. This extension is not completely straightforward and
improves the existing result.
