  Two new realizations, denoted $U_{q,x}(\widehat{gl_2})$ and
$U(R_{q,x}(\widehat{gl_2}))$ of the trigonometric dynamical quantum affine
algebra $U_{q,\lambda}(\widehat{gl_2})$ are proposed, based on
Drinfeld-currents and $RLL$ relations respectively, along with a Heisenberg
algebra $\left\{P,Q\right\}$, with $x=q^{2P}$. Here $P$ plays the role of the
dynamical variable $\lambda$ and $Q=\frac{\partial}{\partial P}$. An explicit
isomorphism from $U_{q,x}(\widehat{gl_2})$ to $U(R_{q,x}(\widehat{gl_2}))$ is
established, which is a dynamical extension of the Ding-Frenkel isomorphism of
$U_{q}(\widehat{gl_2})$ with $U(R_{q}(\widehat{gl_2}))$ between the Drinfeld
realization and the Reshetikhin-Tian-Shanksy construction of quantum affine
algebras. Hopf algebroid structures and an affine dynamical determinant element
are introduced and it is shown that $U_{q,x}(\widehat{sl_2})$ is isomorphic to
$U(R_{q,x}(\widehat{sl_2}))$. The dynamical construction is based on the
degeneration of the elliptic quantum algebra $U_{q,p}(\widehat{sl_2})$ of
Jimbo, Konno et al. as the elliptic variable $p \to 0$.
