What is a pA(2) value?
Digging around a bit, I found a few resources:

Design and models for estimating antagonist potency (pA2, Kd and IC50) following the detection of antagonism observed in the presence of intrinsic activity.

and PHARMACODINAMIC on page 30, describes pA2 as:


  indicates affinity of antagonist for receptors


(as Alan Boyd just commented on)