P2X7 receptors function as ATP-gated cation channels but also interact with other proteins as part of a larger signalling complex to mediate a variety of downstream responses dependent upon the cell type in which they are expressed. Receptor mediated membrane permeabilization to large molecules precedes the induction of cell death but remains poorly understood and the mechanisms that underlie differential sensitivity to NAD are also unknown. By studying alternative variants of the mouse P2X7 receptor we show that sensitivity to NAD is mediated via the P2X7k variant which has a much more restricted distribution than the P2X7a receptor, but is expressed in T-lymphocytes. The altered N-terminus and TM1 of the P2X7k receptor enhances the stability of the active state of this variant compared to P2X7a thereby increasing the efficacy of NAD-dependent ADP-ribosylation as measured by ethidium uptake, a rise in intracellular Ca(2+) and the activation of inward currents. Coexpression of P2X7k and P2X7a receptors reduced NAD-sensitivity. P2X7k receptor-mediated ethidium uptake was also triggered by much lower BzATP concentrations and was insensitive to the P451L single nucleotide polymorphism. P2X7k receptor mediated ethidium uptake occurred independently of pannexin-1 suggesting a pathway intrinsic to the receptor. Only for the P2X7aL451 receptor could we resolve a component of dye-uptake dependent upon pannexin-1. Signalling occurred downstream of the activation of caspases rather than involving direct cross talk between the channels. An in-situ proximity assay showed, however, close association between P2X7 receptors and pannexin-1, which would facilitate ATP efflux through pannexin-1 acting in an autocrine manner.