Possible mechanisms mediating the receptor-independent role of G_i in regulating basal AC activity.

<p>(A) βγ-sink hypothesis: In the absence of PTX (top panel), spontaneously active G_i dominates, inhibiting basal AC activity and maintaining low cAMP levels which are incapable of eliciting an inotropic response even after inhibition of PDE3 and 4 (inhibition marked by X). After PTX treatment (bottom panel), G_i is not only inactivated (through ADP-ribosylation, marked by X) removing its spontaneous intrinsic inhibition upon AC, but also sequesters a large proportion of the shared Gβγ pool. This indirectly increases the proportion of receptor-independent spontaneously active G_s, leading to increased basal AC activity and cAMP that is normally readily degraded by PDE3 and 4. However, inhibition of PDE3 and 4 (marked by X) allows for translation of this cAMP increase into an inotropic response (see Fig. 4A). The net result is a shift from predominantly spontaneous G_i activity towards G_s activity, increasing basal AC activity and promoting activation of AC and positive inotropic effects of all inotropic agents working through increased cAMP signalling. (B) NDPK-hypothesis: In the absence of PTX (top panel), NDPK-activation of G_i dominates due to excess of G_i protein levels over G_s, resulting in low cAMP production readily degraded by PDE3 and 4. In the presence of PTX (bottom panel), G_i is inactivated by permanent ADP-ribosylation (marked by X). Thus, NDPK B could predominantly activate G_s, leading to both increased basal contractile force and cAMP accumulation that becomes revealed after PDE3 and 4 inhibition (marked by X).