Abstract

Heterotrimeric G proteins, composed of Gα and Gβγ subunits, transmit signals from cell surface receptors to cellular effector enzymes and ion channels. The Gαo protein is the most abundant Gα subtype in the nervous system, but it is also found in the heart. Its function is not completely known, although it is required for regulation of N-type Ca2+ channels in GH3 cells and also interacts with GAP43, a major protein in growth cones, suggesting a role in neuronal pathfinding. To analyze the function of Gαo, we have generated mice lacking both isoforms of Gαo by homologous recombination. Surprisingly, the nervous system is grossly intact, despite the fact that Gαo makes up 0.2–0.5% of brain particulate protein and 10% of the growth cone membrane. The Gαo−/− mice do suffer tremors and occasional seizures, but there is no obvious histologic abnormality in the nervous system. In contrast, Gαo−/− mice have a clear and specific defect in ion channel regulation in the heart. Normal muscarinic regulation of L-type calcium channels in ventricular myocytes is absent in the mutant mice. The L-type calcium channel responds normally to isoproterenol, but there is no evident muscarinic inhibition. Muscarinic regulation of atrial K+ channels is normal, as is the electrocardiogram. The levels of other Gα subunits (Gαs, Gαq, and Gαi) are unchanged in the hearts of Gαo−/− mice, but the amount of Gβγ is decreased. Whichever subunit, Gαo or Gβγ, carries the signal forward, these studies show that muscarinic inhibition of L-type Ca2+ channels requires coupling of the muscarinic receptor to Gαo. Other cardiac Gα subunits cannot substitute.