Gα(o) is necessary for muscarinic regulation of Ca(2+) channels in mouse heart

Heterotrimeric G proteins, composed of Gα and Gβγ subunits, transmit signals from cell surface receptors to cellular effector enzymes and ion channels. The Gα(o) 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 Ca(2+) channels in GH(3) cells and also interacts with GAP43, a major protein in growth cones, suggesting a role in neuronal pathfinding. To analyze the function of Gα(o), we have generated mice lacking both isoforms of Gα(o) by homologous recombination. Surprisingly, the nervous system is grossly intact, despite the fact that Gα(o) makes up 0.2–0.5% of brain particulate protein and 10% of the growth cone membrane. The Gα(o)−/− mice do suffer tremors and occasional seizures, but there is no obvious histologic abnormality in the nervous system. In contrast, Gα(o)−/− 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 (Gα(s), Gα(q), and Gα(i)) are unchanged in the hearts of Gα(o)−/− mice, but the amount of Gβγ is decreased. Whichever subunit, Gα(o) or Gβγ, carries the signal forward, these studies show that muscarinic inhibition of L-type Ca(2+) channels requires coupling of the muscarinic receptor to Gα(o). Other cardiac Gα subunits cannot substitute.