G-ALPHA(0) IS NECESSARY FOR MUSCARINIC REGULATION OF CA2+ CHANNELS INMOUSE HEART

Heterotrimeric G proteins, composed of Ga and G beta gamma subunits, t ransmit signals from cell surface receptors to cellular effector enzym es and ion channels. The Ga, protein is the most abundant Ga subtype i n 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-t ype Ca2+ channels in GH(3) cells and also interacts with GAP43, a majo r protein in growth cones, suggesting a role in neuronal pathfinding, To analyze the function of G alpha(o), we have generated mice lacking both isoforms of G alpha(o) by homologous recombination, Surprisingly, the nervous system is grossly intact, despite the fact that G alpha(o ) makes up 0.2-0.5% of brain particulate protein and 10% of the growth cone membrane, The G alpha(o)-/- mice do suffer tremors and occasiona l seizures, but there is no obvious histologic abnormality in the nerv ous system, In contrast, G alpha(o)-/- mice have a clear and specific defect in ion channel regulation in the heart, Normal muscarinic regul ation of L-type calcium channels in ventricular myocytes is absent in the mutant mice, The L-type calcium channel responds normally to isopr oterenol, but there is no evident muscarinic inhibition, Muscarinic re gulation of atrial K+ channels is normal, as is the electrocardiogram. The levels of other G alpha subunits (G alpha(s), G alpha(q), and G a lpha(i)) are unchanged in the hearts of G alpha(o)-/- mice, but the am ount of G beta gamma is decreased. Whichever subunit, G alpha(o) or G beta gamma, carries the signal forward, these studies show that muscar inic inhibition of L-type Ca2+ channels requires coupling of the musca rinic receptor to G alpha(o). Other cardiac G alpha subunits cannot su bstitute.