Uncoupling of cardiac muscarinic and β-adrenergic receptors from ion channels by a guanine nucleotide analogue

Abstract

Guanine nucleotide binding proteins, interchangeably called N or G proteins, seem to be tbe primary signal-transducing components of various agonist-induced cell membrane functions^1–3. In the heart, G proteins have been implicated in β-adrenergic modulation of the slow inward Ca²⁺ current. We have investigated the role of G proteins in muscarinic activation of an inwardly rectifying, acetylcholine (ACh)-induced K⁺ current (I_ACh), and β-adrenergic activation of an (isoprenaline)-induced Ca²⁺ current (I_si). Here we report that intracellular application of the non-hydrolysable GTP analogue 5′-guanylylimidodiphosphate (GppNHp) brought about an agonist-induced, antagonist-resistant, persistent activation of I_ACh and I_si. This functional uncoupling of channel from receptor suggests that the muscarinic receptor and the I_Ach channel are separate molecular structures. Membrane conductance responses to sequential activation of muscarinic and β-adrenergic receptors demonstrate that in contrast to the muscarinic inhibition of I_si, muscarinic stimulation of I_ACh is mediated by a G protein via a pathway that does not involve adenylate cyclase. Taken together, the results support the notion that agonist is required to induce GppNHp binding and/or activation of the G proteins. Once triggered by agonist, the control system remains maximally activated, thereby transforming the cell so that it no longer responds to subsequent homologous receptor-mediated signals.