A possible direct effect of guanine nucleotide binding (G) proteins on calcium channels was examined in membrane patches excised from guinea pig cardiac myocytes and bovine cardiac sarcolemmal vesicles incorporated into planar lipid bilayers. The guanosine triphosphate analog, GTP gamma S, prolonged the survival of excised calcium channels independently of the presence of adenosine 3',5'-monophosphate (cAMP), adenosine triphosphate, cAMP-activated protein kinase, and the protein kinase C activator tetradecanoyl phorbol acetate. A specific G protein, activated Gs, or its alpha subunit, purified from the plasma membranes of human erythrocytes, prolonged the survival of excised channels and stimulated the activity of incorporated channels. Thus, in addition to regulating calcium channels indirectly through activation of cytoplasmic kinases, G proteins can regulate calcium channels directly. Since they also directly regulate a subset of potassium channels, G proteins are now known to directly gate two classes of membrane ion channels.

Properties of the delayed outward current (IK) in ventricular myocytes of the guinea-pig were studied using the whole cell clamp method. The experiments were performed under conditions in which IK was enhanced by application of isoproterenol while the Ca2+ current was eliminated by Ca2+-removal and by the addition of Cd2+. The reversal potential (Erev) of IK, determined from the current tails, was about 10 mV less negative than the K+ equilibrium potential. This was estimated by examining the reversal potential of the inward rectifier K+ current in Ba2+-containing solution, or from the Nernst equation. The Erev--log[K+]o relationship had a slope of 49 mV per tenfold change in [K+]o. In Na+-free solution, Erev became more negative. Thus, although the major charge carriers in IK are K+ ions, Na+ ions may also contribute in part to this current. The PNa/PK ratio in IK, calculated by applying a Goldman-Hodgkin-Katz relation to the reversal potential, was 0.016. The activation of IK during depolarization showed a sigmoidal time course at the onset, while the time course of the current tails was monoexponential at voltages more negative than-50 mV, but biexponential at more positive voltages. These observations can be explained by the conductance equation of the Hodgkin-Huxley type in which the kinetic variable is raised to the second power. These and other features of IK observed in the ventricular cells are discussed in comparison to the properties of similar current systems reported in other cardiac preparations.

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