Modulation of beta-adrenergic responses of chloride and calcium currents by external cations in guinea-pig ventricular cells.

1. The catecholamine-induced Cl- current and the Ca2+ current were recorded in the single ventricular cells of guinea-pig hearts, using the whole-cell patch clamp technique combined with internal perfusion. Dependence of the beta-adrenergic responses on external monovalent cations was investigated. The Cl- current was recognized by measuring the reversal potential of the agonist-induced current. 2. The amplitude of the Cl- current, activated by 1 microM adrenaline or 0.01-0.1 microM isoprenaline, was decreased when the external Na+ concentration ([Na+]o) was reduced by replacement with Tris+. The conductance of the catecholamine-induced Cl- current was proportional to the logarithm of the [Na+]o over a range of 15-140 mM. When the conductance was plotted against the concentration of Tris+, a dose-dependent inhibition of the Cl- response by Tris+ was suggested with a half-maximum concentration of 95 mM. 3. The inhibitory effect of the Na+ substitute TEA+ on the Cl- current was not affected by either increasing the buffer for the internal Ca2+ (10 mM BAPTA) or for the pH (50 mM HEPES). 4. In the relationship between agonist concentration and the Cl- conductance, the half-maximum concentration (K1/2) of isoprenaline was 0.013 microM in the control Na+ solution, and was shifted to 0.07, 0.08, 0.1 and 0.3 microM in the Li+, Cs+, TEA+ and Tris+ external solutions, respectively. The maximum slope conductance was not significantly affected, except for a slight depression on the Tris+ solution. When the current was induced by adrenaline, qualitatively the same finding was obtained; K1/2 was 0.15 and 3.2 microM in the Na+ and Tris+ solutions, respectively. 5. As a substitute for the external Na+, sucrose seemed to be inert. The activation of the inward Cl- current was conserved in the 300 mM sucrose solution ([Cl-]o = 8 mM) with a K1/2 value of 0.015 microM isoprenaline. 6. The Cl- current, when activated by either an external application of forskolin (0.2-10 microM) or an internal perfusion of cyclic AMP (100-500 microM), was not affected by replacing external Na+ with other cations. Activation of the Cl- current by 0.2-5 microM histamine was also insensitive to a substitution of Na+. These findings indicate that the inhibition by the Na+ substitute is at a point before the activation of GTP-binding protein. 7. The effects of Na+ substitution were not affected by varying the Na+ concentration (0-115 mM) in the internal solution, excluding an involvement of a change in the [Na+]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Synergistic action of cyclic GMP on catecholamine-induced chloride current in guinea-pig ventricular cells.

1. Effects of cyclic GMP on the catecholamine-induced chloride current (ICl) were studied using the whole-cell patch-clamp technique combined with internal perfusion in single ventricular myocytes dispersed from guinea-pig heart. 2. When ICl was activated by submaximal doses of isoprenaline (0.01-0.1 microM), adrenaline (0.5-1 microM) and histamine (0.2-0.5 microM), intracellular dialysis with cyclic GMP (10-100 microM) induced an extra increase of ICl. No further increase of ICl was induced by cyclic GMP when ICl was maximally activated. In the absence of agonists, cyclic GMP failed to induce ICl. 3. The enhancement by cyclic GMP was also observed when ICl was activated by external application of 0.2-1.0 microM-forskolin or by internal dialysis with a pipette solution containing 50-200 microM-cyclic AMP. 4. In contrast to cyclic GMP, 10-1000 microM-dibutyryl cyclic GMP and 8-bromo-cyclic GMP were ineffective in modifying ICl. 5. Milrinone (1-10 microM), a specific inhibitor of a kind of phosphodiesterase which is inhibited by cyclic GMP, also enhanced ICl activated by submaximal doses of isoprenaline. Milrinone itself did not activate ICl. 6. When ICl was enhanced by 5 microM-milrinone, an additional application of cyclic GMP failed to increase ICl. In the presence of cyclic GMP, milrinone failed to enhance ICl. 7. The above findings on ICl are analogous to the enhancement by cyclic GMP of the beta-adrenergic stimulation of the Ca2+ current reported in the same preparation, and support the hypothesis that in mammalian cardiac cells cyclic GMP potentiates elevation of cyclic AMP induced by beta-adrenergic agents, and thereby increases the amplitudes of ionic currents.

Beta-adrenergic and muscarinic regulation of the chloride current in guinea-pig ventricular cells.

1. Single guinea-pig ventricular cells were voltage clamped using the patch clamp method combined with the pipette-perfusion technique. The voltage-dependent current systems were mostly blocked, and the background membrane conductance was measured by applying ramp pulses. 2. beta-Adrenergic effectors and related substances such as adrenaline, isoprenaline, forskolin or internal application of cyclic AMP induced a current component which showed a reversal potential near the expected Cl- equilibrium potential as well as an outward rectification in the I-V relation. It is suggested that the activation of this Cl- current was due to phosphorylation of the channel protein or related structure by the cyclic AMP-dependent protein kinase. Coincidentally with the activation of the Cl- current, the membrane capacitance of the cell decreased reversibly. 3. Acetylcholine (ACh) depressed the responses induced by beta-adrenergic stimulation and forskolin, but failed to interfere with the one induced by cyclic AMP. 4. The dose dependence of the Cl- current activation by isoprenaline or forskolin was fitted by the Hill equation, with a coefficient of 1.9 and a half-maximum concentration K 1/2 = 13 nM for isoprenaline, and with a Hill coefficient of 3 and a K 1/2 = 1.2 microM for forskolin. In the presence of 5.5 microM-ACh the dose-response relation shifted to higher doses; K 1/2 was 65 nM for isoprenaline and 3.6 microM for forskolin. 5. Washing out ACh in the presence of isoprenaline frequently caused transient overshoots of the response. When a saturating concentration of isoprenaline was used, this rebound was not observed. 6. The internal application of cyclic GMP enhanced the response of the Cl- current induced by isoprenaline or adrenaline. 7. When cyclic AMP was applied internally, the response was small in most cells. When the cell was superfused with 20 microM-IBMX (3-isobutyl-1-methylxanthine), the Cl- current was consistently induced by the application of cyclic AMP. It is suggested that phosphodiesterase activity strongly buffered the influx of cyclic AMP through the patch pipette tip. 8. We suggest that the compensatory interaction between the beta-adrenergic stimulation and the muscarinic inhibition is at the membrane level, most probably via GTP-binding proteins in activating adenylate cyclase.