On the mechanism of beta-adrenergic regulation of the Ca channel in the guinea-pig heart

Phosphorylation of the purified receptor for calcium channel blockers by cAMP kinase and protein kinase C

The dihydropyridine receptor purified from rabbit skeletal muscle contains three proteins of 165, 55 and 32 kDa. cAMP kinase and protein kinase C phosphorylate the 165-kDa and the 55-kDa proteins. At identical concentrations of each protein kinase, cAMP kinase phosphorylates the 165-kDa protein faster than the 55-kDa protein. Protein kinase C phosphorylates preferentially the 55-kDa protein. cAMP kinase incorporates up to 1.6 mol phosphate/mol protein into the 165-kDa protein and 1 mol/mol into the 55-kDa protein upon prolonged incubation. At a physiological concentration of cAMP kinase 1 mol phosphate is incorporated/mol 165-kDa protein within 10 min, suggesting a physiological role of this phosphorylation. Protein kinase C incorporates up to 1 mol phosphate/mol into the 55-kDa protein and less than 1 mol/mol into the 165-kDa protein. Tryptic phosphopeptide analysis reveals that cAMP kinase phosphorylates two distinct peptides in the 165-kDa protein, whereas protein kinase C phosphorylates a single peptide in the 165-kDa protein. cAMP kinase and protein kinase C phosphorylate three and two peptides in the 55-kDa protein, respectively. Mixtures of the tryptic phosphopeptides derived from the 165-kDa and 55-kDa proteins elute according to the composite of the two elution profiles. These results suggest that the 165-kDa protein, which contains the binding sites for each class of calcium channel blockers and the basic calcium-conducting structure, is a specific substrate for cAMP kinase. The 55-kDa protein apparently contains sites preferentially phosphorylated by protein kinase C.

Cell-to-cell coupling studied by diffusional methods in myocardial cells

The diffusion of large molecular substances from cell to cell in multicellular and enzymatically isolated cell pairs is described. Permeability of the gap junctional membrane to these molecules and the critical diffusing diameter of the myocardial gap junctional channel are discussed.

On the mechanism of histamine induced enhancement of the cardiac Ca2+ current

In guinea pig ventricular myocytes, the effect of histamine on the slow Ca2+ current (ICa) was studied and the following results were obtained: (1) Superfusion of cells with histamine resulted in a dose-dependent enhancement of the amplitude of ICa. The threshold concentration of histamine was 10(-8) M, half maximal increase occurred at 3 X 10(-7) M and maximal enhancement (about 3-4-fold) at 5 X 10(-6) M. (2) The histamine effect was greatly reduced by the H2 antagonist cimetidine (10(-5) M) but only slightly by the H1 antagonist diphenhydramine (10(-5) M). (3) Effects of isoprenaline (ISP) and histamine at maximal effective concentrations on ICa were not additive, suggesting that both agents use the same intracellular pathway. Intracellular infusion of a blocker of the cAMP-dependent protein kinase, Rp-cAMPS (10(-4) M), prevented the histamine effect. (4) The involvement of GTP-dependent transducer proteins was studied by cell dialysis with several GTP derivatives. Intracellular application of the stable GDP-analogue, GDP-beta-S, reduced the histamine effect on ICa, whereas the stable GTP analogue, GTP-gamma-S, mimicked the histamine effect.

On the relationship between V max of slow responses and Ca-current availability in whole-cell clamped guinea pig heart cells

The relationship between Ca current availability and maximum rate of rise (V max) of slow responses was determined in the same single guinea pig ventricular heart cell under voltage and current clamp conditions (whole-cell clamp technique). The results are as follows. (1) Cell capacitance measured in 32 cells from the current response to a fast ramp voltage-clamp pulse (119.6 +/- 4.6 pF, mean +/- SE) or from Vmax values at a holding potential of -50 or -40 mV (118.6 +/- 5.3 pF) are identical. (2) In control conditions ([Ca]o 1.8, [K]o 4 and [Cs]i 140 mM), voltage-dependence of steady-state inactivation of Ca current (ICa) or Vmax are similar up to -35 mV. However, Vmax significantly (P less than 0.005) underestimates ICa availability at more positive potentials. At -30 mV, ICa and Vmax amplitudes represent respectively 35.6 and 22.4% (n = 14) of their maximum value. (3) In the presence of 50 nM isoprenaline, Vmax and the underlying ICa are respectively increased by 79.2 +/- 13.8% and 71.2 +/- 13.8% (n = 15). No statistically significant deviation from linearity is then observed. (4) When Vmax amplitude is expressed as a function of ICa density, an almost linear relationship is observed for Vmax values between 0 and 25 V/s. Vmax is then best described by the equation: Vmax (V/s) = 1.043 ICa (pA/pF) -0.514 (46 cells). (5) We conclude that, under conditions that minimize outward currents, Vmax of slow responses accurately measures ICa amplitude, except when ICa is decreased to less than 40% of its maximum control amplitude (i.e., below 4 pA/pF). At that point, Vmax underestimates ICa.

Effects of calcium release from sarcoplasmic reticulum on membrane currents in guinea pig atrial cardioballs

(1) Ca current (ICa) and membrane currents related to Ca-entry during activation of ICa have been studied in cultured atrial myocytes from hearts of adult guinea pigs by means of patch clamp pipettes. The pipettes were filled with solutions containing citrate (65 mM) as major Ca-chelating compound and Cs ions in order to block K currents. (2) In myocytes dialysed with such solutions a monophasic time course of inactivation of ICa is observed, which is 1-2 orders of magnitude slower as compared to studies on intact cardiac cells or cells dialysed with EGTA as only Ca-chelating compound. (3) During long-lasting or repetitive depolarization a second component of ICa inactivation, apart from the slow decay observed in cells dialysed with such solutions, can be seen. This component of inactivation is not related to the depolarization as such but to loading of the cells with Ca2+. Whenever the rapid component of inactivation occurs, a transient inward current (Iti) after repolarization to the holding potential (-40 to -50 mV) is recorded. Both, ICa inactivation and Iti can be mimicked by extracellular application of caffeine (5-10 mM), suggesting both current changes to be caused by a rise in Cai due to Ca release from sarcoplasmic reticulum. In the presence of caffeine the rapid component of ICa-inactivation and Iti are abolished. (4) In addition to ICa inactivation and activation of Iti sarcoplasmic Ca release causes openings of a novel ion channel with large conductance (greater than 200 pS), the function of which is unknown. (5) The results are consistent with the concept of Cai-dependent inactivation of Ca current, which can be caused either by Ca-entry or by Ca-release from the SR. The transient inward current is likely to reflect a process of Ca-removal from the cell, namely Na-Ca exchange.

Catecholamines modulate the delayed rectifying potassium current (IK) in guinea pig ventricular myocytes

The effects of isoproterenol and norepinephrine on the delayed, outward potassium current, IK, were tested in single, dialyzed guinea pig heart cells where complications from a restricted extracellular space are minimized and internal K concentration is controlled. The average IK reversal potential in control cells was -80 +/- 2.1 mV (N = 4; [K]o = 4.5 mM; [K]i = 120 mM) indicating a high degree of K selectivity (PNa/PK less than or equal to 0.01). In paired experiments, both isoproterenol and norepinephrine increased IK without changing the reversal potential, indicating that the selectivity of the channels is not altered by these agents. The results explain the shortening of action potential duration observed at high concentrations (Quadbeck and Reiter, 1975); and suggest that an increase in IK by catecholamines may serve to limit the degree of calcium current-induced action potential prolongation during increased sympathetic tone and rapid heart rates.

A novel holder allowing internal perfusion of patch-clamp pipettes

We describe a simple pipette holder which allows, within a single experiment, multiple exchanges of the solution inside "gigaseal" glass pipettes commonly used for electrical studies of single cells or isolated membrane patches. The design minimizes electrostatic and mechanical perturbations associated with perfusion by integrating into the holder a reservoir which is connected to a perfusion pipette fabricated from flexible, resilient quartz tubing. The tip of the perfusion pipette can be pulled to any diameter and positioned precisely within the main patch-pipette by sliding the reservoir along a guide in the holder. An open reservoir for suction driven solution exchange, and a closed reservoir for pressure driven solution exchange were developed. For the open system, the speed of solution exchange was studied as a function of the tip diameter of the perfusion pipette (approximately 22 s for a 40 micron tip diameter). Both systems were characterized using atrial myocytes (a) by examining the effects of intracellular applications of cAMP or of the catalytic subunit of protein kinase A on calcium currents in the whole cell recording mode and (b) by studying the effects of local applications of acetylcholine (ACh) on single channel currents in the isolated membrane patch mode.

Electrophysiological and other aspects of excitation-contraction coupling and uncoupling in mammalian airway smooth muscle

In this article the electrophysiological events which are believed to underly agonist-induced contraction and relaxation of airway smooth muscle are reviewed, with special emphasis on the indispensable role of the Ca ion. The contribution made by Na, K, Ca and Cl to, and the role that the electrogenic Na:K-dependent ATPase plays in, the maintenance of the resting membrane potential in both normal and sensitised airway smooth muscle cells is described together with the permeability changes that occur in the plasmalemma in response to excitatory and inhibitory agonists. In addition, the currently available evidence for the existence of potential-sensitive and receptor-operated Ca channels in respiratory smooth muscle, and how such channels may be involved in the regulation of airway calibre, is critically assessed.

Effect of adrenergic agonists on Ca2+-channel currents in single vascular smooth muscle cells

Ca2+-channel currents have been measured in enzymatically dispersed single smooth muscle cells of the rabbit ear artery using the whole-cell patch clamp technique. Inward currents were elicited by depolarizing test pulses from a holding potential of -50 mV. These currents were activated from -30 mV onward and reached full activation around 0 mV. alpha-Adrenergic agonists did not affect the background current measured at the holding potential, but markedly reduced the peak amplitude of the voltage-activated Ca2+-channel currents. This alpha-adrenergic inhibition also occurred in cells which were internally perfused with solutions containing either 10 microM cAMP, 10 microM cGMP or 0.1 mM GTP, but became irreversible when the pipette solution contained a non-hydrolyzable GTP-analog. The action of beta-agonists on the voltage-activated Ca2+-channel currents was variable, and ranged from no effect at all to a 50% reduction of the current. It is concluded that alpha-agonists do not open receptor-operated Ca2+-channels in these smooth muscle cells. The inhibition of the voltage-activated Ca2+-currents does not seem to be mediated through changes in cyclic nucleotide levels, but might be mediated through G-proteins. Its physiological relevance remains however unclear. The action of beta-agonists is consistent with their relaxing effect, but the reason for the non-uniform response has not been elucidated.

Cyclic guanosine 3',5'-monophosphate regulates the calcium current in single cells from frog ventricle

1. The effect of intracellular perfusion with cyclic AMP and cyclic GMP on Ca2+ current (ICa) was studied in single cells isolated from frog ventricle using the whole-cell patch-clamp technique and a perfused pipette. 2. Intracellular perfusion with cyclic GMP (0.1-20 microM) had no effect on the basal ICa. However, when ICa was increased by isoprenaline or by intracellular perfusion with cyclic AMP, perfusion with cyclic GMP (20 microM) reduced ICa by an average of 67%. The effect of cyclic GMP on ICa elevated by cyclic AMP was reversible. A half-maximal effect of cyclic GMP was observed at 0.6 microM. Cyclic GMP had no significant effect on the shape of the ICa current-voltage relationship. 3. The effect of cyclic GMP was specific to the 3',5' form; 2',3'-cyclic GMP had no effect. 4. The effect of cyclic GMP was apparently not mediated by stimulation of cyclic-GMP-dependent protein kinase because 8-bromo-cyclic GMP, a very potent activator of the protein kinase, was without effect. 5. Cyclic GMP had no effect on ICa elevated by the non-hydrolysable 8-bromo-cyclic AMP. The effect of cyclic GMP on cyclic-AMP-elevated ICa was partially blocked by the phosphodiesterase inhibitor, methylisobutylxanthine. Thus, it was hypothesized that the effect of cyclic GMP was mediated by hydrolysis of cyclic AMP as a result of a stimulation of a cyclic nucleotide phosphodiesterase by cyclic GMP. 6. The dose-response curve for cyclic AMP on ICa was well fitted by the Michaelis equation with a K50 (i.e. concentration of cyclic AMP at which response is 50% of the maximum) of 0.7 microM and a maximal 11-fold stimulation of ICa. Cyclic GMP shifted the curve one log unit to the right and decreased the maximal stimulation to 8.6-fold. Thus, the effect of cyclic GMP appeared uncompetitive. 7. The products of cyclic AMP and cyclic GMP hydrolysis, 5'-AMP and 5'-GMP, had no effect on ICa. Furthermore, strong buffering of intracellular pH did not reduce the effect of cyclic GMP. 8. It is proposed that cyclic-GMP-stimulation of a cyclic nucleotide phosphodiesterase may be one of several mechanisms by which acetylcholine regulates ICa.

Regulation of the cardiac calcium channel by protein phosphatases

The calcium current (ICa) through the L-type channel in cardiac ventricular cells is enhanced by phosphorylation of a channel protein [Kameyama, M., Hofmann, F. & Trautwein, W. (1985) Pflügers Arch. Eur. J. Physiol. 405, 285-293]. We investigated the possible contribution of the 'catalytic subunits' of protein phosphatase 1 and 2A in the down-regulation of the cardiac calcium channel. Single guinea-pig ventricular myocytes were voltage clamped and the following results were obtained. (1) Intracellular perfusion of the myocyte with the catalytic subunits of protein phosphatase 1 (2 microM) as well as 2A (2.3 microM) completely abolished the increase of ICa induced by isoprenaline (0.05 microM) but did not decrease the basal level of ICa. Alkaline and acid phosphatases were without detectable effect. (2) Cell dialysis with the modulator of protein phosphatase 1 (inhibitor-2) under control conditions (without addition of isoprenaline) caused a slow significant increase of ICa. (3) The time course for the wash-out of the isoprenaline effect was considerably prolonged in the presence of high concentrations of inhibitor-2. (4) Perfusion of the myocyte under basal conditions with adenosine 5'-[gamma-thio]triphosphate led to a slow increase of ICa. Additional superfusion of the cell with a threshold concentration of isoprenaline (0.01 microM) resulted in a rapid increase of ICa which could not be washed out during at least 10 min. From these results we make the following conclusions. (1) The calcium channel from guinea-pig myocytes is regulated by phosphorylation-dephosphorylation. (2) The catalytic subunits of the protein phosphatases 1 as well as 2A, purified from rabbit skeletal muscle, catalyse the down-regulation of the channel. (3) Indirect evidence suggests that endogenous protein phosphatase 1 contributes only partially to the dephosphorylation of the calcium channel in the intact myocyte.

Modulation of ionic currents in smooth muscle balls of the rabbit intestine by intracellularly perfused ATP and cyclic AMP

The effects of intracellularly perfused ATP and cyclic-AMP (c-AMP) on ionic currents recorded from fragmented smooth muscle cells (smooth muscle ball; SMB) were investigated, using the single electrode whole cell voltage clamp method. The Ca2+ current was distinguished from K+ currents, using pipette solution containing Cs+, TEA+ and 4 mM EGTA. ATP enhanced the Ca2+ current dose-dependently between 0.3 and 10 mM, and slightly slowed the slow component of the decay of the Ca2+ current, while the steady-state inactivation curve remained unaffected. Intracellular application of 5'-adenylyl-imidodiphosphate (AMP-PNP; 1 mM) inhibited the Ca2+ current by competition with ATP, but c-AMP (up to 300 microM) had no effect. With a high-K+ solution containing 0.3 mM EGTA and ATP in the pipette and physiological salt solution in the bath, a net inward current with transient (Ca2+ dependent) and delayed (Ca2+ independent) K+ outwart currents were evoked. Increased concentrations of ATP (above 1 mM) but not c-AMP (up to 100 microM) in the pipette enhanced the transient K+ outward current. Neither agent had any effect on the delayed outward current. When repetitive stimulations of intervals shorter than 5 s were applied, the amplitude of the transient outward current was markedly reduced, and 100 microM c-AMP partially prevented this attenuation. ATP may act on the Ca2+ channel either by phosphorylating the channel protein or by other ATP requiring mechanisms, independently from those induced by the action of c-AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between beta-adrenoceptors and calcium channels in human ventricular myocardium

The stoichiometric relationship between adrenoceptors and saturable binding sites for 1,4-dihydropyridines in calcium channels was investigated in human ventricular myocardium. Membrane particles were prepared from heart specimens of patients undergoing open heart surgery. The patients suffered from hypertrophic obstructive cardiomyopathy (HOCM) or mitral valve disease. Using [3H]-prazosin and [125I]-2-beta-hydroxy-3-iodiphenyl-ethyl-aminoethyl tetralone ([125I]-HEAT) as labels we detected only a marginal density of alpha 1-adrenoceptors, regardless of disease. No alpha 2-adrenoceptors were detected with [3H]-rauwolscine. In HOCM patients we estimated 72 +/- 10 fmol mg-1 (n = 12) beta-adrenoceptors labelled with [3H]-(-)-dihydroalprenolol and 74 +/- 5 fmol mg-1 (n = 2) beta-adrenoceptors labelled with [125I]-(-)-iodocyanopindolol; the equilibrium dissociation constants KD, were 1.2 +/- 0.2 nmol l-1 for [3H]-(-)-dihydroalprenolol and 7 +/- 1 pmol l-1 for [125I]-(-)-iodocyanopindolol. In patients with mitral valve disease we estimated 84 +/- 11 fmol mg-1 (n = 3) labelled with [3H]-(-)-dihydroalprenolol and 66 +/- 13 fmol mg-1 (n = 2) labelled with [125I]-(-)-iodocyanopindolol. The KD values were 1.8 +/- 0.6 nmol l-1 for [3H]-(-)-dihydroalprenolol and 8 +/- 2 pmol l-1 for [125I]-(-)-iodocyanopindolol. In 14 HOCM patients we estimated 107 +/- 12 fmol mg-1 calcium channel sites labelled with [3H]-nimodipine with a KD of 280 +/- 4 pmol l-1. In 5 patients with mitral valve disease the density of calcium channel sites labelled with [3H]-nimodipine was 78 +/- 5 fmol mg-1 with a KD of 290 +/- 20 pmol l-1, In HOCM patients the density of calcium channel sites labelled with the benzoxadiazol 1,4-dihydropyridine ([3H]-(+)-PN 200-110) was 1.6 fold of that labelled with [3H]-nimodipine with a KD of 84 +/- 11 pmol l-1. In a group of 4 HOCM patients in which calcium channels were labelled with [125I]-iodipine, the density of sites was 1.37 +/- 0.07 fold the density of sites labelled by [3H]-(+)-PN 200-11-. The KD value of [125I]-iodipine was 246 +/- 16 pmol-1. (+)-PN 200-110 was approximately 100 fold more potent than (-)-PN 200-110 as a competitor of [125I]-iodipine binding. For the HOCM group a significant correlation was found between beta-adrenoceptor density and calcium channel density, whereas in the mitral valve group no such correlation was found. This does not prove that there is causal interaction leading to a relationship between the density of beta-adrenoceptors and calcium channels. However, because positive inotropic effects of catecholamines mediated by beta-adrenoceptors are associated with opening of calcium channels, this suggests that the density of both beta-adrenoceptors and calcium channels could be co-regulated.

Purification of a functional receptor for calcium-channel blockers from rabbit skeletal-muscle microsomes

The dihydropyridine receptor was purified from rabbit skeletal muscle microsomes in the presence of [3H]nitrendipine plus diltiazem or [3H](+)PN 200-110 to an apparent density of 1.5-2 nmol binding sites/mg protein. Sodium dodecyl sulfate gel electrophoresis in the absence of reducing agents yielded three peptide bands of 142, 56 and 30 kDa in a relative ratio of 11:1:1.3, whereas in the presence of 40 mM dithiothreitol bands of 142, 122, 56, 31, 26 and 22 kDa were obtained in a relative ratio of 5.5:2.2:1:0.9:14:0.09. This gel pattern was observed regardless of whether the receptor was purified as a complex with nitrendipine plus diltiazem or with (+)PN 200-110. cAMP-dependent protein kinase phosphorylated preferentially the 142-kDa band up to a stoichiometry of 0.82 +/- 0.07 (15) mol phosphate/mol peptide. The 56-kDa band was phosphorylated only in substoichiometric amounts. [3H]PN 200-110 bound at 4 degrees C to one site with apparent Kd and Bmax values of 9.3 +/- 1.7 nM and 2.2 +/- 0.3 (3) nmol/mg protein, respectively. The binding was stereospecific and was not observed in the presence of 1 mM EGTA. Desmethoxyverapamil interfered with the binding of [3H]PN 200-110 in an apparent allosteric manner. (-)Desmethoxyverapamil inhibited the binding of [3H]PN 200-110 at 37 degrees C and stimulated it at 18 degrees C. In agreement with these results, (-)desmethoxyverapamil increased the dissociation rate of [3H]PN 200-110 from 0.29 min-1 to 0.38 min-1 at 37 degrees C and decreased it threefold from 0.046 min-1 to 0.017 min-1 at 18 degrees C. The (+)isomer of desmethoxyverapamil inhibited PN 200-110 binding at all temperatures tested. d-cis-Diltiazem stimulated the binding of [3H]PN 200-110 at 37 degrees C with an apparent EC50 of 1.4 microM and decreased the dissociation rate from 0.29 min-1 to 0.11 min-1. The stimulatory effect of d-cis-diltiazem was temperature-dependent and was seen only at temperatures above 18 degrees C. These results suggest that the purified dihydropyridine receptor retains the basic properties of the membrane-bound receptor and contains separate sites for at least dihydropyridines and phenylalkylamines.

The protein-specific phosphatase 1 antagonizes the beta-adrenergic increase of the cardiac Ca current

In isolated ventricular cells from the adult guinea pig heart the slow Ca current was recorded during beta-adrenergic stimulation and during cell dialysis with a protein-specific phosphatase-1 (PPase-1). The increase in the amplitude of ICa during bath application of isoprenaline (5 X 10(-8) M) could be completely reversed by dialysing the cell with 2 microM PPase-1. Lower enzyme concentrations produced smaller effects. The control amplitude of ICa was only little affected by dialysis with PPase-1. The result suggests that PPase-1 is a likely candidate for the downregulation of Ca channels, that Ca channels can open in the dephosphorylated state.

Opposite effects of cyclic GMP and cyclic AMP on Ca2+ current in single heart cells

The slow inward Ca2+ current, ICa, is fundamental in the initiation of cardiac contraction and neurohormonal regulation of cardiac function. It is increased by beta-adrenergic agonists, which stimulate synthesis of cyclic AMP (cAMP) and cAMP-dependent phosphorylation. The neurotransmitter acetylcholine reduces ICa by an unknown mechanism. There is strong evidence that acetylcholine reduces ICa by decreasing adenylate cyclase activity, but cGMP has also been implicated as ACh stimulates cGMP accumulation and activates cGMP-dependent protein kinase. Application of cGMP decreases contractile force, decreases Ca flux, shortens the duration of action potentials and inhibits Ca-dependent action potentials. Other studies, however, have concluded that cGMP levels do not correlate with contractile force and that cGMP has no effect on ICa. We have therefore examined the effects of intracellular perfusion of cGMP on ICa using isolated, voltage-clamped cells from frog ventricle. We find that cGMP has negligible effects on basal ICa, but greatly decreases the ICa that had been elevated by beta-adrenergic agonists or by intracellular perfusion with cAMP. The decrease of ICa is mediated by cAMP hydrolysis via a cGMP-stimulated cyclic nucleotide phosphodiesterase.

Purified dihydropyridine-binding site from skeletal muscle t-tubules is a functional calcium channel

Many excitable cells contain at least two different voltage-dependent Ca channels (L- and T-type). The cardiac, slow, L-type Ca channel is further modulated by cyclic AMP-dependent phosphorylation, which increases the probability of it being open, and is readily blocked by Ca channel blockers including dihydropyridines and phenylalkylamines. The tritiated congeners of these blockers bind in vitro to sites which have the same pharmacological characteristics as those observed in vivo, that is, stereospecific and allosteric interaction between distinct sites. The dihydropyridine-binding site purified from skeletal muscle t-tubules contains three peptides of relative molecular mass (Mr) 142,000 (142K), 56K and 31K. The cAMP kinase incorporates one mol phosphate per mol of the 142K peptide and binding of (+)PN-200/110, a potent Ca antagonist, is allosterically affected by D-cis-diltiazem and verapamil. The purified dihydropyridine-receptor complex has also been incorporated into phospholipid bilayer membranes. Here, we show for the first time that the complex can be reconstituted to form a functional 20-pS Ca channel that retains the principal regulatory, biochemical and pharmacological properties of membrane-bound L-type Ca channels.

An enzymatic mechanism for calcium current inactivation in dialysed Helix neurones

'Wash-out' and inactivation of the Ca current were examined in dialysed, voltage-clamped neurones of Helix aspersa under conditions that isolate the Ca current virtually free of other currents. EGTA or other internal Ca2+ chelators were routinely omitted from the dialysate. The time-dependent loss, or wash-out, of Ca current was slowed by addition to the dialysing solution of agents, such as dibutyryl adenosine 3'-5'-cyclic monophosphate (dibutyryl cyclic AMP), Mg adenosine 5'-triphosphate (ATP) and the catalytic subunit of cyclic-AMP-dependent protein kinase, that promote protein phosphorylation and by EGTA. However, neither the phosphorylation-promoting agents nor internal EGTA prevented wash-out entirely, nor did they significantly restore previously 'washed-out' current. With phosphorylating agents in the dialysing solution, the irreversible development of wash-out was greatly reduced by introduction of leupeptin, an inhibitor of protease activity. Thus, the irreversible component of wash-out appears to result from a Ca-dependent proteolytic process. In the presence of leupeptin alone, Ca current amplitude continued to decline: however, the current could be largely or fully restored with addition of catalytic subunit, dibutyryl cyclic AMP, and Mg ATP to the dialysing solution. Thus, inhibition of proteolysis revealed a reversible component of wash-out that appears to result from dephosphorylation. During perfusion with leupeptin, Mg ATP, dibutyryl cyclic AMP and catalytic subunit the Ca current remained stable for up to several hours without addition of internal Ca2+ buffer. The rate of inactivation of the current that occurs during a depolarizing step showed only a very gradual decline during this time. Under these conditions, perfusion with calcineurin, a Ca-calmodulin-dependent phosphatase, caused a significant increase in the rate of Ca current inactivation. This inactivation was virtually eliminated by introduction of EGTA or by replacement of external Ca2+ with Ba2+, which is consistent with the ion dependency for calmodulin-dependent activation of calcineurin. When ATP in the dialysate was replaced with ATP-gamma-S (adenosine 5'-O-(thiotriphosphate], an analogue that donates a thiophosphate group resistant to hydrolysis, the rate of inactivation slowed. Since Ca-dependent inactivation during step depolarizations is enhanced by conditions that promote dephosphorylation, and Ca current wash-out is slowed by conditions that promote phosphorylation, inactivation and reversible wash-out appear to be related.(ABSTRACT TRUNCATED AT 400 WORDS)

On the mechanism of muscarinic inhibition of the cardiac Ca current

The mechanism of muscarinic inhibition of the Ca-current (ICa) was studied in ventricular myocytes of guinea pig hearts and the following results were obtained. Acetylcholine (ACh) in concentrations up to 10(-4) M had little effect, if any, on ICa in control cells. ACh reduced the isoprenaline (ISP)-induced increase of ICa. The dose-response-relation (ISP concentration vs. ICa density) was shifted by ACh towards higher ISP concentrations. But both, at low and high ISP concentrations ACh had nor or little effect. ACh was ineffective when ICa was increased by dialysing the cell with catalytic subunit of cAMP-dependent protein kinase or cAMP. ACh reduced ICa enhanced by isobutylmethylxanthine or by forskolin. ACh did not depress ICa when the cell was dialysed with the non-hydrolysable GTP-derivative, GMP-PNP. In this condition the beta-adrenergic enhancement of ICa was also absent. Pertussis toxin, which is known to inhibit the inhibitory transducer protein (Ni), abolished the ACh response. We concluded from these results that ACh depresses ICa by inhibiting, via Ni, the cAMP production.

Modulation of Ca current during the phosphorylation cycle in the guinea pig heart

The calcium current (ICa) in the heart is increased by phosphorylation of a protein which is part of, or close to, the Ca channel. The phosphorylation is catalysed by cAMP-dependent protein kinase (cAMP-PK). The question whether dephosphorylated channels are available to open on depolarization was examined in ventricular myocytes of guinea pig by recording whole cell ICa during dialysis with either regulatory (R) subunit of cAMP-PK or protein kinase inhibitor (PKI) or adenosine-5'-(gamma-thio)-triphosphate (ATP gamma S). The following results were obtained: 1) R subunit reduced and PKI reversed the isoprenaline (ISP)-induced enhancement of ICa, suggesting their ability to inhibit cAMP-PK. 2) R subunit and PKI, however, reduced basal (i.e. non beta-adrenergically stimulated) ICa only by about 20%. 3) Dialysis with ATP gamma S resulted in a slow increase in basal ICa, presumably due to dephosphorylation-resistant thiophosphorylation. 4) When, however, the cell was dialyzed with PKI the effect of ATP gamma S was almost completely suppressed, suggesting no detectable phosphorylation related to the channel activity in this condition. These results support the view that even in the dephosphorylated state Ca channels are available to open on depolarization and that phosphorylation by cAMP-PK increases the opening probability.

Mechanism of action of acetylcholine on calcium current in single cells from frog ventricle

Ca currents (ICa) were measured by whole-cell patch clamp in single cells isolated from frog ventricle in which K currents were blocked with intracellular (120 mM) and extracellular (20 mM) Cs. Inward currents elicited by depolarizing voltage steps from a holding potential of -80 mV were blocked completely by 0.5 mM-Cd. The quality of the voltage clamp was assessed using two patch electrodes on a single cell. One electrode was used in the voltage-clamp mode to measure membrane currents and the other in current-clamp to measure membrane potential. Ca currents as large as 2 nA were well clamped in cells as long as 210 micron. Acetylcholine (ACh) had no effect on ICa in the absence of beta-adrenergic stimulation but reduced to control levels ICa elevated by isoprenaline. Nanomolar concentrations of ACh were able to reduce significantly ICa elevated by 2 microM-isoprenaline. ACh had no effect on the shape of the I-V curve, on the reactivation (recovery from inactivation), or on the inactivation of ICa. Although isoprenaline increased ICa by an average of 6.5-fold, it had no effect on the shape of the I-V curve or on the inactivation at test potentials negative to +40 mV. However, isoprenaline slowed the half-reactivation time from a control value of 120 +/- 10 ms (mean +/- S.D.) to 153 +/- 12 ms at -80 mV. The effect of cyclic AMP on ICa was investigated using two patch electrodes, one filled with cyclic AMP. Maximal effects of cyclic AMP were observed with 5 microM-cyclic AMP in the pipette. Maximal ICa was recorded several minutes after breaking the patch with the second electrode. After removing the cyclic-AMP-containing electrode, ICa declined to control levels after approximately 10 min. 5 microM-cyclic AMP in the patch electrode increased ICa by an average of 6.9-fold, but had no effect on the shape of the I-V curve or on inactivation. Cyclic AMP had a slowing effect on reactivation (half-reactivation time = 155 +/- 24 ms) similar to that of isoprenaline. ACh (1-10 microM) did not reduce ICa elevated with cyclic AMP (0.1-20 microM-cyclic AMP in the pipette). With low concentrations of cyclic AMP in the pipette (0.1 microM), isoprenaline augmented ICa, but with 5 microM-cyclic AMP in the pipette, isoprenaline was incapable of increasing ICa further. These results suggest that the decrease of ICa produced by ACh can be explained solely by decreases in cyclic AMP levels.